HVAC glossary
HVAC terms, in plain English.
Short definitions of the words you'll see on estimates, equipment labels, and rebate forms.
- AFUE
- Annual Fuel Utilization Efficiency. The percentage of fuel a furnace turns into usable heat over a year.
- AHRI certificate
- A record from the Air-Conditioning, Heating, and Refrigeration Institute that confirms the rated efficiency of a specific matched indoor and outdoor equipment combination. Learn more
- BTU
- British Thermal Unit. A measure of heat. AC capacity is often listed in BTUs per hour; 12,000 BTU/h equals one ton.
- Capacitor
- An electrical part that helps start and run the compressor and fan motors. A common failure point in hot weather. Learn more
- Condensate drain
- The line that carries water removed from the air away from the indoor unit. Clogs can cause leaks or shut the system off.
- Condenser
- The outdoor unit of a central air conditioner. It releases heat removed from the house.
- Dual-fuel system
- A heat pump paired with a gas furnace, switching between them based on conditions. Learn more
- Ductless mini-split
- A heat pump that sends refrigerant directly to indoor wall, ceiling, or floor units, with no ductwork. Learn more
- EER2
- Energy Efficiency Ratio (2023 test). Cooling efficiency at a fixed hot outdoor temperature. Learn more
- Evaporator coil
- The indoor coil that absorbs heat and moisture from the air. It can freeze if airflow or refrigerant is low.
- HERS rater
- An independent specialist certified to verify HVAC installations, such as duct leakage, for California's Title 24 energy code. Learn more
- Heat pump
- A system that moves heat into the home in winter and out of it in summer, providing heating and cooling with electricity. Learn more
- HSPF2
- Heating Seasonal Performance Factor (2023 test). Heat pump heating efficiency over a season. Learn more
- Inverter / variable-speed
- A compressor or blower that adjusts its speed to match demand, rather than switching fully on and off.
- Manual J
- The ACCA method for calculating a home's heating and cooling load room by room. Learn more
- Manual D
- The ACCA method for designing duct systems so each room gets the right airflow.
- MERV
- Minimum Efficiency Reporting Value. Rates how well an air filter captures particles; higher captures smaller particles. Learn more
- R-410A / R-454B / R-32
- Refrigerants used in air conditioners and heat pumps. Newer systems are moving to lower-global-warming refrigerants such as R-454B and R-32.
- SEER2
- Seasonal Energy Efficiency Ratio (2023 test). Cooling output over a season divided by the electricity used. Learn more
- Short-cycling
- When a system turns on and off too often, usually from oversizing, low refrigerant, or a thermostat problem.
- Static pressure
- Resistance to airflow in the duct system. High static pressure can reduce comfort and strain equipment.
- Title 24
- California's Building Energy Efficiency Standards, which set rules for HVAC replacements including permits and testing. Learn more
- Ton
- A measure of cooling capacity equal to 12,000 BTU per hour.
- Zoning
- Dividing a home into areas with separate thermostats and dampers so each area can be controlled on its own. Learn more
