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HVAC glossary

Plain-English definitions for the terms you'll see on service calls and in manuals. Underlined words in our guides and code charts show these definitions when you tap them.

A B C D E F G H I L M O P R S T W Z

A2L: A refrigerant safety class meaning lower toxicity and mildly flammable. R-32 and R-454B are A2L, so they need A2L-rated tools and extra install rules. Learn more →
AFUE: Annual Fuel Utilization Efficiency: the share of a furnace's fuel that becomes heat over a season. 96% AFUE means 96 cents of every fuel dollar heats the home.
BTU (BTU/h, BTUH): British thermal unit, a measure of heat. Equipment capacity is in BTU per hour (BTU/h); 12,000 BTU/h equals 1 ton.
bubble point: For a blend with glide, the temperature where liquid starts to boil. Use it with liquid-line pressure to figure subcooling. Learn more →
C wire (common wire): The 24-volt common from the transformer. Smart thermostats need it for constant power. Learn more →
CFM: Cubic feet per minute, the amount of air a blower moves. A common target is about 350–400 CFM per ton of cooling. Learn more →
contactor: A heavy-duty relay in the outdoor unit. 24-volt control power on its coil closes contacts that send line voltage to the compressor and fan. Learn more →
DC bus: The high-voltage DC circuit on an inverter board, fed by rectified line voltage. Its capacitors can stay charged after power is off. Learn more →
deep vacuum (evacuation): Pulling a system down to a few hundred microns to remove air and moisture before charging. Confirm with a micron gauge and a decay test. Learn more →
defrost: A heat pump mode that briefly reverses to melt frost off the outdoor coil in heating. Steam and a paused indoor fan are normal.
dew point: For a blend with glide, the temperature where vapor starts to condense. Use it with suction pressure to figure superheat. Learn more →
EEPROM: A memory chip on a control board that stores settings and model data. EEPROM errors usually mean a board or its data is bad.
EEV (electronic expansion valve): An expansion valve opened and closed by the control board with a stepper motor, so it can meter refrigerant precisely.
EPA 608: The federal certification required to buy refrigerant and work on sealed refrigerant systems. Learn more →
flame sensor: A metal rod in the burner flame. The board proves flame by measuring a tiny DC current (microamps) through it. Learn more →
flare connection (flare): A cone-shaped fitting made on the end of copper tubing and tightened onto a threaded nut. The most common leak point on mini-splits. Learn more →
flat-rate pricing (flat rate): Pricing each task at a fixed price quoted up front, instead of billing hours plus parts. Learn more →
glide (temperature glide): The difference between a blended refrigerant's bubble and dew points at the same pressure. R-454B has a small glide; R-410A's is nearly zero.
high limit (limit switch, main limit, primary limit): A safety switch that shuts off the gas if the furnace gets too hot, usually from low airflow. Learn more →
hot surface igniter (HSI, igniter): A ceramic element that glows hot enough to light the gas. Cracks and white spots mean it's failing. Learn more →
HSPF2: The current heating efficiency rating for heat pumps (2023 test method). Higher is more efficient.
inches of water column (in. w.c., inches w.c., "wc, IWC): A unit for very low pressures, used for gas pressure and duct static pressure. 1 psi is about 27.7 inches of water column.
inducer (draft inducer, combustion air inducer): A small blower that pulls combustion gases through the furnace and out the vent before and during burner operation.
inverter: Electronics that turn line power into variable-frequency power so the compressor can speed up and slow down instead of only on and off.
IPM (intelligent power module): The power module on an inverter board that switches high-voltage DC to drive the compressor. IPM faults point to voltage, airflow, the compressor or the board. Learn more →
latent heat (latent load): Heat removed by condensing moisture out of the air, as opposed to sensible heat, which changes the air's temperature.
line set: The pair of copper lines (suction and liquid) that connects the indoor and outdoor units. Learn more →
manifold pressure: Gas pressure after the gas valve, at the burners, measured in inches of water column. Must match the rating plate.
Manual J: The ACCA standard method for calculating a home's heating and cooling load, room by room. Learn more →
MERV: A filter efficiency rating from 1 to 16. Higher catches more but can restrict airflow if the duct system isn't sized for it. Learn more →
microamps (µA): Millionths of an amp. Flame sensor current is measured in microamps; low readings cause flame-loss lockouts.
microfarad (µF, MFD): The unit capacitors are rated in. A run capacitor should read within its printed tolerance, often ±5% or ±6%.
micron (microns): A unit of very low pressure used to measure vacuum. Lower is deeper; 500 microns or below is a common target.
O/B: The thermostat terminal that controls a heat pump's reversing valve. O energizes in cooling on most brands; B energizes in heating on some. Learn more →
PCB (control board, circuit board): Printed circuit board: the electronic control in a furnace, air handler or mini-split.
PFC: Power factor correction: a circuit on inverter boards that smooths how the unit draws power. PFC faults are often supply-voltage problems.
piston (fixed orifice): A fixed metering device with a single hole. Systems with a piston are charged by superheat. Learn more →
pressure switch: A furnace safety that proves the inducer is moving enough air before ignition. Usually the switch isn't the problem; the venting or inducer is. Learn more →
PT chart: A table showing the temperature at which a refrigerant boils or condenses at each pressure. Learn more →
refrigerant recovery: Removing refrigerant from a system into a recovery cylinder instead of venting it, as federal rules require.
reversing valve (4-way valve, four-way valve): The valve that switches a heat pump between heating and cooling by changing the direction of refrigerant flow.
rollout switch (flame rollout): A safety switch near the burners that opens if flame escapes the burner area. Often manual reset. Find the cause before resetting. Learn more →
run capacitor (capacitor): A part that gives a motor's start winding a phase shift so it starts and runs efficiently. Rated in microfarads (µF). Learn more →
saturation temperature (saturation): The temperature at which refrigerant boils or condenses at a given pressure. Read it from a PT chart. Learn more →
SEER2: The current cooling efficiency rating for AC and heat pumps (2023 test method). Higher is more efficient.
subcooling: How many degrees the liquid line is below its saturation temperature. Used to check charge on TXV and EEV systems. Learn more →
superheat: How many degrees the suction vapor is above its saturation temperature. Used to check charge on piston systems and evaporator performance. Learn more →
temperature rise: Supply air temperature minus return air temperature on a furnace. It must fall in the range on the rating plate. Learn more →
thermistor: A temperature sensor whose resistance changes with temperature. Test it against the manufacturer's resistance table. Learn more →
ton (tons): A unit of cooling capacity equal to 12,000 BTU per hour.
total external static pressure (TESP, static pressure): The resistance the blower works against from the ducts, filter and coil, measured in inches of water column. Learn more →
TXV (thermostatic expansion valve): A valve that meters refrigerant into the evaporator to hold a steady superheat. Systems with a TXV are charged by subcooling.
wet bulb: Air temperature measured with a wet wick, which reflects humidity. Indoor wet bulb is used to find target superheat.
zero-crossing: A board circuit that watches the AC voltage pass through zero to time fan and power control. Errors usually mean a power or board problem.