HVAC Glossary

The terms you'll hear from a technician, a diagnosis, or a spec sheet — explained in plain English.

AFUE: Annual Fuel Utilization Efficiency — the percentage of fuel a furnace converts to heat. A 95% AFUE furnace turns 95% of its gas into usable heat.
Air Handler: The indoor unit that houses the blower and, on a heat pump system, the indoor coil — pairs with an outdoor condenser.
Blower Motor: The fan motor that pushes conditioned air through your ductwork and out the vents.
BTU: British Thermal Unit — the standard measure of heating or cooling capacity. One ton of cooling equals 12,000 BTU/hour.
Capacitor: A component that stores electrical charge to help start and run the compressor and fan motors. A leading cause of "won't start" and "clicking" symptoms when it fails.
Compressor: The component in the outdoor unit that pressurizes refrigerant, driving the entire cooling cycle. Often the most expensive part to replace.
Condensate Drain: The pipe that carries water condensation away from the indoor coil. Clogs are a common cause of leaks and system shutoffs.
Condenser Coil: The outdoor coil that releases heat collected from inside the house into the outside air.
Contactor: An electrically controlled relay switch that sends power to the compressor and outdoor fan when the thermostat calls for cooling.
Damper: A movable plate inside ductwork that regulates or redirects airflow, often used in zoned systems.
Defrost Cycle: A brief reversal a heat pump runs in cold weather to melt ice off the outdoor coil — normal, and shouldn't be mistaken for a malfunction if brief.
Ductwork: The network of metal or flexible tubes distributing conditioned air from the system to each room.
Evaporator Coil: The indoor coil where refrigerant absorbs heat from your home's air, cooling it before it's distributed.
Filter Drier: A component in the refrigerant line that removes moisture and debris — replaced during major refrigerant repairs.
Heat Exchanger: The component inside a furnace that transfers heat from combustion gases to the air stream without mixing them. A cracked heat exchanger is a carbon monoxide hazard.
Heat Pump: A system that moves heat rather than generating it, capable of both heating and cooling by reversing its refrigerant flow.
HSPF: Heating Seasonal Performance Factor — the efficiency rating for a heat pump's heating mode, similar in purpose to SEER for cooling.
Limit Switch: A furnace safety switch that shuts off heating elements or the burner if temperatures rise too high, often triggered by restricted airflow.
MERV Rating: Minimum Efficiency Reporting Value — measures how well an air filter captures particles. Higher isn't always better; very high MERV filters can restrict airflow on some systems.
Package Unit: An all-in-one system with the compressor, coils, and blower in a single outdoor cabinet, common on homes without basement or attic space for an air handler.
Plenum: The main box connecting the air handler or furnace to the branching ductwork.
Refrigerant: The chemical (commonly R-410A in modern systems) that cycles between liquid and gas to move heat. Regulated under EPA Section 608; not a DIY-handled substance.
Reversing Valve: The component on a heat pump that switches refrigerant flow direction to change between heating and cooling modes.
SEER / SEER2: Seasonal Energy Efficiency Ratio — the cooling efficiency rating of an AC or heat pump. SEER2 is the updated testing standard used on newer equipment.
Sequencer: A timed relay in electric furnaces that brings heating elements online one at a time rather than all at once.
Short Cycling: When a system turns on and off far more often than normal, wasting energy and wearing out components — usually a sign of an underlying restriction or oversized equipment.
Split System: The most common residential setup: an outdoor condenser paired with an indoor air handler or furnace, connected by refrigerant lines.
Static Pressure: The resistance to airflow inside your ductwork — too high, and the blower struggles to move air, often from restrictive filters or undersized ducts.
Subcooling: A refrigerant measurement technicians use, alongside superheat, to confirm a system has the correct refrigerant charge.
Superheat: A refrigerant temperature measurement technicians use to diagnose charge levels and airflow issues — part of a standard service check, not a DIY reading.
Thermostat: The control that reads room temperature and signals the system to heat, cool, or idle.
Ton (of cooling): A unit of cooling capacity equal to 12,000 BTU/hour — residential systems typically range from 1.5 to 5 tons.
TXV (Thermostatic Expansion Valve): A metering device that regulates refrigerant flow into the evaporator coil, adjusting to load conditions in real time.
Zoning: A ductwork setup using dampers and multiple thermostats to condition different areas of a home independently.