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Get a Free Quote43+ heat pump terms explained in plain language. From COP and SEER2 to cold-climate ratings and refrigerant types — the definitive reference for homeowners evaluating heat pump systems.
A performance certification from the Air-Conditioning, Heating, and Refrigeration Institute. AHRI tests and verifies manufacturer efficiency claims under standardized conditions.
Always look for the AHRI certificate number when comparing equipment. It confirms that the indoor and outdoor units are tested as a matched pair, not rated individually.
The indoor unit of a ducted heat pump system. It contains the evaporator coil, blower fan, and often a supplemental electric heating element.
Air handlers are typically installed in basements, attics, or utility closets. Variable-speed air handlers run quieter and deliver more consistent temperatures than single-speed models.
A heat pump that transfers heat between outdoor air and your home. The most common type of residential heat pump, using a refrigerant cycle to extract heat from outdoor air — even in cold weather.
Modern cold-climate ASHPs can heat effectively down to -15F. They cost $4,000-$8,000 per zone installed — significantly less than ground-source systems.
A supplemental heating source that activates when the heat pump alone cannot meet the thermostat setpoint. Usually electric resistance strips built into the air handler.
Aux heat costs 2-3x more to run than the heat pump itself. If your thermostat shows "Aux" frequently above 30F, your system may be undersized or need service. Aux heat is different from Emergency Heat.
Any secondary heating source that supplements or replaces the heat pump during extreme cold. Options include electric resistance strips, a gas/oil furnace (dual fuel), or a wood stove.
In New England, many homeowners keep an existing boiler or furnace as backup rather than relying solely on electric resistance strips. This "dual fuel" approach can save money during the coldest days.
The outdoor temperature at which a heat pump's heating capacity exactly equals the building's heat loss. Below the balance point, supplemental heat is needed.
A well-insulated home might have a balance point of 15F, while a drafty home could hit 35F. Lowering your balance point through insulation and air sealing reduces reliance on expensive backup heat.
The standard unit of heat energy. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Heat pump capacity is measured in BTUs per hour (BTU/h).
A typical room needs 6,000-12,000 BTU/h of heating capacity. A whole-home system might need 36,000-60,000 BTU/h. One ton of cooling equals 12,000 BTU/h.
Efficiency levels defined by the Consortium for Energy Efficiency. CEE tiers rank equipment from basic (Tier 1) to most efficient (Tier 3/Advanced). Many utility rebates require a specific CEE tier.
For ducted heat pumps: CEE Tier 1 requires SEER2 15.2+ and HSPF2 8.1+. Higher tiers unlock larger rebates. Always check your utility's rebate requirements against the CEE tier list.
A heat pump engineered to maintain high heating capacity at very low outdoor temperatures. Defined by NEEP as maintaining at least 70% of rated capacity at 5F with a COP of at least 1.75.
Cold-climate models use enhanced vapor injection (EVI), larger compressors, and optimized defrost cycles. Brands like Mitsubishi Hyper-Heat, Fujitsu XLTH, and Bosch IDS 2.0 are NEEP-listed ccHP units.
A drain line that removes water produced when warm moist air passes over the cold evaporator coil during cooling mode. Indoor units produce condensate in summer; outdoor units produce it in winter during defrost.
A clogged condensate drain is one of the most common heat pump service calls. Mini-split indoor units have built-in drain pans that must slope toward the drain line.
The ratio of heat energy delivered to electrical energy consumed at a specific outdoor temperature. A COP of 3.0 means the heat pump delivers 3 units of heat for every 1 unit of electricity used.
COP varies with outdoor temperature: a heat pump might have a COP of 4.0 at 47F but drop to 2.0 at 5F. Even at COP 2.0, it is twice as efficient as electric resistance heating (COP 1.0). COP is an instantaneous measurement, unlike seasonal ratings like HSPF.
An automatic process where the heat pump temporarily reverses operation to melt ice buildup on the outdoor coil. During defrost, the system briefly runs in cooling mode to warm the outdoor unit.
Defrost cycles last 1-10 minutes and may occur every 30-90 minutes in cold, humid weather. You may notice steam rising from the outdoor unit — this is normal. Backup heat activates during defrost to maintain indoor comfort.
A heat exchanger that captures waste heat from the refrigerant cycle to preheat domestic hot water. Most common on ground-source heat pumps.
A desuperheater can provide 30-50% of a home's hot water needs at virtually no extra cost during the cooling season. Some air-source systems also offer hot water heat recovery.
The two primary ways to deliver conditioned air. Ducted systems use a central air handler and ductwork. Ductless systems (mini-splits) mount individual units on walls or ceilings in each room.
Ductless mini-splits avoid the 15-25% energy loss typical of ductwork. Ducted systems provide whole-home coverage from one unit. Many homes use a hybrid approach: ducted for the main living area and ductless heads for additions or bedrooms.
The cooling efficiency of a heat pump at a single test condition: 95F outdoor, 80F/67F indoor. Measured in BTU/h per watt. Higher EER means more efficient cooling at peak temperatures.
EER is most relevant in hot climates where the system runs at peak load for extended periods. It is being replaced by EER2 under the new DOE M1 testing standard. Typical residential values range from 10-15 EER.
A thermostat setting that locks out the heat pump compressor and runs only the backup heating source. Use only when the heat pump has failed — it bypasses the efficient compressor entirely.
Emergency heat running on electric resistance strips costs 2-3x more than normal heat pump operation. If you must switch to Em Heat, call for service immediately. It is NOT the same as Auxiliary Heat, which supplements the compressor.
A certification program by the EPA and DOE. Energy Star certified heat pumps meet minimum efficiency thresholds above the federal baseline. Many rebate programs require Energy Star certification.
As of 2026, Energy Star for central heat pumps requires SEER2 15.2+ and HSPF2 8.1+ (split systems). Energy Star Most Efficient designation sets even higher bars and can unlock additional utility incentives.
A heat pump that exchanges heat with the ground rather than outdoor air. Uses buried loops of pipe filled with water or antifreeze solution to tap the earth's stable underground temperature (45-55F year-round).
Geothermal systems achieve COP 3.5-5.0 year-round because ground temperatures are constant. Installation costs $15,000-$30,000+ due to excavation and loop installation. Payback is 7-12 years. Loops last 50+ years.
See Geothermal. A heat pump that uses the stable temperature of the earth (via buried loops) as its heat source in winter and heat sink in summer.
A heat pump and an air conditioner use the identical refrigerant cycle for cooling. The difference: a heat pump includes a reversing valve that allows it to reverse the cycle and heat your home in winter. An AC can only cool.
In cooling mode, a heat pump IS an air conditioner — same efficiency, same comfort. The added reversing valve and controls add $500-$1,500 to the cost compared to an AC-only system. In most climates, the heating savings pay for the difference within 1-2 years.
The total heating output in BTU over a heating season divided by total electricity consumed in watt-hours. Higher is better. HSPF uses the older DOE testing standard and is being phased out.
HSPF values typically range from 8-13 for residential heat pumps. The old federal minimum was HSPF 8.2. Multiply HSPF by 0.85 to approximate the newer HSPF2 rating.
The updated Heating Seasonal Performance Factor using the DOE M1 test procedure (since January 2023). HSPF2 values are lower than HSPF for the same equipment because the new test is more stringent.
HSPF2 is the current standard for comparing heating efficiency. Federal minimum is HSPF2 7.8 (split) / 6.7 (packaged). Energy Star requires HSPF2 8.1+. The best cold-climate units reach HSPF2 10-12.
A compressor that uses a variable-frequency drive (inverter) to adjust its speed continuously rather than cycling fully on and off. This is the key technology behind variable-speed heat pumps.
Inverter-driven compressors match output to actual demand, running at 30-100% capacity as needed. Benefits: 20-40% energy savings vs single-speed, quieter operation, more consistent temperatures, less wear from cycling.
The insulated copper tubing that connects the outdoor condenser unit to the indoor evaporator. Consists of two lines: a larger suction line (insulated) and a smaller liquid line.
Line sets are typically 15-50 feet long. Longer runs reduce efficiency. Pre-charged line sets come with refrigerant sealed inside. Custom installations require brazing, evacuation, and charging by a certified technician.
The engineering process of determining how much heating and cooling capacity a building requires. Proper load calculations prevent oversizing (short-cycling, poor dehumidification) and undersizing (insufficient comfort).
Every qualified heat pump installation begins with a load calculation. It accounts for square footage, insulation, window area, orientation, air infiltration, occupancy, and local climate data.
The outdoor temperature at which the compressor is shut off to prevent damage. Older heat pumps locked out around 25-30F. Modern cold-climate models operate continuously down to -13F to -22F.
If your system has a lockout set too high, you are running expensive backup heat unnecessarily. A qualified technician can adjust the lockout temperature if your equipment supports lower-temperature operation.
The ACCA (Air Conditioning Contractors of America) standard method for calculating residential heating and cooling loads. Manual J considers every factor affecting a home's energy needs.
A proper Manual J uses room-by-room analysis, not "rule of thumb" sizing. It accounts for insulation R-values, window U-factors, air leakage rates, duct losses, internal gains, and local 99% design temperatures. Insist that your installer performs a Manual J before recommending equipment.
The ACCA standard for selecting HVAC equipment based on Manual J results. Manual S matches equipment capacity and performance curves to the calculated loads to ensure proper sizing.
Manual S prevents the common mistake of selecting equipment that is oversized for the load. It considers both sensible and latent loads, and matches equipment at the local design temperature — not just nameplate capacity.
A ductless heat pump system consisting of an outdoor compressor/condenser connected to one or more indoor wall- or ceiling-mounted units (heads). Each indoor unit conditions a single zone independently.
Mini-splits are the most popular heat pump type for retrofits in existing homes because they require no ductwork — just a 3-inch hole in the wall for the line set. A single outdoor unit can support 1-5 indoor heads.
A heat pump system with one outdoor unit connected to multiple indoor units, each controlling the temperature of a separate zone independently. Also called a multi-split.
Multi-zone systems allow different temperatures in different rooms. A 3-zone system might serve the living room, master bedroom, and home office. The outdoor unit modulates to serve only active zones, saving energy when some zones are off.
An electric heating element installed in the base pan of the outdoor unit to prevent condensate from freezing in winter. Essential in cold climates where defrost water can accumulate and freeze.
Without a pan heater, ice can build up in the outdoor unit base, blocking airflow and potentially damaging the unit. Most cold-climate installations in New England include a factory or aftermarket pan heater.
A lower-GWP refrigerant (GWP 675) used in some newer heat pumps, especially from Daikin and other Japanese manufacturers. R-32 is more efficient than R-410A and uses about 30% less charge.
R-32 is mildly flammable (A2L classification) but is widely used in Asia and Europe. It is one of the leading replacements for R-410A in residential heat pumps.
The most common refrigerant in heat pumps installed between 2010-2024. R-410A is an HFC with a global warming potential (GWP) of 2,088. It is being phased down under the AIM Act and Kigali Amendment.
R-410A operates at higher pressures than the older R-22. It does not deplete the ozone layer but has significant climate impact. New equipment manufactured after January 1, 2025 must use lower-GWP alternatives.
The primary replacement for R-410A in US residential heat pumps. R-454B (sold as Opteon XL41) has a GWP of 466 — a 78% reduction from R-410A. It is an A2L (mildly flammable) refrigerant.
Major manufacturers (Carrier, Lennox, Trane, Rheem) are transitioning to R-454B for new equipment. The lower GWP helps comply with AIM Act phase-down targets. R-454B requires updated safety standards (UL 60335-2-40) and may require leak detection in some installations.
The precise amount of refrigerant in the system. Correct charge is critical for efficiency and equipment life. Too much or too little refrigerant degrades performance and can damage the compressor.
Charge is set during installation by weighing refrigerant (factory charge + line set adjustment) or by measuring subcooling and superheat. An improper charge can reduce efficiency by 5-20%.
The total cooling output in BTU over a cooling season divided by total electricity consumed in watt-hours. Higher is better. SEER uses the older DOE testing standard and has been replaced by SEER2.
SEER values typically ranged from 14-25+ for residential heat pumps. The old federal minimum was SEER 14. Multiply SEER by 0.95 to approximate the newer SEER2 rating.
The updated Seasonal Energy Efficiency Ratio using the DOE M1 test procedure (since January 2023). SEER2 values are lower than SEER for the same equipment because the new test adds external static pressure.
SEER2 is the current standard for comparing cooling efficiency. Federal minimum is SEER2 14.3 (split in South/Southwest) or 13.4 (split in North). Energy Star requires SEER2 15.2+. Top mini-splits reach SEER2 20-30+.
A heat pump system with one outdoor unit connected to one indoor unit, conditioning a single room or area. The simplest and most efficient configuration.
Single-zone mini-splits are the highest-efficiency heat pump configuration because the outdoor unit is optimized for exactly one indoor head. A single-zone 12,000 BTU unit can heat a 500-750 sq ft space effectively.
The noise level of a heat pump measured in A-weighted decibels. Lower is quieter. Outdoor units typically range from 50-65 dBA; indoor mini-split heads run 19-32 dBA at low speed.
For context: 40 dBA is a quiet library, 50 dBA is a moderate rainfall, 60 dBA is normal conversation. Outdoor unit noise matters for neighbor relations. Indoor unit noise matters for bedrooms. Inverter-driven units are significantly quieter at partial load.
A metric used in some state rebate programs that combines all heating equipment in a home into one efficiency score. TES percentage represents the fraction of total heating load served by heat pumps.
In states like New Jersey, rebate amounts increase with higher TES percentages. A 100% TES home uses heat pumps for all heating. A 60% TES home uses heat pumps for 60% of its heating load with another source covering the rest.
A unit of cooling capacity equal to 12,000 BTU/h. A "3-ton" heat pump can produce 36,000 BTU/h of cooling. The term originates from the cooling power of melting one ton of ice per day.
Residential heat pumps typically range from 1.5 to 5 tons. Bigger is NOT better — oversized equipment short-cycles, causing poor humidity control, uneven temperatures, and premature wear. Proper sizing via Manual J is essential.
A compressor that can operate at a continuous range of speeds (typically 30-100% of maximum capacity) rather than just on or off. Synonymous with inverter-driven compressor in residential heat pumps.
Variable speed is the single most important technology advancement in modern heat pumps. It enables: precise temperature control (within 0.5F), lower energy consumption, quieter operation, better dehumidification, and extended equipment life from reduced cycling.
A heat pump that exchanges heat with a water body (lake, pond, or well) rather than air or ground loops. Similar in concept to ground-source but uses open or closed water loops.
Water-source heat pumps can be very efficient if a suitable water body is available. Open-loop systems pump groundwater directly; closed-loop systems circulate fluid through submerged pipe loops. Permitting requirements vary by state.
| Rating | Measures | Federal Min. | Energy Star | Best Available |
|---|---|---|---|---|
| SEER2 | Seasonal cooling | 13.4 (North) | 15.2+ | 30+ |
| HSPF2 | Seasonal heating | 7.8 (split) | 8.1+ | 12+ |
| COP | Instantaneous heating | -- | -- | 4.0+ at 47F |
| EER | Peak cooling | -- | -- | 15+ |
SEER and SEER2 both measure seasonal cooling efficiency, but SEER2 uses the updated DOE M1 test procedure (mandatory since January 2023) which adds external static pressure to simulate real-world duct resistance. SEER2 values are lower than SEER for the same equipment. To convert, multiply SEER by approximately 0.95 to get SEER2. For example, a unit rated SEER 20 is roughly SEER2 19.
COP (Coefficient of Performance) is the ratio of heat delivered to electricity consumed at a specific temperature. A COP of 3.0 means you get 3 units of heat for every 1 unit of electricity — 300% efficient. Electric resistance heat has a COP of 1.0. COP matters because it directly determines your operating cost: a higher COP means lower heating bills. COP decreases as outdoor temperature drops, which is why cold-climate heat pumps are designed to maintain high COP at low temperatures.
A cold-climate heat pump (ccHP) is engineered to maintain high heating capacity at very low temperatures. NEEP (Northeast Energy Efficiency Partnerships) defines ccHP as maintaining at least 70% of rated capacity at 5F with a COP of at least 1.75. These units use enhanced vapor injection, larger compressors, and optimized defrost cycles. Popular ccHP models include Mitsubishi Hyper-Heat, Fujitsu XLTH, and Bosch IDS 2.0.
A mini-split is a ductless heat pump with individual wall-mounted indoor units — no ductwork required. A central (ducted) heat pump uses an air handler and ductwork to distribute conditioned air throughout the house. Mini-splits are more efficient (no duct losses), offer zone control, and are easier to retrofit. Central systems provide whole-home coverage and are invisible inside the home. Many installations combine both: ducted for main areas and ductless heads for additions.
Manual J is the ACCA-standard method for calculating how much heating and cooling your specific home needs. Without it, installers guess — and usually oversize the system. An oversized heat pump short-cycles (turns on and off too frequently), causing poor humidity control, uneven temperatures, higher energy bills, and premature equipment failure. A proper Manual J considers your insulation, windows, air leakage, orientation, and local climate to determine the right-sized system.
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