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- A2L
- A safety class for the newest refrigerants, such as R-454B and R-32: low toxicity and only mildly flammable. Equipment built for them includes added safety features.
- Aerosol duct sealing
- A method of sealing leaks from inside the ductwork by blowing a fine sealant through the system, reaching gaps that cannot be reached by hand.
- AFUE
- Annual Fuel Utilization Efficiency. The share of a furnace's fuel that becomes heat in your home. At 80% AFUE, 80% becomes heat and about 20% leaves through the vent.
- Air handler
- The indoor unit of a heat pump or some air conditioning systems, containing the blower, the coil and often backup electric heat. Also called a fan coil.
- BTU
- British Thermal Unit, the standard measure of heat. One BTU is roughly the heat of a single kitchen match. Heating and cooling capacity is measured in BTUs per hour.
- Capacity
- How much heating or cooling a system can deliver, usually in BTUs per hour or tons.
- CFM
- Cubic feet per minute, a measure of airflow. A typical home system moves around 350 to 400 CFM for each ton of cooling.
- Compressor
- The heart of an air conditioner or heat pump, in the outdoor unit. It pumps refrigerant around the system.
- Condensate
- The water drawn out of the air as it is cooled. It collects in a drain pan and flows away through the condensate line.
- Condenser coil
- The coil in the outdoor unit that releases heat from the refrigerant. Also called the outdoor coil.
- Damper
- A movable plate inside the ductwork that opens and closes to control airflow. Zoning systems use motorized dampers.
- Downflow furnace
- A furnace that draws air in at the top and sends warm air out at the bottom, often used where ducts run under the floor.
- Dual fuel
- A system that pairs a heat pump with a gas furnace, using whichever is more economical for the conditions.
- Ductless mini-split
- A system with an outdoor unit connected to one or more wall- or ceiling-mounted indoor units, with no ductwork.
- Ductwork
- The channels that carry air from your system to each room and back.
- ECM
- Electronically commutated motor. A high-efficiency, variable-speed motor used in modern blowers. It uses far less electricity than older motors and runs more quietly.
- EER2
- Energy Efficiency Ratio, 2023 test method. Cooling efficiency at peak heat. California sets a minimum for new air conditioners.
- ERV and HRV
- Energy recovery and heat recovery ventilators. They bring in fresh outdoor air while recovering energy from the stale air they send out.
- Evaporator coil
- The indoor coil where refrigerant absorbs heat from your home's air, cooling it and removing moisture.
- GWP
- Global warming potential. How strongly a gas traps heat compared with carbon dioxide. Newer refrigerants have a much lower GWP than older ones.
- Heat exchanger
- In a furnace, the sealed metal chamber that transfers heat from the burner to your home's air while keeping combustion gases separate.
- Heat pump
- A system that both cools and heats by moving heat into or out of your home with electricity.
- HERS rater
- An independent specialist who tests and verifies certain HVAC installations, such as duct leakage, under California's energy code.
- Horizontal furnace
- A furnace that lies on its side, drawing air in at one end and sending it out the other; common in attics.
- HSPF2
- Heating Seasonal Performance Factor, 2023 test method. The heating efficiency of a heat pump over a season. Higher is better.
- Humidifier
- A device that adds moisture to dry indoor air.
- Humidity
- The amount of moisture in the air. Too little feels dry; too much feels sticky and encourages mold.
- HVAC
- Heating, ventilation and air conditioning.
- Hydronic heating
- Heating that uses hot water from a boiler, circulated to radiators, baseboards or tubing in the floor.
- Load calculation
- A room-by-room assessment of how much heating and cooling a home needs. The right way to size a system.
- MERV
- Minimum Efficiency Reporting Value. A filter's rating for catching particles, from 1 to 16. Higher numbers catch finer particles, but your system must be able to handle the added resistance.
- Packaged system
- A system with all its components in one cabinet, installed on the roof or beside the building.
- R-22, R-410A, R-454B, R-32
- Successive generations of home refrigerant. See our guide: Refrigerant changes explained.
- Refrigerant
- The fluid that carries heat through an air conditioner or heat pump.
- Refrigerant lines
- The two copper pipes connecting the outdoor and indoor units. Also called the line set.
- Reversing valve
- The valve in a heat pump that changes the direction of refrigerant flow to switch between cooling and heating.
- SEER2
- Seasonal Energy Efficiency Ratio, 2023 test method. Cooling efficiency over a whole season. Higher is better.
- Split system
- A system with components in two places: an outdoor unit and an indoor unit such as a furnace or air handler.
- Thermostat
- The control that tells your system when to heat or cool. Programmable and smart thermostats follow a schedule you set.
- Ton
- A measure of cooling capacity equal to 12,000 BTUs per hour. Most homes need between about 2 and 5 tons.
- Two-stage
- Equipment that runs on a lower setting most of the time and a higher one when needed, for steadier comfort.
- Upflow furnace
- A furnace that draws air in at the bottom and sends warm air out the top, common in closets and basements.
- Variable-speed
- Equipment that adjusts its output in small steps to match what your home needs, for the most even, quiet comfort.
- Zoning
- Dividing a home into areas with their own thermostats, so each can be heated or cooled separately.