HVAC Terms in Plain English

Capacitor, tonnage, static pressure: HVAC talk can sound like another language. Learn a handful of these words and a diagnosis gets much easier to follow, and to question.

HVAC Terms, A to Z

Air handler Airflow

The indoor cabinet that holds the blower, and usually the evaporator coil and electric heat strips, and moves air through the ducts. Homes with a gas furnace use the furnace’s blower instead.

Blower motor Airflow

The motor that spins the indoor fan and pushes air through the ducts in both heating and cooling. When it fails, no air comes out of the vents at all.

BTU Ratings

British thermal unit: the heat it takes to warm a pound of water by one degree Fahrenheit. Heating and cooling capacity is rated in BTUs per hour.

Capacitor Cooling

A small can-shaped part that stores a charge and gives the compressor and fan motors the jolt they need to start. When one weakens, the outdoor unit hums but won’t start until an AC repair replaces it.

Compressor Cooling

The pump in the outdoor unit that squeezes refrigerant and drives it around the system. It’s the costliest part to replace, so when one fails on an older system, replacing the system often makes more sense.

Condensate drain line Cooling

The pipe, usually white PVC, that carries water from the indoor coil’s drain pan to a drain or outdoors. Houston’s humidity keeps it busy all summer, and flushing out the algae that clog it is part of maintenance.

Condenser Cooling

The outdoor unit, where a fan pulls air through a coil to dump the heat taken out of the house; it also holds the compressor. A coil matted with dirt and lint can’t shed heat, so keep plants and clutter a couple of feet back.

Contactor Controls

An electrical switch in the outdoor unit that closes when the thermostat calls for cooling and sends power to the compressor and fan. Pitted contacts, or fire ants packed between them, can keep the unit from starting or leave it running nonstop.

Ductwork Airflow

The supply and return ducts that carry air between the system and the rooms, often flexible duct in the attic. Flex duct can sag, kink or pull apart at the joints, and sealing them with mastic is a core part of ductwork repair.

Evaporator coil Cooling

The indoor coil, on top of the furnace or inside the air handler, where cold refrigerant soaks up heat and moisture from the air passing over it. The moisture drips into the drain pan, and when airflow drops, the coil can freeze solid.

Filter rating Ratings

How fine a dust a filter catches, usually given as a MERV number. Higher numbers catch finer particles but can choke the airflow on a system that wasn’t built for them.

Flame sensor Heating

A thin metal rod in the burner flame that proves the gas has lit. A sooty one makes the furnace light, run a few seconds and quit, and cleaning it is a quick heating repair.

Furnace Heating

The indoor unit that burns natural gas, or uses electric elements, to warm the air before the blower sends it through the ducts. Its burners, ignitor, flame sensor and heat exchanger all get checked in a fall tune-up.

Heat exchanger Heating

The metal chamber in a gas furnace that passes the burners’ heat into the air while keeping the combustion gases apart from it. A crack can leak carbon monoxide into the airflow, one more reason to keep a working CO alarm near the bedrooms.

Heat pump Heating

An outdoor unit that cools in summer and runs in reverse in winter to pull heat from the outdoor air, with electric heat strips as backup. Its air feels milder than a furnace’s, but truly cold air in heat mode calls for heating repair.

Ignitor Heating

The part that lights the burners in a modern gas furnace, usually a hot surface ignitor that glows orange. These crack with age, and a cracked one leaves the furnace going through its start-up routine without ever lighting.

Manual J load calculation Ratings

The industry method for working out how much heating and cooling a house needs, room by room, from its size, insulation, windows and air leaks. It, not a rule of thumb per square foot, should size a new system.

Refrigerant Cooling

The fluid that carries heat out of the house in a sealed loop, so a system that’s low has a leak somewhere. Older units use R-22, which is no longer made; newer ones use R-410A, R-454B or R-32.

SEER and SEER2 Ratings

Seasonal energy efficiency ratio: how much cooling a system delivers per unit of electricity over a season, and higher is better. SEER2, used since 2023, is tested at more realistic airflow, so the same unit scores a little lower on it.

Short cycling Controls

A system that switches on and off every few minutes instead of running full cycles. The usual suspects are an oversized unit, a clogged filter, low refrigerant or a thermostat in a bad spot, and in humid weather it leaves the air damp.

Static pressure Airflow

The resistance the blower pushes against from the ducts, filter, coil and grilles, measured a bit like blood pressure. Too high means a strained blower and weak airflow, and the usual ductwork fix is a bigger return or resized runs.

Thermostat Controls

The wall control that tells the system when to heat, cool or run the fan, based on the temperature where it hangs. In direct sun, over a lamp or near a supply vent, it reads the wrong temperature and the whole house follows.

Tonnage Ratings

Cooling capacity, not weight: one ton removes 12,000 BTU of heat an hour, and most homes use between 1.5 and 5 tons. Bigger isn’t better, since an oversized system short-cycles and leaves the house clammy.