HVAC Glossary

A plain-English guide to the terms HVAC contractors use, from SEER2 and auxiliary heat to reversing valves, short cycling and R-22.

By the Knoxville HVAC Services editorial team · Last reviewed October 14, 2026

What do common HVAC terms mean?

These are the words an HVAC contractor is most likely to use when diagnosing a problem or quoting a new system. Each definition is written in plain language, so you can follow the conversation and ask better questions.

Air handler: The indoor cabinet that holds the blower and, in most systems, the evaporator coil. It moves air across the coil and through the ducts. With a heat pump it often holds electric backup heat as well.
Auxiliary heat: Backup heat, usually electric heat strips, that a heat pump turns on when it cannot keep up on its own or during a defrost cycle. A thermostat may show it as Aux. It costs more to run than the heat pump itself.
BTU: British thermal unit, a measure of heat energy. Heating and cooling equipment is rated in BTUs per hour, which describes how much heat it can add to or remove from a home.
Capacitor: A small electrical part that gives the compressor and fan motors the boost they need to start and keeps them running. It is one of the most common parts to fail. It stores a charge even with the power off, so it is not a DIY repair.
Compressor: The pump in the outdoor unit that pressurizes refrigerant and moves it through the system. It is the most expensive single part, so a compressor failure often raises the repair or replace question.
Condensate drain: The pipe and pan that carry away water the indoor coil pulls out of the air. If it clogs, water can overflow, or a safety float switch may shut the system off.
Condenser: The outdoor unit of a central air conditioner, holding the compressor, the outdoor coil and the fan. It releases the heat taken out of the house. On a heat pump the same unit also collects heat in winter.
Defrost cycle: A normal heat pump routine in cold, damp weather. The system briefly reverses to melt frost off the outdoor coil, which can cause steam and a short pause in heating.
Dual fuel: A system that pairs an electric heat pump with a gas or propane furnace. The heat pump handles most heating and the furnace takes over in colder weather.
Ductwork: The metal or flexible channels that carry air from the air handler to the rooms and back. ENERGY STAR says a typical house loses about 20 to 30 percent of duct airflow to leaks, holes and poor connections.
Evaporator coil: The indoor coil, usually on top of the furnace or inside the air handler. Cold refrigerant in the coil absorbs heat and moisture from the air blowing across it.
Heat pump: A system that cools like an air conditioner and heats by running in reverse, moving heat from outdoor air into the house. ENERGY STAR says an air-source heat pump can deliver up to three times more heat energy than the electricity it uses.
HSPF2: Heating Seasonal Performance Factor 2, the current rating for a heat pump's heating efficiency over a typical season. A higher number means more efficient heating.
Load calculation (Manual J): A room-by-room calculation of how much heating and cooling a house needs, based on its size, insulation, windows and orientation. Manual J is the method from the Air Conditioning Contractors of America. It is how a contractor should size new equipment.
MERV filter: A filter rated on the Minimum Efficiency Reporting Value scale. A higher MERV traps smaller particles but can also restrict airflow, so ask the contractor what rating your system can handle.
Mini-split: A ductless heat pump with an outdoor unit and one or more indoor heads mounted on a wall or ceiling. ENERGY STAR notes that mini-splits need no ductwork, which suits additions and rooms the ducts do not reach.
R-22: An older refrigerant, also called HCFC-22. The EPA has phased out most HCFCs, with some use continuing only to service existing equipment, which makes leaks on R-22 systems a bigger decision.
R-410A: A refrigerant used in many air conditioners and heat pumps that replaced R-22 systems. It is not interchangeable with R-22, so an R-22 system cannot simply be refilled with it.
Refrigerant: The fluid that carries heat between the indoor and outdoor coils. It is not used up like fuel. ENERGY STAR notes that too much or too little makes a system less efficient and shortens its life. Handling it requires EPA Section 608 certification.
Return and supply: Supply vents blow conditioned air into rooms. Return grilles pull room air back to the system to be heated or cooled again. Blocked returns starve the system of air.
Reversing valve: The valve in a heat pump that switches the direction of refrigerant flow, which is what lets the same system heat in winter and cool in summer. A stuck valve can leave a heat pump blowing cool air in heating mode.
SEER2: Seasonal Energy Efficiency Ratio 2, the current rating for an air conditioner's or heat pump's cooling efficiency over a typical season. A higher number means less electricity for the same cooling.
Short cycling: A system that turns on and off every few minutes without finishing a full cycle. Causes include an oversized unit, a thermostat problem, a clogged filter or an electrical fault. ENERGY STAR warns oversized equipment may cycle too often and wear out sooner.
Ton: A unit of cooling capacity for air conditioners and heat pumps. Bigger is not better: a unit should be sized by a load calculation, not by the size of the old one.
Zoning: Splitting a home into areas with their own thermostats, using motorized dampers in the ducts or separate systems. It helps houses with floors or wings that heat and cool unevenly.

Where do these definitions come from?

Efficiency, duct and sizing guidance follows ENERGY STAR. Heat pump terms follow ENERGY STAR's heat pump guide and the Department of Energy. Refrigerant terms follow the EPA. To see the terms in use, read the AC repair, heat pump repair and AC replacement guides.