GLOSSARY

HVAC terms in plain English

Common acronyms and terms that show up on estimates, manufacturer spec sheets, and rebate forms. Written for homeowners, not for industry insiders. If something on your quote isn't here, ask us.

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AFUE Annual Fuel Utilization Efficiency
Efficiency rating for gas furnaces, expressed as a percentage. 80 percent AFUE means 80 percent of the fuel becomes heat in your home; 20 percent goes up the flue. High-efficiency condensing furnaces hit 90 to 98 percent AFUE. Bay Area gas furnaces installed since 2015 are typically 92 to 96 percent AFUE.
Air Handler AHU — Air Handling Unit
The indoor portion of a split HVAC system. Contains the blower, evaporator coil, and (in heat pump systems) electric heat strips. Sits in a closet, attic, garage, or basement. Connected to the outdoor condenser via refrigerant line set and to the home via ductwork.
BTU British Thermal Unit per hour (BTU/h)
Unit of heating or cooling capacity. 12,000 BTU equals 1 ton of cooling. A typical 4-ton AC produces 48,000 BTU of cooling per hour. Furnace heating capacity is also measured in BTU; a typical Bay Area furnace runs 60,000 to 100,000 BTU heating input.
Capacitor Run capacitor / start capacitor
Electrical component that stores and releases energy to start and run AC motors. Run capacitors degrade with heat and age. Failed capacitor is the single most common AC repair we see; replacement runs $150 to $300.
Coil Evaporator coil / Condenser coil
Heat-exchange surfaces in an HVAC system. Evaporator coil is indoors (cools air); condenser coil is outdoors (releases heat). Both made of copper tubes with aluminum fins. Coils need periodic cleaning; buildup degrades efficiency.
Condensate Condensate / Condensate drain
Water that drips off the evaporator coil during cooling (dehumidification byproduct). Drains through a PVC line to outside or to a floor drain. Clogged condensate lines cause water leaks and trigger float switch shutdowns. Common service call.
Condenser Outdoor condenser unit
The outdoor portion of a split HVAC system. Contains the compressor, condenser coil, and fan. Compresses refrigerant and releases heat to outside air (cooling mode) or absorbs heat from outside air (heat pump heating mode).
Contactor Compressor contactor
An electrically-controlled switch that connects power to the outdoor compressor when the thermostat calls for cooling. Pitting wear from arcing is common past 8 years; pitted contactors arc on startup and accelerate compressor wear. Replacement $150 to $300.
EER / SEER / SEER2 Energy Efficiency Ratio / Seasonal Energy Efficiency Ratio
Cooling efficiency ratings. SEER averages seasonal performance; SEER2 (2023+) uses revised test conditions and runs roughly 6 to 8 percent lower for the same equipment. A 14 SEER unit measured under the older standard is roughly equivalent to a 13.4 SEER2 unit measured under the current standard. Don't compare SEER directly to SEER2.
ENERGY STAR ENERGY STAR certified
Government efficiency program. ENERGY STAR-qualified HVAC equipment meets minimum efficiency thresholds and qualifies for many rebate programs. Look for the blue logo on equipment nameplates and product literature.
Heat Exchanger Heat exchanger (gas furnace)
Steel chamber inside a gas furnace that separates combustion gases from breathable air. Air passes over the hot exterior of the exchanger; combustion byproducts vent up the flue. A cracked heat exchanger leaks carbon monoxide into the home and is a safety shutdown.
Heat Pump Air-source heat pump
HVAC system that moves heat rather than burning fuel. Cools in summer (heat moves from indoors to outdoors) and heats in winter (heat moves from outdoor air to indoors). Modern heat pumps deliver 2 to 4 units of heat per unit of electricity, far more efficient than gas furnaces or electric resistance.
HSPF / HSPF2 Heating Seasonal Performance Factor
Heating efficiency rating for heat pumps. HSPF2 is the current standard (2023+). Higher HSPF2 means more heat per watt of electricity over a heating season. Typical modern heat pumps run 7.5 to 9.0 HSPF2.
IRA Tax Credit (Section 25C) Energy Efficient Home Improvement Credit
Federal tax credit for energy-efficient home improvements including heat pumps. Section 25C expired December 31, 2025 under the One Big Beautiful Bill Act and is no longer available for air-source heat pump installs. Section 25D for geothermal remains.
Manual J ACCA Manual J load calculation
Industry-standard residential HVAC load calculation method. Accounts for insulation, window area, orientation, ceiling height, infiltration, internal gains, and climate to determine actual heating and cooling load. Required for proper system sizing. Skipping Manual J leads to oversized systems that short-cycle.
MERV Minimum Efficiency Reporting Value
Air filter efficiency rating. Higher MERV catches smaller particles but creates more airflow resistance. MERV 8 is standard for most homes; MERV 11 to 13 for allergies or wildfire smoke; over MERV 13 only if your blower can handle the pressure drop.
Mini-Split Ductless mini-split
A heat pump system that doesn't use ductwork. One outdoor condenser connects to one or more indoor air handlers (heads) via small refrigerant line sets. Each indoor head serves a zone with its own thermostat. Best for older homes without existing ducts, additions, and zoned comfort.
R-22 / R-410A / R-454B Refrigerant generations
R-22 (phased out 2020): older refrigerant, no longer manufactured, reclaimed-only at high cost. R-410A: 2010s-2020s standard, still common in existing systems. R-454B: current low-GWP standard for new installs as of 2024-2025. Different refrigerants need different equipment; no field conversions between them.
Refrigerant Line Set Suction line / liquid line
Two copper tubes (one larger, one smaller) running between the outdoor condenser and indoor air handler. Carry refrigerant in liquid and vapor states. On heat pump retrofits, line sets are sometimes flushed and reused; sometimes replaced. We pressure-test on every retrofit to decide.
SEER2 Seasonal Energy Efficiency Ratio (2023+ standard)
See EER / SEER / SEER2. SEER2 uses revised M1 test conditions starting 2023; values run 6 to 8 percent lower than equivalent SEER for the same equipment.
Static Pressure External static pressure (ESP)
Air pressure measurement inside HVAC ductwork, in inches of water column. Most residential systems are designed for 0.5 inch ESP; readings above 0.8 indicate restriction (dirty filter, crushed duct, undersized return). We measure static pressure at every tune-up to catch developing problems.
Sub-panel Electrical sub-panel
Secondary electrical panel that branches off the main panel to add circuit capacity. Often needed for heat pump conversions in older Bay Area homes where the main panel maxes out at 100A or 125A. Sub-panel install runs $1,800 to $4,500 depending on routing.
Ton Ton of cooling capacity
Unit of HVAC capacity equal to 12,000 BTU per hour. A 3-ton AC produces 36,000 BTU/h of cooling. Typical Bay Area home sizing is roughly one ton per 600 to 800 square feet, but actual sizing comes from a Manual J load calculation.
Variable-Speed Variable-speed compressor / blower
HVAC equipment that runs at multiple speeds rather than just on/off. Provides better comfort (less temperature swing), better humidity control, quieter operation, and higher efficiency. Most modern premium equipment (Daikin Aurora, Carrier Infinity, Mitsubishi Hyper-Heat) is variable-speed.
Zone / Multi-Zone Zoned HVAC system
An HVAC system that controls multiple temperature zones independently — usually via dampers in ducted systems or multiple indoor heads on mini-splits. Allows different rooms or floors to hold different temperatures. Common in larger Bay Area homes (Alamo, Blackhawk, Orinda).

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