Glossary

HVAC and Indoor Air Glossary

The words technicians and spec sheets use, explained.

An outdoor condenser unit for a central air conditioner beside a brick house

The outdoor condenser: H Padleckas, CC BY-SA 3.0

TermMeaning
AFUEAnnual Fuel Utilization Efficiency: how much of a furnace's fuel becomes heat.
BTUBritish thermal unit, a measure of heat.
Ton (cooling)12,000 BTU per hour of cooling capacity.
SEER2 / EER2Cooling efficiency ratings under the 2023 US test method.
HSPF2Heat pump heating efficiency rating.
CFMCubic feet per minute of airflow.
CADRClean air delivery rate of a portable air cleaner.
MERVFilter efficiency rating from 1 to 16.
HEPAHigh-efficiency particulate air filter: at least 99.97% of 0.3 micron particles.
PM2.5Fine particles 2.5 microns or smaller.
VOCsVolatile organic compounds released by paints, cleaners and furnishings.
Return airAir pulled back from rooms to the HVAC system.
Supply airConditioned air delivered to rooms.
PlenumA box or chamber connecting equipment to ducts.
Air handlerThe indoor unit with the blower and coil.
CondenserThe outdoor unit that releases heat in cooling mode.
Evaporator coilThe indoor coil that absorbs heat and removes moisture.
RefrigerantThe fluid that carries heat in AC and heat pumps.
CondensateWater removed from the air by the cooling coil.
Static pressureResistance to airflow in the ducts.
Manual JIndustry method for calculating a home's heating and cooling load.
ERV / HRVVentilators that exchange stale and fresh air while recovering heat or moisture.
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Facts on this page were checked against these sources (updated Oct 12, 2026). Spot an error? Email hello@fireempireshop.com.

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