HVAC glossary for homeowners

This glossary explains the terms you'll hear from HVAC contractors, see on quotes, and read on equipment labels, in plain English. Each entry tells you what the term means and why it matters when you're hiring a local heating and cooling company.

Terms are grouped by topic: equipment types, parts, efficiency ratings, refrigerants, heating, ductwork, indoor air quality, thermostats, service and repair, installation rules, contractor and pricing terms, and common problems. If a contractor uses a word you don't recognize, look it up here before you sign anything.

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A to Z index

Systems and equipment types

HVAC: Heating, Ventilation, and Air Conditioning. The umbrella term for everything that heats, cools, filters, and moves air in your home. Some contractors add "R" (HVAC-R) for refrigeration.
Central air conditioning: A whole-home cooling system with an outdoor unit (condenser) and an indoor unit (evaporator coil, usually mounted on the furnace or air handler), connected by refrigerant lines and delivering cool air through ducts.
Split system: Any system where the equipment is divided between an outdoor unit and an indoor unit. Most central AC and heat pump systems in homes are split systems.
Packaged unit (package unit): All the heating and cooling components in one cabinet, usually placed on the roof or a concrete pad beside the house. Common in homes without basements or attics and in warmer regions.
Heat pump: An outdoor unit that looks like an AC but runs in both directions: it pulls heat out of the house in summer and pulls heat from outdoor air into the house in winter. Very efficient in mild and moderate climates. Newer cold-climate models work well below freezing.
Air-source heat pump: The standard type of heat pump, exchanging heat with the outdoor air. When someone says "heat pump" without qualifying it, this is what they mean.
Geothermal (ground-source) heat pump: A heat pump that exchanges heat with the ground or a well through buried loops instead of outdoor air. Highest efficiency of any system but expensive to install because of excavation or drilling.
Dual-fuel (hybrid) system: A heat pump paired with a gas furnace. The heat pump handles mild weather; the furnace takes over when it gets cold enough that gas is cheaper or more effective. Often the most cost-effective setup in mixed climates.
Furnace: Heating equipment that warms air and blows it through ducts. Gas furnaces burn natural gas or propane; electric furnaces use heating elements; oil furnaces burn heating oil.
Air handler: The indoor unit that contains the blower fan and, usually, the evaporator coil. Homes with a heat pump or electric heat have an air handler instead of a furnace. It may also hold electric heat strips.
Ductless mini-split: A heat pump or AC with a slim outdoor unit and one or more wall-, ceiling-, or floor-mounted indoor "heads," connected by refrigerant lines with no ductwork. Ideal for additions, garages, sunrooms, older homes without ducts, or rooms that are always too hot or cold.
Multi-zone mini-split: One outdoor unit serving several indoor heads, each with its own temperature control.
Boiler: Heats water (or makes steam) and sends it through radiators, baseboard heaters, or in-floor tubing. Boilers heat with water, not air, so they don't provide cooling or filtration.
Hydronic heating: Any heating system that moves heat with hot water, such as a boiler with radiators or radiant floor tubing.
Radiant floor heating: Tubing (hydronic) or electric mats under the floor that warm the surface and the room from below.
Window unit / room air conditioner: A self-contained AC that sits in a window or through-the-wall sleeve and cools one room. Not a whole-home solution but cheap and easy to install.
Portable air conditioner: A wheeled AC that vents hot air through a hose to a window. Less efficient than a window unit of the same size.
PTAC (packaged terminal air conditioner): The through-the-wall heating/cooling units common in hotels; occasionally used in sunrooms or converted spaces.
Evaporative cooler (swamp cooler): Cools air by blowing it across wet pads. Works only in dry climates and adds humidity to the home.
Rooftop unit (RTU): A packaged unit installed on a roof. Common on commercial buildings and some homes.
Variable-speed / inverter system: Equipment whose compressor and fan can run at many speeds instead of just on and off. Quieter, more efficient, and better at holding a steady temperature and humidity. Most mini-splits and premium central systems are inverter-driven.
Single-stage, two-stage, and modulating: Describes how many output levels the equipment has. Single-stage is all-or-nothing; two-stage has a low and high setting; modulating (variable-capacity) adjusts in small steps. More stages generally mean better comfort and efficiency at a higher purchase price.
Communicating system: A matched system where the thermostat, indoor unit, and outdoor unit share data over a digital link rather than simple on/off wires. Usually requires all parts to be from the same brand.
Matched system: An outdoor unit and indoor coil/air handler that the manufacturer has tested and rated together. Mismatched parts can lose efficiency and void warranties.
Whole-house fan: A large fan in the attic ceiling that pulls cool evening air through open windows and pushes hot air out through the attic. Not an air conditioner.
Attic fan / powered attic ventilator: Exhausts hot air from the attic only, to reduce heat load on the house. Different from a whole-house fan.

Key components and parts

Compressor: The heart of an AC or heat pump, located in the outdoor unit. It pumps refrigerant through the system. It's the most expensive single part; a failed compressor on an older unit often means replacing the whole outdoor unit.
Condenser (condensing unit): The outdoor unit of a central AC or heat pump. It contains the compressor, condenser coil, and fan, and releases heat pulled from the house into the outside air.
Condenser coil: The coil in the outdoor unit where refrigerant releases heat. Dirt, grass clippings, and cottonwood fluff on this coil hurt efficiency; cleaning it is part of a tune-up.
Evaporator coil (indoor coil, A-coil): The coil inside the indoor unit where refrigerant absorbs heat from your home's air, cooling and dehumidifying it. Shaped like an "A" in most furnaces. Leaks in this coil are a common and expensive repair.
Refrigerant lines (line set): The two insulated copper pipes connecting the indoor and outdoor units: the larger suction line and the smaller liquid line.
Blower (blower motor): The fan in the furnace or air handler that pushes air through the ducts. Types include PSC (single-speed), ECM (variable-speed, more efficient), and X13 (a constant-torque middle option).
Fan motor (condenser fan motor): The motor spinning the fan on top of the outdoor unit. Different from the blower motor indoors.
Capacitor: A small cylinder that stores electrical charge to start and run the compressor and fan motors. Capacitors wear out with heat and age; a failed one is one of the most common and cheapest AC repairs. "Dual run capacitor" serves both compressor and fan.
Contactor: An electrical relay in the outdoor unit that switches high-voltage power to the compressor and fan when the thermostat calls for cooling. Pitted or stuck contactors cause units to not start or not shut off.
Control board (circuit board): The electronic brain inside the furnace or air handler that sequences the ignition, blower, and safety checks. Boards can fail from power surges or moisture.
Transformer: Steps household voltage down to the 24 volts used by the thermostat and controls.
Heat exchanger: The metal chambers in a gas furnace where combustion heat transfers to the air without mixing exhaust into it. A cracked heat exchanger can leak carbon monoxide and usually means the furnace must be replaced.
Burners: Where gas mixes with air and ignites inside a furnace.
Ignitor (hot surface ignitor): A ceramic or silicon element that glows red-hot to light the burners. Replaced the standing pilot light in modern furnaces. Ignitors are brittle and commonly fail.
Pilot light: A small constantly burning flame that lights the burners in older furnaces and water heaters.
Flame sensor: A thin metal rod that confirms the burners lit. A dirty flame sensor causes the furnace to start and shut down repeatedly; cleaning it is a routine fix.
Gas valve: Controls gas flow to the burners; opens when the control board calls for heat.
Inducer motor (draft inducer): A small fan that pulls combustion gases through the heat exchanger and out the flue before the burners light.
Flue / vent pipe: Carries exhaust gases from a gas or oil furnace outdoors. Metal for standard furnaces; PVC for high-efficiency condensing furnaces.
Limit switch (high-limit switch): A safety that shuts the furnace off if it overheats, often because of a clogged filter or blocked airflow.
Pressure switch: A safety that confirms the inducer is venting properly before allowing ignition.
Rollout switch: A safety that trips if flame escapes the burner area.
Reversing valve: The valve in a heat pump that flips refrigerant flow between heating and cooling mode.
Expansion valve (TXV / TEV) or piston: Meters refrigerant into the evaporator coil. A TXV (thermostatic expansion valve) adjusts flow automatically and is more efficient; a fixed piston (orifice) is simpler and cheaper.
Filter drier: A canister in the refrigerant line that traps moisture and debris. Should be replaced whenever the system is opened.
Accumulator: A tank on heat pumps that keeps liquid refrigerant from reaching the compressor.
Crankcase heater: Keeps oil in the compressor warm in cold weather so it starts safely.
Hard-start kit: An add-on capacitor and relay that gives an aging compressor an extra jolt to start. A stopgap, not a cure.
Condensate drain line: The PVC pipe that carries water removed from the air by the evaporator coil to a drain or outdoors. Clogs cause water leaks and shutoffs.
Condensate pump: A small pump that lifts condensate to a drain when gravity drainage isn't possible, such as in basements.
Drain pan (primary and secondary/emergency pan): Catches condensate under the coil. The secondary pan under an attic unit protects the ceiling if the primary drain clogs.
Float switch (wet switch): Shuts the system off when the drain pan fills, preventing water damage. Cheap insurance for attic installations.
Heat strips (electric resistance heat, heat kit): Electric heating elements inside an air handler that provide backup or emergency heat for a heat pump. Expensive to run.
Disconnect box: The switch box on the wall near the outdoor unit that cuts power for service. Required by code.
Whip: The short flexible electrical conduit between the disconnect box and the outdoor unit.
Service valves: Valves on the outdoor unit where technicians connect gauges to check refrigerant pressures.
Schrader valve: The small tire-style valve technicians use to access refrigerant. Leaks here are common and cheap to fix.
Pad (condenser pad): The concrete or composite base the outdoor unit sits on. Should be level; a tilted unit strains the compressor.
Plenum: The large sheet-metal box on the supply and return sides of the furnace or air handler where ducts connect.
Thermistor / sensor: Temperature sensors that report to the control board. A bad sensor can cause wrong readings and erratic operation.

Efficiency ratings, sizing, and measurements

BTU (British Thermal Unit): The basic unit of heating and cooling energy: the heat needed to raise one pound of water by 1°F. Equipment capacity is rated in BTUs per hour (BTU/h). A furnace might be 80,000 BTU/h; a window AC 8,000 BTU/h.
Ton (tonnage): A measure of cooling capacity. One ton = 12,000 BTU/h of cooling. Residential systems run from 1.5 to 5 tons. A "3-ton unit" is 36,000 BTU/h. Tonnage has nothing to do with weight.
SEER (Seasonal Energy Efficiency Ratio): The older cooling-efficiency rating for AC and heat pumps: cooling output over a season divided by electricity used. Higher is better. You'll still see SEER on equipment made before 2023.
SEER2: The current cooling-efficiency rating, in effect since January 2023. It uses a tougher test that better reflects real ductwork, so a SEER2 number is roughly 4-5% lower than the equivalent SEER. Federal minimums are 13.4 SEER2 in the northern U.S. and 14.3 SEER2 in the South and Southwest for most sizes. Good equipment runs 16-20+ SEER2.
EER / EER2 (Energy Efficiency Ratio): Cooling efficiency at a single hot outdoor temperature (95°F) rather than across a season. More telling than SEER for very hot climates. EER2 is the current version.
HSPF / HSPF2 (Heating Seasonal Performance Factor): Heating efficiency for heat pumps across a season. Higher is better. HSPF2 is the current version; the federal minimum is 7.5 HSPF2.
COP (Coefficient of Performance): Heat output divided by energy input at one moment. A heat pump with a COP of 3 delivers three units of heat for every unit of electricity. That makes it three times as efficient as an electric space heater.
AFUE (Annual Fuel Utilization Efficiency): Furnace and boiler efficiency: the share of fuel that becomes heat in your home. An 80% AFUE furnace sends 20% of the fuel's heat up the flue; a 96% AFUE condensing furnace loses only 4%. Federal minimum for new gas furnaces is 80% AFUE. A 2023 federal rule raises it to 95% for most non-weatherized gas furnaces starting in late 2028.
Condensing furnace (high-efficiency furnace): A furnace at 90%+ AFUE that pulls extra heat from exhaust until water condenses out. Vents through PVC pipe and needs a condensate drain.
ENERGY STAR: An EPA label for equipment that meets efficiency levels well above the federal minimum. Often required to qualify for rebates.
CEE tiers: Efficiency levels set by the Consortium for Energy Efficiency, used by utilities and the federal tax credit to define which equipment qualifies for incentives.
AHRI certificate / AHRI number: A certificate from the Air-Conditioning, Heating, and Refrigeration Institute confirming the rated efficiency of a specific outdoor unit + indoor coil combination. Ask for it; rebates and tax credits usually require it.
Nameplate / data plate: The label on equipment listing model, serial number, capacity, refrigerant type, and electrical requirements. The serial number usually encodes the manufacture date.
Load calculation: The engineering process for figuring out how much heating and cooling a house actually needs, based on square footage, insulation, windows, orientation, climate, and more. The industry standard method is Manual J (see Installation).
Oversizing: Installing equipment with more capacity than the home needs. Common and harmful: oversized AC cools fast but shuts off before removing humidity, short-cycles, and wears out sooner. Bigger is not better.
Undersizing: Equipment too small to keep up on the hottest or coldest days. Less common than oversizing.
Rule of thumb sizing: Estimating size by square footage alone (e.g., "one ton per 500 sq ft"). Unreliable; a good contractor does a real load calculation instead.
CFM (cubic feet per minute): Airflow volume. Central systems typically need about 350-450 CFM per ton of cooling. Too little airflow freezes coils and overheats furnaces.
Static pressure: The resistance the blower works against to push air through ducts, filters, and coils, measured in inches of water column (in. w.c.). Most residential equipment is designed for 0.5 in. w.c. total; many homes measure far higher because of undersized ducts or restrictive filters. High static pressure shortens equipment life. A good technician measures it.
Delta T (temperature split): The difference between air temperature entering the return and leaving the supply. On a healthy AC it's usually 16-22°F. Technicians use it as a quick health check.
Superheat and subcooling: Refrigerant temperature measurements technicians use to confirm the system has the correct charge. If a tech "checks the charge," this is what they're measuring.
Design temperature: The outdoor temperature the system is sized for, based on local climate data (for example, 97°F in summer for Houston). Systems aren't sized for the single hottest day on record.
Degree days: A measure of how much heating or cooling a climate needs over a season. Used in load calculations and energy audits.
Sensible vs. latent cooling: Sensible cooling lowers air temperature; latent cooling removes moisture. In humid climates, latent capacity matters as much as tonnage.
Watts / kilowatt-hours (kWh): Electrical power and energy. Your electric bill charges by kWh; efficiency ratings tell you how many BTUs you get per watt.
Therm: A unit of natural gas energy equal to 100,000 BTU. Gas bills are charged by the therm or by CCF (100 cubic feet, roughly one therm).
Amps, volts, and breaker size: Outdoor units usually run on 208/240 volts and require a dedicated breaker sized to the unit's nameplate. Electrical upgrades can add to installation cost.
Sound rating (decibels, dB): Outdoor units are rated in dB; 50-60 dB is quiet, 70+ is loud. Variable-speed units are quietest.
Payback period: How long energy savings take to repay the extra cost of higher-efficiency equipment. Useful when deciding between a standard and a premium system.

Refrigerant and cooling terms

Refrigerant: The chemical that circulates between the indoor and outdoor coils, absorbing heat inside and releasing it outside. Often called "Freon," which is actually a brand name.
Refrigeration cycle: The loop of compression, condensing, expansion, and evaporation that moves heat. All air conditioners and heat pumps work this way; they don't create cold, they move heat.
R-22 (HCFC-22, "Freon"): The refrigerant in most systems installed before 2010. Production and import were banned in the U.S. in 2020 because it damages the ozone layer. Remaining supply is reclaimed and very expensive. If your system uses R-22, a major refrigerant repair usually isn't worth it.
R-410A (Puron): The standard refrigerant from about 2010 through 2024. Ozone-safe but a potent greenhouse gas. New R-410A systems could no longer be manufactured after January 1, 2025, but existing systems can still be serviced and recharged for years.
R-32 and R-454B: The two new lower-global-warming refrigerants used in equipment made from 2025 onward. Both are "A2L" mildly flammable, which requires updated installation practices and leak sensors. Different brands chose different ones; they are not interchangeable with R-410A.
A2L refrigerant: The safety class for R-32 and R-454B: low toxicity, low flammability. Safe in homes when installed to code.
GWP (Global Warming Potential): How much a refrigerant warms the atmosphere compared to CO2. R-410A is about 2,088; R-454B about 466; R-32 about 675. The AIM Act phase-down is pushing the industry toward lower-GWP options.
Drop-in refrigerant: A substitute meant to replace another refrigerant without modifying the system. Manufacturers generally do not approve these; using one can void the warranty.
Refrigerant charge: The amount of refrigerant in the system. A properly charged system has the exact amount the manufacturer specifies. "Low on charge" means there is a leak. Refrigerant doesn't get used up.
Recharge / top-off: Adding refrigerant. A legitimate temporary fix, but adding refrigerant without finding and fixing the leak is throwing money away and is now restricted under EPA rules for larger leaks.
Leak search / leak detection: Finding where refrigerant is escaping, using electronic sniffers, UV dye, nitrogen pressure tests, or bubble solution.
Nitrogen pressure test: Filling the system with nitrogen and watching for pressure drop to confirm it is leak-free before charging. Standard practice on a good installation.
Evacuation / vacuum: Pulling all air and moisture out of the refrigerant lines with a vacuum pump before adding refrigerant. Skipping or rushing this step is a leading cause of early compressor failure. A quality install pulls to 500 microns or lower.
Recovery: Legally capturing old refrigerant into a tank instead of venting it. Federal law requires it.
Reclaim: Cleaning recovered refrigerant back to new-product purity for resale.
Suction line / liquid line: The two refrigerant pipes. The suction line (larger, insulated) carries cool gas back to the compressor; the liquid line (smaller) carries warm liquid to the indoor coil.
Line set flush: Cleaning old refrigerant lines when reusing them with a new system. Required when switching refrigerant types if lines aren't replaced.
Brazing: Joining copper refrigerant lines with a torch and silver alloy. Good technicians flow nitrogen while brazing to keep the inside of the pipe clean.
Head pressure / suction pressure: The high- and low-side refrigerant pressures technicians read on their gauges to diagnose problems.
Dehumidification: Moisture removal that happens naturally as air passes over a cold evaporator coil. Oversized or single-stage systems dehumidify poorly because they run in short bursts.
Relative humidity (RH): The percentage of moisture in the air relative to what it can hold at that temperature. Most homes are comfortable at 40-55% RH indoors.
Dew point: The temperature at which moisture condenses out of air. Explains why coils sweat and why ducts in hot attics can drip.
Freeze-up / frozen coil: Ice forming on the evaporator coil, usually from low refrigerant, a dirty filter, or weak airflow. The system must thaw before it can be diagnosed.
Compressor lockout: A safety that stops the compressor when pressures are unsafe. Often shows up as "the outdoor unit won't come on."
Cooling capacity: The rated amount of heat the system can remove per hour (BTU/h or tons). Actual capacity drops as outdoor temperature rises.
Cooling load: The amount of heat your house gains that the system must remove. Determined by a load calculation.

Heating terms

Forced-air heating: Heat delivered by blowing warm air through ducts. Furnaces and heat pumps are forced-air systems; boilers and radiant floors are not.
Natural gas vs. propane (LP): Both fuel the same furnace models, but the burners and gas valve must be set up for the right fuel with a conversion kit. Propane is delivered to a tank; natural gas comes by pipeline.
Electric resistance heat: Heat made by running electricity through coils, as in electric furnaces, baseboard heaters, space heaters, and heat-pump heat strips. Simple and reliable but the most expensive way to heat per BTU.
Heating stages: Like cooling stages: single-stage furnaces run full blast or off; two-stage run at about 65% most of the time; modulating furnaces adjust output in 1% steps for the steadiest temperatures.
Auxiliary heat (aux heat): Backup heat that runs alongside a heat pump when it can't keep up on its own, usually electric heat strips. Seeing "AUX" on your thermostat in very cold weather is normal; seeing it constantly means something is wrong or the system is undersized.
Emergency heat (EM heat): A thermostat setting that shuts off the heat pump entirely and runs only the backup heat. Use it only if the heat pump has failed; it is expensive to run.
Supplemental heat: General term for any secondary heat source that helps the main system.
Balance point: The outdoor temperature below which a heat pump can no longer meet the home's heating need and backup heat kicks in. Typically 25-35°F for standard heat pumps; cold-climate models push it far lower.
Cold-climate heat pump: A heat pump engineered to deliver full or near-full capacity at 5°F and to operate down to −15°F or lower. Look for the ENERGY STAR Cold Climate designation.
Defrost cycle: In heating mode, frost builds on the outdoor coil. The heat pump briefly reverses into cooling mode to melt it, and you may see steam rising from the unit. Normal; the indoor heat strips usually cover the temporary cool air.
Heat pump "cool air" complaint: Heat pumps blow air around 90-100°F, cooler than a gas furnace's 120-140°F, so the air can feel lukewarm even though it is heating the house. This is normal.
Heating capacity: The heat output of the equipment in BTU/h. For heat pumps, capacity drops as the outdoor temperature drops, which is why balance point matters.
Input vs. output BTU: A furnace's input is the fuel it burns; output is the heat delivered. An 80,000 BTU input furnace at 80% AFUE delivers 64,000 BTU output.
Heat rise (temperature rise): The temperature increase across a furnace's heat exchanger, printed on the nameplate as a range (e.g., 35-65°F). Outside that range signals an airflow problem.
Sequencer: Turns on electric heat strips one at a time so they don't overload the circuit.
Standing pilot vs. electronic ignition: Older furnaces keep a pilot flame burning constantly; modern ones ignite on demand with a hot surface ignitor or spark, saving fuel.
Direct vent / sealed combustion: A furnace that draws combustion air from outside through a dedicated pipe rather than from inside the home. Safer and more efficient.
Combustion air: The air a gas appliance needs to burn fuel. Furnaces in tight closets need a source of it by code.
Backdrafting: Exhaust gases spilling back into the home instead of going up the flue, often because a tight house or exhaust fans create negative pressure. A carbon monoxide hazard.
Carbon monoxide (CO): A colorless, odorless, deadly gas produced by incomplete combustion. Every home with gas appliances should have CO alarms, and furnaces should be inspected yearly.
Combustion analysis: Testing the furnace exhaust with an electronic analyzer to confirm safe, efficient burning. A sign of a thorough technician.
Boiler terms: radiators, baseboards, zone valves, circulator pump, expansion tank: Radiators and baseboards release heat from hot water; zone valves direct water to different areas; the circulator pump moves the water; the expansion tank absorbs pressure changes as water heats.
Steam heat: An older boiler type that sends steam rather than hot water to radiators. Requires periodic bleeding and water-level checks.
Baseboard heater: A long, low heater along the wall, either electric or hydronic.
Space heater: A portable electric or fuel heater for one room. Not a substitute for a working central system and a leading cause of home fires.
Setback: Lowering the thermostat while away or asleep to save fuel. Works well for furnaces; heat pumps do best with small setbacks so backup heat doesn't kick in during recovery.
Heat loss: How fast a house loses heat to the outdoors through walls, windows, roof, and air leaks. Determines furnace or heat pump size.
Fuel switching / electrification: Replacing gas heating with an electric heat pump. Often paired with rebates and tax credits.

Ductwork, airflow, and ventilation

Ductwork (ducts): The network of channels that carries conditioned air from the equipment to rooms and back. Poor ductwork wastes 20-30% of a system's output in a typical home, so a new system installed on bad ducts will underperform.
Supply duct / supply register: Ducts and vents that deliver heated or cooled air into rooms. Registers usually have adjustable louvers.
Return duct / return grille: Ducts and grilles that pull room air back to the equipment to be filtered and conditioned again. Homes with too few returns have pressure problems and uneven comfort.
Register, grille, diffuser: Register: a supply vent with a damper. Grille: a fixed vent, usually on a return. Diffuser: a ceiling supply that spreads air in multiple directions.
Trunk and branch: A duct layout with a large main trunk line and smaller branch ducts running to each room.
Flex duct: Flexible, wire-coiled, insulated plastic duct. Cheap and easy to install but restricts airflow if kinked, sagging, or stretched around tight bends.
Rigid duct (sheet metal duct): Galvanized steel ducting. More expensive but delivers better airflow and lasts longer.
Duct board: Rigid fiberglass board used to build plenums and ducts.
Duct sealing: Closing leaks at joints and seams with mastic (a paste) or UL 181 foil tape. Not cloth "duct tape," which fails quickly.
Aeroseal: A brand-name process that seals duct leaks from the inside by blowing an aerosolized sealant through the system.
Duct insulation: Wrap or lining, rated by R-value (R-6 and R-8 are common), that keeps air in attic or crawlspace ducts from gaining or losing heat.
Duct leakage test (duct blaster test): Pressurizing the ducts to measure how much air leaks out. Required by code for new construction in many areas.
Duct cleaning: Vacuuming and brushing the inside of ducts. Useful after renovation, rodent problems, or visible mold; otherwise the EPA says it hasn't been shown to improve health for most homes. Beware of "$99 whole-house" offers used as a foot in the door.
Damper: A movable plate inside a duct that adjusts or shuts off airflow. Manual dampers are set by hand; motorized dampers are controlled by a zoning system.
Balancing (air balancing): Adjusting dampers and registers so each room gets the right share of air. Fixes hot and cold rooms without new equipment.
Zoning: Dividing the home into areas with separate thermostats and motorized dampers so each zone is conditioned independently. See Thermostats and controls.
Bypass duct: In some zoning systems, a duct that dumps excess air back to the return when only one zone is open. Modern practice avoids them.
Boot: The sheet-metal fitting that connects a duct to a register in the floor, wall, or ceiling.
Takeoff: The fitting where a branch duct connects to the trunk.
Jumper duct / transfer grille: A short duct or wall grille that lets air escape a closed bedroom back to a central return, preventing pressure imbalances.
Return air pathway: Any route that lets air get back to the return, including undercut doors, jumper ducts, and transfer grilles.
Panned joist return: A return "duct" made by nailing sheet metal across floor joists. Common in older homes and very leaky.
Plenum: The main box where the ducts meet the equipment on the supply and return sides.
Filter rack / filter slot: Where the air filter sits, usually at the return plenum or behind a return grille.
Media cabinet: A larger housing for a 4-5-inch-thick pleated filter, which lasts longer and restricts airflow less than 1-inch filters.
Airflow restriction: Anything that slows air: dirty filters, closed vents, kinked flex duct, undersized returns, dirty coils. Causes high static pressure and equipment strain.
Ventilation: Bringing fresh outdoor air in and stale indoor air out. Tight modern homes need mechanical ventilation; older homes leak enough on their own.
Exhaust fan: Bathroom, kitchen, and dryer fans that push moisture and odors outside. They should vent outdoors, not into the attic.
Make-up air: Air brought in to replace what large exhaust fans (range hoods, dryers) push out, so the house doesn't go negative and backdraft appliances.
Fresh air intake: A duct that brings a controlled amount of outdoor air to the return side of the system.
ERV / HRV: Energy recovery ventilator / heat recovery ventilator. Fans that exchange stale indoor air for fresh outdoor air while transferring heat (HRV) or heat and moisture (ERV) between the two streams. ERVs suit humid climates; HRVs suit cold, dry ones.
Air changes per hour (ACH): How many times the air in a space is replaced each hour. Used in ventilation design and blower-door tests.
Blower door test: A fan sealed in a doorway measures how leaky the house is. Part of an energy audit.
Building envelope: The walls, roof, floor, windows, and doors that separate inside from outside. Sealing and insulating it reduces the load on HVAC equipment.
Air sealing: Closing gaps in the envelope with caulk, foam, and weatherstripping. Cheap and often the best first step before buying new equipment.
Negative / positive pressure: When more air leaves than enters a space (negative) or vice versa (positive). Imbalances cause drafts, backdrafting, and moisture problems.
Stack effect: Warm air rising and leaking out the top of the house while pulling cold air in low. Increases heating bills in winter.
Conditioned space vs. unconditioned space: Areas the system heats and cools (living areas) vs. those it doesn't (attics, crawlspaces, garages). Equipment and ducts in unconditioned space lose energy.
Encapsulated attic / sealed crawlspace: Insulating the attic roofline or sealing the crawlspace so equipment and ducts sit in conditioned space. Improves efficiency and equipment life.

Indoor air quality and filtration

IAQ (Indoor Air Quality): The overall cleanliness and comfort of the air inside your home: particles, humidity, gases, and odors. Contractors sell "IAQ products" as add-ons; some are worthwhile, some are not.
Air filter: The disposable or washable panel that traps dust before it reaches the equipment. Its first job is protecting the coil and blower; cleaner air is the bonus. Change 1-inch filters every 1-3 months, thick media filters every 6-12 months.
MERV (Minimum Efficiency Reporting Value): A 1-16 scale for how well a filter captures particles. MERV 1-4 catches only large dust; MERV 8 is a reasonable minimum; MERV 11-13 catches fine particles, smoke, and most allergens. Higher MERV restricts airflow more, so check that your system can handle it. Many 1-inch filter slots can't go above MERV 11 without raising static pressure.
MPR and FPR: Retail rating scales from 3M (MPR) and Home Depot (FPR) that roughly map to MERV. MPR 1000-1900 ≈ MERV 11-13; FPR 7-10 ≈ MERV 11-13.
HEPA filter: High-Efficiency Particulate Air filter, capturing 99.97% of particles at 0.3 microns. True HEPA is too restrictive for most central systems and is usually found in portable air purifiers or dedicated bypass units.
Pleated filter: A filter with accordion folds for more surface area. Better than flat fiberglass filters at the same size.
Fiberglass filter: The cheap blue or green flat filter. Protects the equipment but does little for air quality.
Washable / electrostatic filter: Reusable filters that trap particles with static charge. Convenient but generally rated low (MERV 4-8) and must be fully dry before reinstalling.
Media filter / media air cleaner: A 4- to 5-inch-thick pleated filter in its own cabinet. Higher MERV with less airflow restriction; the best upgrade for most homes.
Electronic air cleaner: A powered unit that charges particles so they stick to collector plates. Effective when clean; plates need washing regularly.
UV light (UV germicidal lamp): An ultraviolet lamp mounted near the evaporator coil to keep mold and bacteria from growing on the wet coil surface. Helps keep the coil clean; does little to sterilize moving air because exposure time is too short. Bulbs need replacing every 1-2 years.
Air purifier / air scrubber: A broad term for add-on devices using UV, ionization, photocatalytic oxidation, or plasma to treat air. Evidence varies widely by technology; some produce ozone as a byproduct. Ask for independent test data before buying.
Ionizer / bipolar ionization: Releases charged ions to make particles clump and fall out of the air. Marketing outpaces independent evidence; some units create ozone.
Ozone: A lung irritant. Any device that intentionally generates ozone should not be used in occupied homes. California bans ozone-producing air cleaners.
Activated carbon filter: Absorbs odors and gases (VOCs) that particle filters miss. Often combined with a HEPA filter in portable purifiers.
VOCs (Volatile Organic Compounds): Gases released by paints, cleaners, furniture, and building materials. Ventilation and carbon filtration reduce them; particle filters don't.
PM2.5: Particles smaller than 2.5 microns, such as smoke and fine dust. They penetrate deep into the lungs and are the main target of MERV 13 and HEPA filtration.
Allergens: Pollen, dust mite debris, pet dander, and mold spores. Most are captured by MERV 11+ filters.
Mold: Grows where moisture and organic material meet: wet coils, drain pans, damp ducts, and humid crawlspaces. Fixing the moisture source matters more than any treatment.
Humidifier (whole-house): Adds moisture to the air in winter through the ductwork. Bypass and fan-powered types use the furnace's warm air; steam humidifiers boil water and work independently. Common in dry, cold climates; rarely needed in humid regions.
Dehumidifier (whole-house): Removes moisture through the duct system independently of cooling. Valuable in humid climates during mild weather when the AC doesn't run enough to dry the air.
Humidistat: A control that switches a humidifier or dehumidifier on and off at a set humidity level.
Hygrometer: A humidity gauge. Inexpensive ones help you know if your home is in the 40-55% comfort range.
CO detector / CO alarm: Sounds when carbon monoxide reaches dangerous levels. Required near sleeping areas in most codes; replace every 5-10 years.
Radon: A radioactive gas from soil that enters through foundations. Not an HVAC issue directly, but radon mitigation systems use fans and vents; testing is recommended for every home.
Air quality monitor: A device that reads PM2.5, CO2, VOCs, humidity, and temperature so you can see when to ventilate or filter.
CO2 (carbon dioxide) level: A marker of how stale indoor air is. Above about 1,000 ppm usually means the home needs more ventilation.
Portable air purifier: A standalone unit with a fan and HEPA/carbon filter for one room. Check the CADR rating for room size.
CADR (Clean Air Delivery Rate): How much filtered air a portable purifier delivers per minute. Match it to the room: roughly two-thirds of the room's square footage is a common guideline.
Sick building syndrome: Health complaints linked to time in a building, usually from poor ventilation, humidity, or contaminants.
Coil cleaning: Removing dust and biological growth from the evaporator coil. Improves efficiency and air quality; a dirty coil can smell like dirty socks ("dirty sock syndrome") when the system starts.

Thermostats, controls, and zoning

Thermostat: The control that senses room temperature and tells the equipment when to run. It should be mounted on an interior wall away from sun, drafts, lamps, and kitchens.
Setpoint: The temperature you've asked the thermostat to hold.
Manual (non-programmable) thermostat: Holds one setpoint until you change it. Simple and cheap.
Programmable thermostat: Lets you schedule different setpoints by time and day. Saves energy if you actually program it.
Smart thermostat: A Wi-Fi thermostat you control from a phone, which learns your habits, adjusts automatically, reports energy use, and can respond to utility demand-response programs. Brands include Nest, ecobee, and Honeywell Home. Many utilities offer rebates.
Learning thermostat: A smart thermostat that builds its own schedule from how you adjust it.
Geofencing: A smart-thermostat feature that uses your phone's location to set back the system when everyone leaves and recover before you return.
Remote sensor: A small wireless sensor placed in another room so the thermostat can average temperatures or prioritize the room you're in.
Communicating thermostat: A brand-specific thermostat that exchanges detailed data with the equipment. Required to unlock all features of some variable-speed systems; usually not replaceable with a generic smart thermostat.
C-wire (common wire): The wire that provides continuous 24-volt power to a thermostat. Many smart thermostats need one; older homes may lack it, requiring an adapter or new wire.
Thermostat wiring (R, W, Y, G, C, O/B): Standard wire labels: R = power, W = heat, Y = cooling, G = fan, C = common, O/B = heat pump reversing valve. Helpful when comparing thermostat compatibility.
Heat pump thermostat: A thermostat with the extra terminals and logic needed to control a heat pump's reversing valve and auxiliary heat. A standard furnace thermostat won't work correctly on a heat pump.
Fan setting: Auto vs. On: Auto runs the blower only while heating or cooling. On runs it constantly, which improves air mixing and filtration but adds electricity cost and, in humid climates, can re-evaporate moisture off the coil back into the house.
Circulate mode: A smart-thermostat option that runs the fan for a set number of minutes per hour as a middle ground.
Deadband / differential / swing: How far the temperature drifts from the setpoint before the system turns on or off. A wider swing means fewer cycles; a narrower one means steadier temperature.
Hold / temporary hold / permanent hold: Overrides the schedule until the next programmed change (temporary) or indefinitely (permanent).
Recovery (smart recovery, adaptive recovery): The thermostat starts heating or cooling early so the home reaches the scheduled temperature at the scheduled time.
Compressor protection / minimum off time: A built-in delay (often 5 minutes) that prevents the compressor from restarting too quickly. If the AC doesn't come on immediately, this is usually why.
Lockout: A thermostat or control setting that prevents a stage from running under certain conditions, such as locking out the heat pump below a set outdoor temperature on a dual-fuel system.
Zoning system: Motorized dampers in the ducts plus a zone control panel and multiple thermostats, so different areas of the house are conditioned independently. Fixes the classic upstairs-hot, downstairs-cold problem.
Zone control panel: The board that receives calls from each zone thermostat and opens the right dampers and equipment stages.
Zone damper: A motorized damper that opens or closes a duct branch on command from the zone panel.
Outdoor temperature sensor: Lets the thermostat display outdoor temperature and make dual-fuel or heat pump lockout decisions.
Demand response: A utility program that briefly adjusts your smart thermostat during peak demand in exchange for bill credits. You can usually override it.
Time-of-use rates: Electricity priced higher during peak hours. Smart thermostats can pre-cool before the peak to save money.
Home automation integration: Connecting the thermostat to Alexa, Google Home, Apple HomeKit, or SmartThings for voice control and routines.
Thermostat calibration: Adjusting a thermostat that reads a few degrees off. Most digital thermostats have an offset setting.
Mercury thermostat: The old round dial thermostats with a mercury switch. Still work, but mercury must be disposed of properly. Don't throw them in the trash.
Line-voltage thermostat: A thermostat that switches full 120/240 volts directly, used with baseboard heaters. Not interchangeable with low-voltage (24V) thermostats.

Maintenance, service, and repair

Tune-up (preventive maintenance, seasonal maintenance): A scheduled inspection and cleaning, ideally once for cooling in spring and once for heating in fall. A real tune-up includes cleaning coils, checking refrigerant pressures, testing capacitors and electrical connections, checking the drain, measuring airflow and temperature split, and inspecting the heat exchanger. A 15-minute visit is not a tune-up.
Maintenance agreement / service plan / club membership: A prepaid annual contract for one or two tune-ups, usually with perks like priority scheduling, repair discounts, and waived diagnostic fees. Worth it if the contractor actually does thorough work.
Diagnostic fee / service call fee / trip charge: The flat fee to come out and find the problem, typically $75-$200. Often waived or credited if you approve the repair. Ask before booking.
Diagnosis: Identifying the cause of a problem. A good diagnosis names the failed part and explains why it failed, not just what to replace.
Flat-rate pricing: Fixed prices per repair from a price book, regardless of how long it takes. Predictable, but compare against other quotes for big jobs.
Time and materials (T&M): Billing by the hour plus parts. Less common in residential HVAC today.
Emergency / after-hours service: Repairs outside normal hours, usually at a premium rate. Check whether "24/7 service" means a live technician or just an answering service.
Repair vs. replace: The judgment call when equipment fails. A common rule: if the repair costs more than 50% of a new system, or the unit is over 12-15 years old and the repair is major, replacement usually makes more sense. Another guideline multiplies repair cost by equipment age; over $5,000 leans toward replacing.
Second opinion: Getting another contractor's diagnosis before a major repair or replacement. Reputable companies expect it.
Short cycling: The system turning on and off in rapid bursts instead of running steady cycles. Causes include oversizing, a dirty filter, low refrigerant, a failing thermostat, or a frozen coil. Wears out compressors fast.
Long cycling / constant running: The system running nonstop without reaching setpoint. Points to undersizing, low refrigerant, dirty coils, duct leaks, or extreme weather.
Frozen coil / iced-up unit: Ice on the indoor coil or refrigerant lines. Turn the system off and let it thaw (run fan only), then call for service. Usually low airflow or low refrigerant.
Refrigerant leak repair: Finding and fixing the leak, then evacuating and recharging. Coil leaks often mean coil replacement.
Capacitor replacement: One of the most common repairs. Typically $150-$400 installed.
Contactor replacement: Another common, relatively inexpensive electrical repair.
Blower motor replacement: Replacing the indoor fan motor. ECM motors cost several times more than standard PSC motors.
Compressor replacement: Replacing the outdoor compressor. Expensive enough that on out-of-warranty units over 10 years old, replacing the whole outdoor unit or system usually makes more sense.
Evaporator coil replacement: Replacing a leaking indoor coil. Often $1,500-$3,500; on an R-22 or older R-410A system, replacement of the whole system may be the better investment.
Heat exchanger replacement: Rarely done; a cracked heat exchanger on an older furnace almost always means a new furnace.
Ignitor / flame sensor service: Common furnace repairs. Cleaning a flame sensor may cost under $150; replacing an ignitor a bit more.
Drain line cleaning / drain flush: Clearing algae and sludge from the condensate line, usually with a vacuum or nitrogen. Pouring a cup of vinegar down the line every few months helps prevent clogs.
Coil cleaning: Cleaning the outdoor condenser coil (rinse with hose, coil cleaner) and indoor evaporator coil (harder; sometimes requires pulling the coil).
Filter change: The single most important homeowner maintenance task. Note the airflow arrow on the filter frame; it points toward the equipment.
Refrigerant recharge: Adding refrigerant after a leak repair, charged by the pound. Prices vary widely by refrigerant type.
Blower wheel cleaning: Removing caked dust from the blower fan blades, which restores airflow.
Electrical inspection: Checking wires, connections, contactors, and capacitors for wear, corrosion, and overheating.
Amp draw: The electrical current a motor or compressor pulls. Technicians compare it to the nameplate rating to judge health.
Megohm test: Testing the insulation of compressor windings to predict failure.
Bearing failure / motor noise: Grinding, squealing, or rattling from motors usually means bearings are wearing out.
Hard start / slow start: The compressor struggling to start, often with a hum or dimming lights. Signals a weak capacitor or compressor.
Tripped breaker: The circuit breaker shutting off power to the unit. Reset it once; if it trips again, stop and call. Something is drawing too much current.
Reset button / rollout reset: Some safeties have a manual reset. If it trips repeatedly, there is a real problem.
Error code / fault code: Blinking-light or display codes on the furnace control board or thermostat that point to a specific fault. A legend is usually on the inside of the panel.
Lockout mode: A furnace safety state after several failed ignition attempts. Power-cycling clears it temporarily; the cause still needs fixing.
Thermostat wire short / low-voltage short: A damaged control wire that blows the fuse on the control board. Often caused by rodents or lawn equipment.
Rodent damage: Chewed wires, nests in outdoor units, and droppings in ducts are common service findings.
Warranty repair: Repairs covered under the manufacturer's parts warranty. You typically still pay labor unless you have a labor warranty. Registration within 60-90 days of installation is usually required for the full term.
Home warranty: A third-party service contract that covers appliances and systems for a fee per claim. Coverage and contractor quality vary widely; read the exclusions.
Sub-cooling / superheat check: How technicians verify the correct refrigerant charge. If your tech only "tops it off by feel," find another tech.
Annual furnace inspection: Recommended for all gas furnaces: heat exchanger check, combustion analysis, burner cleaning, safety tests.
Pre-season checkup: Scheduling maintenance before summer or winter demand so problems are caught before the first heat wave or cold snap.
Lifespan: Typical service life: central AC and heat pumps 12-17 years, gas furnaces 15-25 years, boilers 20-30 years, mini-splits 15-20 years. Coastal, humid, or poorly maintained systems fall on the short end.

Installation, permits, and regulations

Manual J: The ACCA standard for residential load calculation. It determines the heating and cooling capacity your home actually needs, room by room. Ask every bidding contractor whether they performed one; a quote based on the old unit's size or square footage alone is a red flag.
Manual S: The ACCA standard for selecting equipment that matches the Manual J load, including allowances for how much oversizing is acceptable.
Manual D: The ACCA standard for duct design. Used when installing new ductwork or fixing bad ducts.
Manual T: The ACCA standard for selecting and placing registers and grilles.
ACCA: Air Conditioning Contractors of America, the trade association that publishes the Manual J/S/D/T standards and quality installation guidelines.
Quality installation (QI): An ACCA/ANSI standard for how equipment should be installed: correct sizing, proper airflow, correct refrigerant charge, sealed ducts, and verified performance. Installation quality affects efficiency and lifespan as much as the equipment brand.
Commissioning / start-up: Testing and adjusting a newly installed system: verifying airflow, refrigerant charge, temperature split, static pressure, and safety controls. Ask for the start-up sheet with the measured numbers.
Permit: Authorization from your city or county to install or replace HVAC equipment. Required almost everywhere for new systems and often for major repairs. A contractor who suggests skipping the permit is cutting corners; unpermitted work can complicate home sales and insurance claims.
Inspection: A visit from the local building inspector to verify permitted work meets code. Usually scheduled by the contractor after installation.
Building code / mechanical code: Local rules (based on the International Residential Code, International Mechanical Code, and International Fuel Gas Code) covering clearances, venting, electrical, condensate, and refrigerant safety.
Energy code (IECC): Rules for minimum efficiency, duct sealing, and insulation on new construction and major renovations.
DOE minimum efficiency standards: Federal minimums for new equipment, set by the Department of Energy. Current: 13.4 SEER2 (North) / 14.3 SEER2 (South and Southwest) for central AC; 14.3 SEER2 and 7.5 HSPF2 for heat pumps; 80% AFUE for gas furnaces. Regions are defined by state.
Regional standards (North / South / Southwest): The DOE divides the country into three regions with different cooling minimums. Texas and the Southeast are in the South region; Arizona, New Mexico, Nevada, and California are the Southwest.
EPA Section 608: The federal rule requiring anyone who handles refrigerant to be certified. Ask to see the technician's card.
AIM Act: The 2020 law directing the EPA to phase down high-GWP refrigerants like R-410A. It's why new equipment uses R-32 or R-454B.
Refrigerant transition (2025): As of January 1, 2025, new residential AC and heat pump systems must use lower-GWP refrigerants. R-410A systems manufactured before then could still be installed through 2025 in most cases. Existing R-410A systems can be serviced indefinitely.
Leak repair rule: EPA rules require repairing leaks in larger systems above certain thresholds. Residential systems are mostly exempt, but venting refrigerant is illegal for anyone.
Federal tax credit (25C, Energy Efficient Home Improvement Credit): A tax credit for qualifying high-efficiency heat pumps, furnaces, and ACs. The credit ended for equipment placed in service after December 31, 2025. Check with a tax professional before counting on any federal credit.
Utility rebate: Cash back from your electric or gas utility for installing qualifying equipment, smart thermostats, or duct sealing. Usually requires an AHRI certificate and a licensed contractor.
Manufacturer rebate / seasonal promotion: Discounts from equipment brands, common in spring and fall.
State and local incentives: Programs like home electrification rebates (HEAR/HOMES) administered by states. Availability varies; check the DSIRE database or your state energy office.
Contractor license: A state or local license required to perform HVAC work. Texas, for example, licenses HVAC contractors through TDLR. Verify the license number on the quote.
Mechanical drawings / scope of work: Written description of exactly what will be installed and done. Vague quotes lead to disputes.
Line set replacement vs. reuse: New refrigerant lines are best; reusing existing lines is acceptable if they're the right size, undamaged, and properly flushed. The quote should say which.
Pad, stand, or bracket: How the outdoor unit is mounted. Coastal and flood-prone areas may require elevated stands.
Clearance: Required open space around outdoor units (usually 12-24 inches on sides, 5 feet above) and around furnaces for airflow and service.
Condensate disposal: Code requires condensate to drain to an approved location with a secondary pan or float switch for equipment above finished space.
Electrical upgrade: Larger equipment or electrification may require a new breaker, wire, or panel. Should be quoted up front.
Gas line sizing: Furnaces need adequate gas pipe size and pressure; upgrades sometimes accompany new installs.
Venting upgrade: High-efficiency furnaces need new PVC venting; switching from an 80% to a 95% furnace often requires re-lining or abandoning the old chimney flue used by the water heater ("orphaned water heater").
Orphaned water heater: When a furnace is removed from a shared chimney, the water heater alone may not vent properly. A good contractor addresses this.
Crane lift: Needed for rooftop packaged units. Adds to installation cost.
Change-out: Industry slang for a like-for-like replacement of existing equipment.
Retrofit: Installing new equipment in an existing home, as opposed to new construction, often with ductwork modifications.
Final walkthrough: The contractor showing you the completed work, thermostat operation, filter location, and maintenance needs before you sign off.

Contractor, pricing, and warranty terms

Licensed, bonded, and insured: Licensed: holds the state or local contractor license. Bonded: carries a surety bond that can compensate you if the contractor fails to complete the job or violates regulations. Insured: carries general liability and workers' compensation so you aren't liable for damage or injuries on your property. Ask for proof of all three.
NATE certification: North American Technician Excellence, the leading independent certification for HVAC technicians, earned by passing specialty exams. A strong signal of competence, though many good techs aren't certified.
EPA 608 certification: Legally required for anyone handling refrigerant. Universal certification covers all equipment types.
Factory-authorized / dealer: A contractor approved by a manufacturer to sell and install its equipment, sometimes with training requirements and extended warranty perks. Examples include Carrier Factory Authorized Dealer and Trane Comfort Specialist.
Manufacturer brands and tiers: Major brands include Carrier/Bryant, Trane/American Standard, Lennox, Rheem/Ruud, Goodman/Amana/Daikin, York, and Bosch. Many pairs share the same factory and parts. Installation quality matters more than brand.
Estimate / quote / proposal: A written price for the work. A good proposal lists equipment model numbers, capacity, efficiency ratings, scope of work, what's included (permit, disposal, thermostat, line set, pad), warranty terms, and total price.
Free estimate: Most contractors quote replacements for free but charge a diagnostic fee for repairs.
Good-better-best pricing: Three quote tiers (basic single-stage, mid two-stage, premium variable-speed). Useful for comparison; watch for pressure toward the top tier.
Flat-rate vs. itemized pricing: Most residential contractors quote one total. You can still ask for the equipment cost separately to compare quotes fairly.
Installed price: The full price including equipment, labor, materials, permits, and disposal. Compare installed prices, not equipment prices.
Parts warranty: The manufacturer's coverage on components, typically 10 years on most parts and sometimes lifetime on the compressor or heat exchanger, but only if registered (usually within 60-90 days). Unregistered warranties often drop to 5 years.
Labor warranty: Covers the contractor's labor to replace failed parts. Manufacturers usually cover parts only; labor comes from the contractor, typically 1-2 years included, with longer terms sold as an extended warranty.
Extended warranty / labor plan: A paid contract extending labor coverage to match the parts warranty, often 10 years. Check who backs it: the contractor, the manufacturer, or a third party.
Workmanship warranty: The contractor's guarantee that the installation itself was done right, covering leaks at brazed joints, duct connections, and similar issues.
Warranty registration: Registering the equipment with the manufacturer after installation. Some contractors do it for you; confirm and keep the confirmation.
Transferable warranty: Whether the warranty passes to a new homeowner. Many do, sometimes with a fee.
Pro-rated warranty: Coverage that decreases over time, common with compressors on budget brands.
Satisfaction guarantee / comfort guarantee: A promise to fix or remove the system if you're not satisfied within a set period. Read the terms.
Financing: Loans or payment plans for new systems, often through a lender the contractor partners with. "0% APR" offers usually carry a dealer fee baked into the price; ask for the cash price too.
Deposit: Payment up front, typically 10-50%. Never pay in full before installation.
Change order: A written amendment when the scope or price changes mid-job. Insist on it in writing.
Scope creep / upsell: Adding products or services beyond what's needed. Common upsells: UV lights, surge protectors, duct cleaning, "whole-house" purifiers. Some are worthwhile; ask why you need each one.
Comfort advisor / home comfort consultant: The sales title for the person who quotes a replacement. Some are technicians; many are salespeople on commission.
Service technician vs. installer: Technicians diagnose and repair; installers put in new systems. Different skill sets.
Commission-based pricing: Technicians paid by what they sell. Not inherently bad, but it can encourage unnecessary repairs or replacements. Ask how the company pays its techs.
Private equity-owned contractor: Many local HVAC companies have been acquired by large investment groups and kept their local names. Service quality and pricing may change after acquisition.
Subcontractor: Another company hired by your contractor to do part of the job. Ask who will actually be in your home.
Lien waiver: A document confirming the contractor and suppliers have been paid, so no one can place a lien on your home. Reasonable to request on large jobs.
BBB rating / reviews: Better Business Bureau grades and online reviews (Google, Yelp, Angi). Look at how the company responds to complaints, not just the star count.
Bait-and-switch: Advertising a low price ($59 tune-up, $2,999 system) and then pressuring you into something much more expensive. A red flag.
Red-tagging: A technician or gas utility shutting off equipment as unsafe, usually for a suspected cracked heat exchanger or gas leak. Legitimate when real; get a second opinion if it comes with an immediate replacement quote.
Disposal / haul-away: Removing and recycling the old equipment, including legal refrigerant recovery. Should be included in the quote.
Model number / serial number: Model identifies what it is; serial identifies the specific unit and usually encodes the manufacture date. Get both on your invoice.
Equipment tier (builder grade, mid-grade, premium): Builder grade is the least expensive single-stage equipment used in new construction; premium is variable-speed communicating equipment. Reliability differences are smaller than salespeople imply; comfort and efficiency differences are real.
Price range benchmarks: Typical installed prices (2026, varies by region): central AC replacement $5,000-$12,000; gas furnace $4,000-$9,000; full system $9,000-$20,000; heat pump $6,000-$16,000; ductless mini-split $3,500-$6,000 per zone. Wide ranges reflect equipment tier, home difficulty, and local labor rates.

Common problems and what they usually mean

The entries below match the way homeowners describe problems to the most likely causes, from simplest to most serious. Check the simple things first; call a licensed contractor for anything involving gas, refrigerant, or electrical components.

"AC is running but blowing warm air": Thermostat set to fan or heat; dirty filter; frozen evaporator coil; outdoor unit off (tripped breaker, failed capacitor, or contactor); low refrigerant from a leak; failed compressor.
"The outdoor unit won't turn on": Tripped breaker or disconnect; blown low-voltage fuse; failed capacitor (often with a humming sound); bad contactor; float switch tripped by a clogged drain; thermostat wiring fault.
"The outdoor unit runs but the indoor fan doesn't": Failed blower motor or blower capacitor; bad control board; clogged filter tripping a limit switch.
"Indoor fan runs but nothing else happens": Thermostat in fan-on mode; outdoor unit lost power; safety lockout.
"The AC never shuts off": Undersized system, extreme heat, low refrigerant, dirty coils, leaky ducts, or a thermostat problem. Constant running on a 100°F day may be normal if it holds temperature.
"The system turns on and off constantly": Short cycling: oversized equipment, dirty filter, low refrigerant, frozen coil, failing thermostat, or a thermostat placed in a bad spot.
"Ice on the copper lines or the indoor unit": Frozen coil from restricted airflow (dirty filter, closed vents, failed blower) or low refrigerant. Turn cooling off, run fan only to thaw, then call.
"Water around the indoor unit": Clogged condensate drain, cracked drain pan, failed condensate pump, or frozen coil melting. In attics, a full secondary pan means the primary drain is blocked.
"Water dripping from a ceiling under the attic unit": Overflowing drain pan, usually a clogged drain. Turn the system off immediately to limit damage.
"Furnace blows cold air": Thermostat fan-on; pilot or ignitor failure; dirty flame sensor causing shutdown; overheating tripping the limit switch; gas supply issue; heat pump in defrost mode (normal).
"Furnace clicks but won't light": Failed hot surface ignitor, dirty flame sensor, gas valve problem, or a pressure switch preventing ignition because of a venting issue.
"Furnace starts, runs a few minutes, and shuts off": Dirty flame sensor (most common), overheating from a clogged filter or blocked vents, or a flue or pressure switch fault.
"Heat pump is blowing lukewarm air": Normal; heat pumps deliver 90-100°F air. If the house isn't holding temperature, check filter, outdoor coil, and whether aux heat is working.
"Steam or smoke coming from the outdoor unit in winter": Almost always the normal defrost cycle, not a fire.
"AUX heat keeps showing on the thermostat": Normal in very cold weather or after a large setpoint change; constant use in mild weather points to a heat pump problem or a stuck reversing valve.
"Some rooms are hot, others cold": Duct design or leakage, undersized returns, closed dampers, lack of zoning, sun exposure, or insulation gaps. Balancing, duct sealing, zoning, or a mini-split for the problem room are common fixes.
"Upstairs is always hotter": Heat rises, attic ducts gain heat, and single-thermostat systems can't compensate. Zoning or a dedicated upstairs system solves it.
"Weak airflow from the vents": Dirty filter, blower problem, duct leaks or disconnections, kinked flex duct, closed dampers, dirty evaporator coil, or a frozen coil.
"Strange smells": Burning dust at first heat of the season (normal, brief); "dirty socks" musty smell (mold on coil); rotten eggs (gas leak: leave and call the gas company); electrical burning (shut off and call); sewage (drain trap issue).
"Strange noises": Banging or booming at furnace startup (delayed ignition, needs service); squealing (belt or bearing); grinding (motor bearings); clicking that repeats (relay or contactor); hissing (refrigerant leak or duct leak); rattling (loose panel or debris in the outdoor unit); bubbling (refrigerant leak or drain issue).
"High energy bills": Aging equipment, low refrigerant, dirty coils, leaky ducts, poor insulation, wrong thermostat settings, or running fan-on continuously.
"House is humid even with the AC on": Oversized system short-cycling, single-stage equipment in mild weather, fan set to On, duct leaks pulling in attic air, or a coil not cold enough. A whole-house dehumidifier or a variable-speed system helps.
"House is too dry in winter": Common with forced-air heat in cold climates. A whole-house humidifier targets 30-40% RH in winter.
"Thermostat screen is blank": Dead batteries, tripped breaker, blown low-voltage fuse on the control board, or a float switch cutting power.
"Thermostat reads a different temperature than the room feels": Poor thermostat location, drafts, calibration offset, or stratified air (use fan circulate mode).
"Breaker keeps tripping": Failing compressor or motor drawing too much current, shorted wiring, or an undersized breaker. Do not keep resetting it.
"Carbon monoxide alarm went off": Leave the house, call 911 or the gas company, then have the furnace and any gas appliances inspected before using them again.
"Pilot light won't stay lit": Faulty thermocouple, dirty pilot, or gas supply issue on an older furnace or water heater.
"Mini-split is leaking water inside": Clogged drain line or a failed condensate pump on units installed below the drain level.
"Mini-split blinking lights": An error code; count the blinks and check the manual or call for service.
"My system is 15+ years old and needs a big repair": Get a replacement quote alongside the repair quote. Newer equipment often pays back part of its cost in energy savings, and parts for older refrigerants are costly and scarce.
"Contractor says I need a whole new system after a minor problem": Get a second opinion, especially if the technician didn't test anything or the quote came with same-day pressure.