HVAC terms, in plain English
68 terms you'll hear from a technician, see on a quote, or read on an equipment label — each with what it means and why it matters in a Central Kentucky home.
A.
- Refrigerants
A2L Refrigerant
An A2L refrigerant is one classified under ASHRAE Standard 34 as lower toxicity (class A) and lower flammability (class 2L), meaning it is hard to ignite and burns slowly if it does. R-32 and R-454B, the main replacements for R-410A in new U.S. home systems, are both A2L. Equipment built for them follows the UL 60335-2-40 safety standard.
What it means for you - Ratings, Sizing & Money
AFUE
AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace's fuel energy that becomes usable heat in the home over a typical year. An 80% AFUE gas furnace loses about a fifth of its fuel energy, mostly up the vent, while a 95% AFUE condensing furnace captures extra heat from its exhaust and wastes far less.
What it means for you - Ratings, Sizing & Money
AHRI Certificate (Matched System)
An AHRI certificate is a record from the AHRI Certification Directory listing the certified efficiency ratings, such as SEER2, EER2 and HSPF2, for one specific combination of outdoor unit, indoor coil or air handler, and sometimes furnace. Each matched combination has its own AHRI reference number, so a mismatched pairing may not carry the advertised rating.
What it means for you - Airflow & Indoor Air
Air Filter
An air filter is the replaceable panel, pleated or fiberglass, that sits in a return grille or a filter slot at the furnace or air handler and catches dust, lint, pollen, and other particles before they reach the blower and coil. It protects the equipment first; its MERV rating describes how well it captures smaller particles.
What it means for you - Parts & Controls
Air Handler
An air handler is the indoor cabinet of a split heat pump or air conditioning system that houses the blower motor, the evaporator coil, controls and often the filter rack. It moves conditioned air through the supply ducts and pulls air back through the returns. Electric resistance heat strips inside many air handlers provide auxiliary and emergency heat.
What it means for you - Heat Pumps, Geothermal & Ductless
Auxiliary Heat
Auxiliary heat is backup heat that a heat pump's thermostat turns on automatically when the heat pump alone cannot keep up, such as below the balance point, during a defrost cycle or after a large thermostat setback. In all-electric systems it usually comes from resistance heat strips in the air handler; in dual-fuel systems, from a gas furnace.
What it means for you
B.
- Heat Pumps, Geothermal & Ductless
Balance Point
The balance point is the outdoor temperature at which a heat pump's heating capacity equals a home's heat loss. Above it, the heat pump can heat the house alone; below it, supplemental heat such as electric strips or a furnace must make up the difference. It depends on both the equipment's capacity and the home's insulation and air sealing.
What it means for you - Parts & Controls
Blower Motor (ECM)
A blower motor is the electric motor that turns the indoor fan wheel in a furnace or air handler, moving air across the heat exchanger or evaporator coil and through the home's ductwork. Older permanent split capacitor (PSC) motors run at fixed speeds, while electronically commutated motors (ECMs) vary their speed and typically use less electricity.
What it means for you
C.
- Parts & Controls
Capacitor
A capacitor is an electrical component that stores and releases energy to start and run the motors in air conditioners, heat pumps and furnace blowers. A run capacitor keeps the compressor and fan motors turning efficiently, a start capacitor adds a brief boost at compressor startup, and a dual run capacitor serves the compressor and condenser fan together.
What it means for you - Furnaces & Gas Heat
Carbon Monoxide (CO)
Carbon monoxide (CO) is a colorless, odorless, poisonous gas produced when fuels such as natural gas, propane, wood or gasoline burn incompletely. Household sources include leaking or back-drafting furnaces, blocked flues, wood stoves and portable generators. EPA notes exposure causes headaches, dizziness, confusion and nausea and can be fatal at high levels.
What it means for you - Cooling & Air Conditioning
Central Air Conditioner
A central air conditioner is a ducted cooling system that cools an entire home from one set of equipment. An outdoor condensing unit and an indoor evaporator coil, linked by copper refrigerant lines, pull heat and moisture from indoor air, and a blower delivers the cooled air through supply ducts. Efficiency is rated in SEER2; capacity in tons.
What it means for you - Cooling & Air Conditioning
Compressor
A compressor is the motor-driven pump in an air conditioner or heat pump that squeezes low-pressure refrigerant vapor into hot, high-pressure vapor, keeping refrigerant circulating through the whole system. It sits in the outdoor unit and comes in single-stage, two-stage and variable-speed designs. It is usually the most expensive single component to replace.
What it means for you - Parts & Controls
Condensate Drain Line
A condensate drain line is the pipe, usually PVC, that carries water away from an air conditioner's or heat pump's evaporator coil drain pan, and from high-efficiency condensing furnaces, to a floor drain, the outdoors, or a condensate pump. Algae, sludge, and debris can clog it, backing water into the pan or triggering a safety shutoff.
What it means for you - Cooling & Air Conditioning
Condenser Coil
A condenser coil is the finned outdoor heat exchanger inside a condensing unit where hot, high-pressure refrigerant gives up the heat it absorbed indoors. The condenser fan draws outdoor air across the aluminum fins and tubing, and the refrigerant condenses from vapor back into liquid as it cools. A heat pump's outdoor coil serves this role in cooling mode.
What it means for you - Cooling & Air Conditioning
Condensing Unit
A condensing unit is the outdoor half of a split air conditioning system. It houses the compressor, condenser coil, condenser fan motor and electrical components including the capacitor and contactor. Refrigerant vapor carrying heat from indoors is compressed inside the cabinet, and the coil and fan then release that heat to the outdoor air.
What it means for you - Parts & Controls
Contactor
A contactor is an electrically operated switch in the outdoor unit of an air conditioner or heat pump that sends line voltage to the compressor and condenser fan. When the thermostat calls for cooling or heating, a 24-volt control signal energizes its coil and pulls the contacts closed; when the call ends, the contacts open and the unit stops.
What it means for you
D.
- Heat Pumps, Geothermal & Ductless
Defrost Cycle
A defrost cycle is the routine a heat pump runs in heating mode to melt frost from its outdoor coil. The control board briefly switches the system into cooling to warm the coil, usually stops the outdoor fan and often turns on backup heat indoors. Visible steam rising from the outdoor unit during defrost is normal.
What it means for you - Heat Pumps, Geothermal & Ductless
Desuperheater
A desuperheater is a small auxiliary heat exchanger built into many geothermal heat pumps that captures superheated refrigerant gas leaving the compressor and uses its surplus heat to preheat domestic hot water, usually feeding a conventional water heater tank. It produces the most hot water during cooling season, when that heat would otherwise go into the ground.
What it means for you - Furnaces & Gas Heat
Draft Inducer Motor
A draft inducer motor is the small fan motor on a gas furnace that pulls combustion gases through the heat exchanger and pushes them out the vent. It starts before ignition, and a pressure switch must confirm proper draft before the igniter and gas valve are allowed to operate. If the inducer fails, the furnace locks out.
What it means for you - Heat Pumps, Geothermal & Ductless
Dual-Fuel System
A dual-fuel system, or hybrid heat pump, pairs an electric heat pump with a gas or propane furnace sharing the same ductwork and controls. The heat pump handles all cooling and heating in milder weather; below a set switchover temperature, the controls shut the heat pump off and run the furnace instead. The two normally do not heat together.
What it means for you - Airflow & Indoor Air
Duct Sealing
Duct sealing is closing the leaks, gaps, and loose connections in a home's ductwork, usually at joints, boots, takeoffs, and plenum connections, using mastic sealant or foil-backed metal tape rather than cloth duct tape. ENERGY STAR estimates a typical home's duct system loses about 20 to 30 percent of the air moving through it.
What it means for you - Heat Pumps, Geothermal & Ductless
Ductless Mini-Split
A ductless mini-split is a heat pump with an outdoor compressor connected by a slim refrigerant line set to one or more indoor heads mounted on walls or ceilings. Each head heats and cools its own zone without ductwork. Most use variable-speed inverter compressors, and single-zone and multi-zone configurations are both common.
What it means for you - Airflow & Indoor Air
Ductwork
Ductwork is the system of supply and return ducts that moves air between a furnace, heat pump, or air handler and the rooms of a home. Supply runs deliver heated or cooled air; returns bring room air back to be filtered and reconditioned. Ducts are typically sheet metal, flexible duct, or duct board, insulated where they cross unconditioned space.
What it means for you
E.
- Ratings, Sizing & Money
EER2
EER2 (Energy Efficiency Ratio 2) is the U.S. Department of Energy rating for an air conditioner's or heat pump's cooling output per watt of electricity at one hot test condition, 95°F outdoors, rather than averaged across a season. It describes peak-heat performance. Federal EER2 minimums apply only to air conditioners installed in the Southwest region.
What it means for you - Heat Pumps, Geothermal & Ductless
Emergency Heat
Emergency heat is a manually selected thermostat setting on heat pump systems that shuts off the outdoor heat pump and heats the home with backup heat only, usually electric resistance strips or, in dual-fuel systems, a gas furnace. Unlike auxiliary heat, which engages automatically, emergency heat is meant for a failed or damaged heat pump.
What it means for you - Airflow & Indoor Air
Energy Recovery Ventilator (ERV)
An energy recovery ventilator (ERV) is a balanced whole-house ventilation appliance that exchanges stale indoor air for filtered outdoor air through a core that transfers both heat and some water vapor between the two airstreams. That moisture transfer helps keep indoor humidity steadier than a heat recovery ventilator, which transfers heat only.
What it means for you - Refrigerants
EPA Section 608 Certification
EPA Section 608 certification is the federal credential, under Section 608 of the Clean Air Act, for technicians who maintain, service, repair or dispose of equipment that could release refrigerants. EPA offers Type I, Type II, Type III and Universal certification; Type II covers high-pressure equipment, including residential split air conditioners and heat pumps.
What it means for you - Ratings, Sizing & Money
Equipment Warranty
An HVAC equipment warranty is a manufacturer's written promise to replace defective parts, such as a compressor or heat exchanger, for a stated period. It usually covers parts only; labor to diagnose and install them is separate and comes from the installing contractor or an extended plan. Many manufacturers require product registration within a set window.
What it means for you - Cooling & Air Conditioning
Evaporator Coil
An evaporator coil is the indoor heat exchanger of an air conditioner or heat pump, mounted atop a furnace or inside an air handler. Cold refrigerant in its tubes absorbs heat from air the blower pushes across its fins, and moisture condenses on the coil and drains away. In heat pump heating mode, the same coil releases heat instead.
What it means for you
F.
G.
- Furnaces & Gas Heat
Gas Furnace
A gas furnace is a heating appliance that burns natural gas or propane, passes the heat through a metal heat exchanger and blows warmed air through a home's ductwork. Combustion gases are vented outdoors through a flue. Furnace efficiency is rated by AFUE, the percentage of fuel energy delivered to the home as usable heat.
What it means for you - Furnaces & Gas Heat
Gas Piping (Gas Line)
Gas piping is the system of pipes, fittings, shutoff valves and appliance connectors that carries natural gas or propane from the meter or tank to furnaces, water heaters, ranges, dryers and other appliances. Common materials include black steel and corrugated stainless steel tubing (CSST). Sizing, installation and pressure testing follow a fuel gas code such as NFPA 54.
What it means for you - Heat Pumps, Geothermal & Ductless
Geothermal Heat Pump
A geothermal heat pump, also called a ground-source heat pump, moves heat between a home and the ground through a buried loop of pipe filled with water or antifreeze solution. Because ground temperature stays far more stable than outdoor air, it gathers winter heat and rejects summer heat more efficiently than an air-source heat pump.
What it means for you - Heat Pumps, Geothermal & Ductless
Ground Loop
A ground loop is the buried piping of a geothermal heat pump that circulates water or an antifreeze solution to exchange heat with the earth. Closed loops are laid horizontally in trenches, vertically in drilled boreholes or submerged in a pond; open loops instead draw well water through the system and return or discharge it.
What it means for you
H.
- Furnaces & Gas Heat
Heat Exchanger
A heat exchanger is the metal chamber or set of tubes inside a gas furnace that keeps burning fuel and its exhaust separate from the air circulating through the home. Heat passes through the metal walls into the airflow while combustion gases vent outdoors. A cracked heat exchanger can let those gases, including carbon monoxide, into the airstream.
What it means for you - Heat Pumps, Geothermal & Ductless
Heat Pump
A heat pump is an electric heating and cooling system that moves heat rather than burning fuel. In winter it pulls heat from outdoor air and delivers it indoors; in summer a reversing valve runs the refrigerant cycle backward to remove heat like an air conditioner. Heating efficiency is rated in HSPF2 and cooling efficiency in SEER2.
What it means for you - Furnaces & Gas Heat
Hot Surface Igniter
A hot surface igniter is a silicon carbide or silicon nitride element that glows hot enough to light the gas at a furnace's burners. During a heating call it heats for several seconds, then the gas valve opens and the burners ignite. Hot surface ignition replaced the standing pilot light on most residential gas furnaces built in recent decades.
What it means for you - Ratings, Sizing & Money
HSPF2
HSPF2 (Heating Seasonal Performance Factor 2) is the U.S. Department of Energy rating for how much heat a heat pump delivers per watt-hour of electricity over a typical heating season, measured under the M1 test procedure adopted in 2023. Since January 1, 2023, new split-system heat pumps nationwide must be rated at least 7.5 HSPF2.
What it means for you - Ratings, Sizing & Money
HVAC Financing
HVAC financing is a loan or credit plan that lets a homeowner pay for heating and cooling equipment, installation or major repairs over time instead of all at once. Rates, terms and approval are set by the lender, not the contractor, and depend on the applicant's credit. Applications are often started through the contractor.
What it means for you - Ratings, Sizing & Money
HVAC Maintenance Plan
An HVAC maintenance plan is a service agreement in which a contractor performs scheduled tune-ups and inspections on a home's heating and cooling equipment, often with member terms such as priority scheduling or repair discounts. Visits typically cover filters, airflow, electrical components, refrigerant charge, condensate drainage and, for furnaces, combustion safety checks.
What it means for you
K.
- Ratings, Sizing & Money
Kentucky HVAC License
A Kentucky HVAC license is the state credential for heating, ventilation and air conditioning work, issued by the Department of Housing, Buildings and Construction's Division of HVAC under KRS Chapter 198B. The two license levels are master HVAC contractor, who supervises all work and pulls permits, and journeyman HVAC mechanic, who works under a master.
What it means for you - Ratings, Sizing & Money
Kentucky HVAC Permit
A Kentucky HVAC permit is a construction permit for installing, altering or replacing heating, ventilation and air conditioning equipment, issued under KRS Chapter 198B and state regulation 815 KAR 8:070 through the Department of Housing, Buildings and Construction's Division of HVAC. Permits go to licensed master HVAC contractors, with a narrow homeowner exception, and include inspections.
What it means for you
L.
M.
- Ratings, Sizing & Money
Manual J Load Calculation
A Manual J load calculation is the Air Conditioning Contractors of America (ACCA) method for working out how much heating and cooling a specific house needs, room by room. It accounts for insulation, windows, orientation, air leakage, duct location and local design temperatures, and ACCA describes it as the ANSI-recognized standard for residential load calculation.
What it means for you - Airflow & Indoor Air
MERV Rating
MERV (Minimum Efficiency Reporting Value) is a 1-to-16 scale, from the ASHRAE Standard 52.2 test method, rating how well an air filter captures particles between 0.3 and 10 microns. Higher numbers capture more fine particles such as pollen fragments and smoke, but denser media can add airflow resistance, so the right MERV depends on the system.
What it means for you - Central Kentucky Home & Climate
Mixed-Humid Climate (Zone 4A)
A mixed-humid climate is a region with both a meaningful heating season and a hot, humid cooling season. The U.S. Department of Energy's Building America program defines it as more than 20 inches of annual precipitation, roughly 5,400 heating degree days or fewer, and winter monthly averages below 45°F. Kentucky falls in IECC Climate Zone 4A.
What it means for you
O.
P.
R.
- Refrigerants
R-22 (Freon)
R-22, also called HCFC-22 and widely known by the brand name Freon, is an ozone-depleting refrigerant used in most U.S. home air conditioners and heat pumps installed before 2010. Under the Clean Air Act phaseout, U.S. production and import of R-22 ended in 2020. Existing systems may keep running on recovered, reclaimed or stockpiled supply.
What it means for you - Refrigerants
R-410A
R-410A is the HFC refrigerant blend, sold under trade names including Puron, used in most U.S. home air conditioners and heat pumps installed from about 2010 through 2024. Its global warming potential exceeds EPA's limit of 700 for new residential systems, so manufacturing and import of new R-410A split systems ended January 1, 2025.
What it means for you - Refrigerants
R-454B
R-454B is a refrigerant blend of R-32 and R-1234yf used in many new residential air conditioners and heat pumps as a lower-global-warming replacement for R-410A. EPA's reference table lists its GWP at 465, under the limit of 700 for new residential systems. It is classified A2L, meaning lower toxicity and lower flammability.
What it means for you - Refrigerants
Refrigerant
Refrigerant is the fluid sealed inside an air conditioner or heat pump that carries heat from one place to another, absorbing it as it evaporates in one coil and releasing it as it condenses in the other. It is not consumed in normal operation. Common residential refrigerants include R-22, R-410A, R-32 and R-454B.
What it means for you - Refrigerants
Refrigerant Leak
A refrigerant leak is an escape of refrigerant from an air conditioner or heat pump's sealed circuit, usually through corroded coils, worn joints or vibrating line connections. Because refrigerant is not used up, a system that is low has a leak. Symptoms include weak heating or cooling, longer run times, iced coils and hissing sounds.
What it means for you - Central Kentucky Home & Climate
Relative Humidity
Relative humidity (RH) is the amount of water vapor in the air compared with the most that air can hold at its current temperature, expressed as a percentage. Warmer air holds more moisture, so RH rises as air cools. EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent.
What it means for you - Ratings, Sizing & Money
Repair vs. Replace
Repair vs. replace is the decision a homeowner faces when heating or cooling equipment fails or falls behind: fix it, keep it running while monitoring its condition, or install new equipment. A sound answer rests on a whole-system diagnosis weighing age, repair cost, efficiency, comfort, warranty status, reliability and likely remaining life, not just the failed part.
What it means for you - Heat Pumps, Geothermal & Ductless
Reversing Valve
A reversing valve is the solenoid-controlled valve in a heat pump's outdoor unit that switches the direction of refrigerant flow, changing the system between heating and cooling. It also shifts the system into cooling briefly during a defrost cycle. A stuck valve or failed solenoid can leave a heat pump blowing cool air in heat mode.
What it means for you
S.
- Ratings, Sizing & Money
SEER2
SEER2 (Seasonal Energy Efficiency Ratio 2) is the U.S. Department of Energy rating for how much cooling an air conditioner or heat pump delivers per watt-hour of electricity over a typical cooling season. It replaced SEER in 2023 under a tougher test procedure. Higher numbers mean less electricity for the same cooling.
What it means for you - Cooling & Air Conditioning
Short Cycling
Short cycling is when an air conditioner, heat pump or furnace starts and stops in rapid, frequent cycles instead of completing normal run times. Common causes include oversized equipment, a clogged filter, a frozen evaporator coil, incorrect refrigerant charge, an overheating furnace and a misplaced or failing thermostat. It adds wear and reduces comfort and efficiency.
What it means for you - Parts & Controls
Smart Thermostat
A smart thermostat is a Wi-Fi connected wall control that schedules heating and cooling, can be adjusted from a smartphone app and often learns occupancy patterns or uses remote sensors. Many report runtime and energy use, send maintenance or fault alerts and support heat pump, multi-stage and dual-fuel equipment. ENERGY STAR certifies models that meet its criteria.
What it means for you - Cooling & Air Conditioning
Split System
A split system is an air conditioner or heat pump divided between two cabinets: an outdoor condensing unit containing the compressor and condenser coil, and an indoor furnace or air handler containing the evaporator coil and blower. Insulated copper refrigerant lines and low-voltage control wiring connect the halves, and the pair is rated together as a matched system.
What it means for you - Airflow & Indoor Air
Static Pressure
Static pressure is the resistance air meets as the blower pushes it through ducts, filters, coils, and registers, measured in inches of water column (in. w.c.). Total external static pressure combines readings taken before and after the furnace or air handler; when it exceeds the equipment's rated maximum, airflow drops and components work harder.
What it means for you
T.
- Parts & Controls
Thermostat
A thermostat is the wall control that senses indoor temperature and tells heating and cooling equipment when to start and stop. Types include manual, programmable, Wi-Fi smart and communicating models. Thermostats for heat pumps also manage the reversing valve, auxiliary heat and emergency heat, and many control fan settings, humidity or system alerts.
What it means for you - Ratings, Sizing & Money
Tonnage (BTU/h)
Tonnage is the cooling capacity of an air conditioner or heat pump, where one ton equals 12,000 BTU/h (British thermal units per hour) of heat moved. Residential systems commonly range from 1.5 to 5 tons. Tonnage measures capacity, not efficiency, and the right size for a home comes from a Manual J load calculation.
What it means for you
V.
W.
- Airflow & Indoor Air
Whole-Home Air Cleaner
A whole-home air cleaner is filtration or air-cleaning equipment built into a forced-air system so it treats all the air the blower moves. Common types are deep-pleated media cabinets that hold 4- or 5-inch filters and electronic air cleaners. EPA notes in-duct air cleaners only work while the system fan is running.
What it means for you - Airflow & Indoor Air
Whole-Home Dehumidifier
A whole-home dehumidifier is a ducted appliance, usually installed beside the furnace or air handler or in a basement or crawlspace, that pulls moisture from the air for the entire house and drains it away continuously. It runs on a humidistat independently of the air conditioner, controlling humidity when cooling demand is low.
What it means for you - Airflow & Indoor Air
Whole-Home Humidifier
A whole-home humidifier is a device mounted on the furnace or air handler ductwork that adds water vapor to heated air for the entire house. Common types are bypass and fan-powered evaporative units, which pass air over a water-fed pad, and steam units. A humidistat controls output so indoor air stays comfortable without condensing on cold windows.
What it means for you
Z.
Want the bigger picture?
The glossary defines the words. The Learning Center walks through the decisions — repair or replace, heat pumps in Kentucky winters, humidity, and buying a new system.
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