Glossary · GetHeatPumped

A2L refrigerant: A safety classification meaning "mildly flammable, low toxicity". Includes R-32 and R-454B, the standard replacements for R-410A in 2025 onwards.
ASHP: Air-source heat pump. The most common kind. Moves heat between indoor air and outdoor air.
AUX heat: Auxiliary heat. Electric resistance "strips" inside a ducted air handler. Run at COP 1.0, which is about three times more expensive per BTU than the heat pump itself. The thermostat decides when to fire them.
Balance point: The outdoor temperature at which the heat pump alone can no longer meet the home's heating load. Below this point, aux heat or a backup fuel covers the gap.
BTU (British Thermal Unit): The amount of heat needed to raise one pound of water by 1°F. The base unit of HVAC capacity. 1 BTU/h is a rate; 12,000 BTU/h equals one ton of cooling.
ccASHP: Cold-climate air-source heat pump. NEEP certifies units that meet a 1.75 COP at 5°F at maximum capacity. The certified list lives at ashp.neep.org.
Commissioning: The post-install verification that the system was set up correctly: vacuum at 500 microns held 15+ minutes, refrigerant charge within ±10%, superheat and subcooling measured, electrical connections checked, documented in a report. About 70% of US heat pumps are not commissioned to ACCA standards.
Compressor: The pump in the refrigeration cycle. Squeezes low-pressure cold vapour into high-pressure hot vapour. The most expensive single component to replace, and the one most likely to fail when commissioning is skipped.
Condenser: The corner of the cycle where high-pressure hot vapour gives up its heat (to outdoor air in cooling mode, to indoor air in heating mode) and becomes high-pressure liquid.
COP (Coefficient of Performance): Heat moved divided by electrical energy consumed. COP of 3 means the unit delivers 3 units of heat for every 1 unit of electricity. Heat pumps run at COP 2.5 to 4 most of the year; resistance strips run at COP 1.0.
Defrost cycle: Automatic 5-15 minute reversal that runs every 30-90 minutes in cold humid weather. Briefly switches to cooling mode so hot refrigerant melts frost off the outdoor coil. You'll see steam outside and may feel cool indoor air during it. Demand-defrost (modern) only runs when frost is actually detected.
Delta T: The temperature difference between return and supply air. Heat pumps in heating mode run a 15-25°F delta T. Furnaces run 30-40°F. Lower delta T but longer runtime gives the same or better total heat delivery.
Ducted: A central system that moves conditioned air through ductwork to multiple rooms. Single air handler, single thermostat unless you add zoning.
Ductless mini-split: Indoor head units mounted in each room, connected to one outdoor unit via refrigerant lines. No ducts. Controlled via infrared, not 24V wiring.
Evaporator: The corner of the cycle where low-pressure cold mix absorbs heat (from indoor air in cooling mode, from outdoor air in heating mode) and boils back to low-pressure cold vapour.
GWP (Global Warming Potential): How much heat a kilogram of a gas traps in the atmosphere relative to CO2, over 100 years. R-410A is 2,088. R-32 is 675. R-454B is 466. The EPA AIM Act caps new equipment refrigerants at GWP 700 from 2025.
HEAR / HEEHRA: Home Electrification and Appliance Rebates. Federal program funded by the Inflation Reduction Act, administered by states. Up to $8,000 toward a heat pump for households at or below 150% of area median income.
HSPF2: Heating Seasonal Performance Factor 2. The 2023 update to HSPF; measures average heating efficiency over a season. Higher is better. The minimum federal HSPF2 is 7.5 in northern climates.
Inverter: The variable-speed drive that lets a modern heat pump run the compressor at 30%, 60%, 110% of capacity instead of just on/off. The reason modulating heat pumps are more efficient and last longer than single-stage units.
Manual J: The ACCA-published procedure for calculating heating and cooling loads room by room. Includes infiltration, orientation, glazing, internal gains, and duct losses. The right way to size a system; "match the existing furnace" is the wrong way.
MeasureQuick: A Bluetooth-probe-based commissioning platform. Generates a timestamped digital report that documents vacuum, charge, superheat/subcooling, and static pressure. If your installer uses one, you have evidence. If they don't, you don't.
Metering device: The expansion valve at the top of the refrigeration cycle. Drops high-pressure liquid refrigerant suddenly into a low-pressure cold mix (some liquid, some vapour). This is where the Joule-Thomson effect happens.
PTAC / PTHP: Packaged Terminal Air Conditioner / Heat Pump. The single-box under-the-window units in hotels and apartments. All refrigerant inside one chassis.
R-32 / R-454B: The A2L refrigerants replacing R-410A in new residential and light-commercial equipment after Jan 1, 2025 under the EPA's AIM Act. Lower GWP, mildly flammable but with lower flammability than propane.
Reversing valve: A four-way valve on the compressor's discharge line. Swaps where the high-pressure hot vapour goes (to outdoor coil for cooling, to indoor coil for heating). The reason a heat pump can both heat and cool with the same hardware.
RTU (Rooftop Unit): A packaged heat pump or AC mounted on a commercial roof. All refrigerant components inside one weatherproof box; conditioned air ducts down into the building.
SEER2: Seasonal Energy Efficiency Ratio 2. The 2023 update to SEER; measures average cooling efficiency over a season. Higher is better. Federal minimum is SEER2 14.3-15.2 depending on region.
Short cycling: Rapid on/off operation of an oversized or misconfigured system. Damages compressors and tanks efficiency. Inverter-driven heat pumps should not short-cycle if sized and commissioned correctly.
Split system: A heat pump with separate indoor and outdoor units, connected by refrigerant line set. Standard mini-splits and central air-source heat pumps both qualify.
Ton (of refrigeration): A unit of HVAC capacity. 1 ton = 12,000 BTU/h. Originally the rate of heat extraction equivalent to melting one short ton of ice over 24 hours. Used because it sounds cool.
VRF (Variable Refrigerant Flow): A large split system that can serve many indoor units from one outdoor compressor, each with independent temperature control. Common in commercial buildings; rarer in residential.

Heat Pump Glossary.