HVAC Glossary: AC Terms in Plain English

The terms below are the ones that come up on this site and on service calls. Each definition is the plain version. Where a term matters to a specific problem or decision, it links to the page that goes deeper.

Efficiency ratings

SEER2 stands for Seasonal Energy Efficiency Ratio, measured under the federal test procedure that took effect for equipment made from January 1, 2023. It's the cooling delivered over a typical season divided by the energy used, and higher is more efficient. In Florida, the minimum for a new split-system air conditioner under 45,000 Btu/h is 14.3 SEER2. See SEER2 explained.

SEER is the older version of the same rating. The 2023 test assumes more resistance in the ductwork, so SEER2 numbers run a little lower than SEER numbers for the same equipment. Florida's 14.3 SEER2 minimum corresponds roughly to the old 15 SEER.

EER2 is the Energy Efficiency Ratio at a single hot condition, 95 °F outdoors. Where SEER2 is a season average, EER2 describes efficiency at full load on a hot afternoon.

HSPF2, the Heating Seasonal Performance Factor, is the heating-mode efficiency rating for heat pumps under the 2023 test.

An AHRI certificate is the record that a specific indoor and outdoor combination was tested and rated together. Florida requires replacement systems to be a matched, certified combination.

Sizing and design

Btu/h means British thermal units per hour, the rate at which a system removes heat. Equipment capacity and a home's cooling load are both stated in Btu/h.

A ton is a unit of cooling capacity equal to 12,000 Btu/h. A "3-ton" system removes heat at 36,000 Btu/h, and the number says nothing about the unit's weight.

A load calculation (Manual J) is ACCA's standard method for working out how much cooling a home needs, room by room, from its windows, insulation, air leakage, ducts and occupants. See AC sizing and Manual J.

Sensible and latent load are the two parts of the cooling load. Sensible load is the heat you feel as temperature, and latent load is moisture. In South Florida the latent part is large, which is why humidity control is part of sizing.

Manual S is ACCA's standard for choosing equipment whose real capacity matches the calculated load.

Manual D is ACCA's standard for sizing duct systems so they can carry the air the equipment needs. See airflow and duct design.

Manual T is ACCA's standard for choosing and placing supply registers and return grilles.

Oversizing means installing more capacity than the load calculation calls for. In humid climates it leads to short cycles and damp air. See AC short cycling.

Refrigerant and the cooling cycle

Refrigerant is the fluid that carries heat from inside the house to the outdoor unit. It circulates in a sealed system, so a system that needs refrigerant added has a leak.

R-22 is the older refrigerant, which hasn't been produced or imported in the U.S. since 2020. Existing R-22 systems can still be used and serviced with recovered or previously made refrigerant.

R-410A is the refrigerant in most systems installed from roughly the 2010s until 2025. Existing R-410A systems can be repaired and serviced.

R-454B and R-32 are the lower-GWP refrigerants used in new residential systems. Equipment has to be designed for them, and they aren't used to convert older systems. See the R-454B guide.

GWP, or global warming potential, is how strongly a refrigerant warms the climate compared with carbon dioxide. New residential and light-commercial AC systems must use refrigerants with a GWP below 700.

A2L is a safety classification for refrigerants with low toxicity and lower flammability, including R-454B and R-32. A2L equipment carries red-marked service ports and labels, and it's serviced with tools and procedures meant for it.

EPA Section 608 certification is the federal certification technicians need to service equipment containing regulated refrigerants.

The compressor is the pump in the outdoor unit that moves refrigerant through the system.

The condenser coil is the outdoor coil where refrigerant releases the heat it carried out of the house.

The evaporator coil is the indoor coil where refrigerant absorbs heat from the house's air. It's also where moisture condenses. See frozen coil.

The metering device controls how much refrigerant enters the evaporator. A thermostatic expansion valve (TXV) adjusts that flow automatically, while simpler systems use a fixed orifice.

Superheat is how far the refrigerant vapor leaving the evaporator is above its boiling point. Technicians use it to check the charge on systems with a fixed metering device.

Subcooling is how far the liquid refrigerant leaving the condenser is below its condensing temperature. Technicians use it to check the charge on systems with a TXV.

The line set is the pair of copper refrigerant lines between the indoor and outdoor units: the larger, insulated suction line and the smaller liquid line.

The reversing valve lets a heat pump run its cycle in reverse. It's the practical difference between a heat pump and a cooling-only AC. See heat pumps.

Equipment

A split system is the most common setup, with an outdoor condenser and an indoor air handler or coil connected by a line set. See central air conditioners.

The air handler is the indoor unit that holds the blower, evaporator coil, filter and drain pan. It usually sits in a closet, garage or attic. See air handlers.

The condenser (outdoor unit) is the outdoor half of a split system, containing the compressor, condenser coil and fan.

A package unit puts every component in one outdoor or rooftop cabinet. See package units.

A ductless mini-split connects an outdoor unit to one or more wall-mounted indoor units with no ductwork, and each indoor unit works independently. See ductless mini-splits.

Variable-speed and multi-stage equipment can run at lower output instead of only on or off. Running longer at a lower speed can help with comfort and moisture removal when the system is set up for it.

Air and ducts

Supply ducts carry cooled air to the rooms, and return ducts bring room air back to the air handler. Too little return is a common cause of weak airflow. See weak airflow.

The plenum is the box on the air handler that the ducts connect to. The supply plenum is on the outlet side and the return plenum on the inlet side. See plenums.

The trunk line is the main duct that feeds the smaller branch runs.

Flex duct is the flexible round duct used for most branch runs in Broward attics. Crushed, kinked or disconnected flex is a common reason one room won't cool. See room not cooling.

Registers and grilles are the covers on supply outlets (registers) and return openings (grilles).

CFM, cubic feet per minute, is how much air the system moves.

Static pressure is the resistance air meets as it moves through the system, measured in inches of water column with a manometer. Each air handler has a maximum rated static pressure on its rating plate, and readings above it mean the ducts, filter or coil are choking the airflow.

Temperature split is the difference between the air going into the system and the air coming out. A technician compares the measured split with a target based on humidity and airflow, because there isn't one "normal" number for every house. See how we diagnose AC problems.

Water and drains

Condensate is the water the evaporator coil pulls out of the air. In South Florida a system can remove a lot of it every day.

The primary drain pan sits under the coil, collects condensate and sends it into the drain line.

The auxiliary (secondary) drain pan is a second pan under the air handler that catches overflow. Florida's code requires overflow protection where an overflow could damage the building.

A float switch (water-level switch) shuts the system off when water backs up. When it trips, the AC stops cooling, but the switch is only doing its job. The clog behind it is the real problem. See AC not draining.

The trap is a dip in the drain line near the air handler that keeps air from being pulled back up the drain. Slime and sludge often collect there first. See AC drain line clogs.

A condensate pump is a small pump that lifts condensate when the drain can't run downhill to an outlet.

Electrical parts

Capacitors give the compressor and fan motors the boost they need to start and run. They weaken in the heat and are a common reason an outdoor unit won't start. See AC not turning on.

The contactor is the switch in the outdoor unit that turns the compressor and fan on when the thermostat calls for cooling. Pitted or stuck contacts cause no-start and won't-stop problems.

The disconnect is the switch box near the outdoor unit that cuts its power for service.

The breaker is the overcurrent protection in your electrical panel. A breaker that trips again after a reset is a reason to stop and call. See AC tripping the breaker.

Air quality

Relative humidity (RH) is how much moisture the air holds compared with the most it could hold at that temperature. The EPA recommends keeping indoor RH below 60 percent, ideally 30 to 50 percent. See high humidity in the house.

MERV, the Minimum Efficiency Reporting Value, rates filters from 1 to 16 on how well they capture particles. A higher rating catches more, as long as the system can still move enough air through the filter. See air filtration.

A UV-C lamp is an ultraviolet lamp in the air handler, used mainly to limit biological growth on the coil and drain pan. See UV lights.

Activated carbon is a filter medium that adsorbs some odors and gases. It has limited capacity and needs replacing when saturated.

A whole-home dehumidifier removes moisture independently of cooling, for homes where the AC alone can't keep humidity down. See humidity control.

Codes and permits

HVHZ is the Florida Building Code's High-Velocity Hurricane Zone, which consists of Broward and Miami-Dade counties. Outdoor equipment here has to be installed to resist the code's wind pressures.

A tie-down is the approved anchoring that fastens an outdoor unit or rooftop unit to its pad, stand or curb. It's checked at the final inspection. See AC permits in Broward.

A mechanical permit is the city permit for installing or replacing heating, cooling and ventilation equipment. An AC changeout in Broward needs one.

A matched system is an indoor and outdoor combination that has been tested and rated together. Florida requires replacement systems to be matched.