Air Source Heat Pumps Explained (2026): Air-to-Air vs Air-to-Water

An air source heat pump is the most common kind of heat pump in American homes, and it is what people usually mean when they say "heat pump." It pulls heat out of outdoor air in winter and dumps heat into outdoor air in summer, so one piece of equipment both heats and cools. The two big families are air-to-air (the ducted and ductless systems that dominate the US) and air-to-water (hydronic systems that heat water for radiators and radiant floors, common in Europe and rare here).

The quick answer: An air source heat pump uses a refrigerant cycle to move heat between outdoor air and your home instead of generating heat by burning fuel or running resistance coils. Air-to-air models (ducted splits and ductless mini splits) deliver that heat through air and are the standard in the US; air-to-water models heat a water loop for radiators or radiant floors. Modern inverter and cold-climate units keep useful heating capacity well below 0°F, and every new split heat pump must reach at least 14.3 SEER2 and 7.5 HSPF2. A heat pump is a good fit for most US homes, especially those replacing electric, propane or oil heat, or replacing a central AC that already has ducts.

How an air source heat pump moves heat

Air contains heat energy even at 0°F. A heat pump uses a refrigerant that boils at a very low temperature to collect it. In heating mode the outdoor coil acts as the evaporator: liquid refrigerant absorbs heat from outdoor air and turns to vapor. The compressor raises that vapor's pressure and temperature well above room temperature, the hot vapor condenses in the indoor coil and releases its heat into the house, and an expansion valve drops the pressure so the cycle repeats.

A reversing valve swaps the roles of the two coils so the same unit cools in summer, exactly like a central air conditioner. For a deeper walk-through of the cycle, defrost mode and the reversing valve, see our post on how a heat pump works.

Because the unit moves heat rather than making it, it delivers more heat energy than the electricity it consumes. That ratio is the coefficient of performance (COP): a COP of 3 means three units of heat per unit of electricity, while a resistance heater is always 1. COP falls as outdoor temperature drops, which is why ratings and cold-climate design matter.

Air-to-air heat pumps: ducted and ductless

Ducted split systems pair an outdoor heat pump condenser with an indoor air handler, or a furnace with an evaporator coil, connected to existing ductwork. This is the straightforward replacement for a central air conditioner, and matched heat pump and air handler systems are the usual all-electric package.

Ductless mini splits connect an outdoor unit to one or more indoor heads (wall-mounted, ceiling cassette or slim-duct) through a refrigerant line set. They suit additions, older homes without ducts, garages and room-by-room zoning; multi-zone systems run two to five heads from one outdoor unit.

• Ducted: whole-home comfort from one thermostat, central filtration, supports dual fuel with a gas furnace; but duct losses in unconditioned attics and less room-by-room control.
• Ductless: no duct losses, independent zone control, very high efficiency ratings; but visible indoor heads, one head per zone and more filters to clean.

Air-to-water and high-temperature heat pumps

An air-to-water heat pump uses the same outdoor refrigerant cycle but transfers the heat into a water loop that feeds radiant floor tubing, panel radiators, hydronic fan coils or an indirect tank for domestic hot water. In Europe this is the dominant residential design because most homes there heat with water. In the US it is a niche product for homes that already have hydronic heat or are building new with radiant floors.

The main constraint is water temperature. A standard air-to-water heat pump is most efficient delivering water around 95-120°F, which suits radiant floors and oversized low-temperature radiators. Old cast-iron radiators and most baseboard systems were designed for 160-180°F boiler water.

That is where the high-temperature heat pump comes in: an air-to-water unit built to supply water at roughly 150-175°F so it can replace a boiler without changing the radiators, using refrigerants such as R-290 (propane) or CO2, cascade compressor arrangements, or vapor-injection compressors. The trade-off is efficiency. The larger the lift between outdoor air and supply water temperature, the lower the COP, so a unit running at 170°F in January uses noticeably more electricity than one feeding a radiant floor. European homeowners search for these because decarbonization rules are pushing boilers out of older radiator homes. In the US the usual paths are cheaper:

• Keep the boiler for the coldest days and add an air-to-air heat pump for cooling and most of the heating season, the hydronic version of dual fuel.
• Replace undersized radiators with larger low-temperature panels or fan-coil convectors so a standard air-to-water unit can run at 120°F or less.
• Add ductwork or mini splits and retire the hydronic system, which is the most common US choice.

If you have a radiator home, get a heat loss calculation and radiator survey first; the supply temperature your radiators actually need at design conditions decides which route is viable. HVAC MORE carries air-to-air equipment, not air-to-water heat pumps.

Inverter and cold-climate models

An inverter heat pump uses a variable-speed compressor that ramps between roughly 25 and 100 percent of capacity instead of switching on and off. It matches output to the load, which means longer, quieter cycles, steadier temperatures, better summer dehumidification and a higher seasonal rating. Single-stage units still work well in mild climates and cost less; two-stage units sit in between.

Cold-climate heat pumps add enhanced vapor injection, larger outdoor coils and refined defrost logic so the unit keeps a large share of its rated heating capacity at 5°F and runs at -5°F to -15°F or below. The ENERGY STAR cold-climate designation requires, among other criteria, a COP of at least 1.75 at 5°F. Older single-stage heat pumps lost most of their capacity by the mid-20s°F and leaned on electric strip heat, which is where their reputation for high winter bills came from.

When comparing cold-climate models, read the manufacturer's extended performance data, not just the nameplate tonnage: heating capacity at 47°F, 17°F and 5°F, and the minimum operating temperature. A 3-ton unit rated 36,000 BTU/h at 47°F may deliver 30,000 at 5°F if it is a true cold-climate design, or 18,000 if it is not.

SEER2, HSPF2 and the ratings that matter

Since January 2023 US heat pumps have been rated under the DOE's M1 test procedure, which produces SEER2 (cooling) and HSPF2 (heating). The test uses higher external static pressure to reflect real ductwork, so SEER2 and HSPF2 run a bit lower than the old SEER and HSPF for the same unit. The federal minimum for a split heat pump is 14.3 SEER2 and 7.5 HSPF2 in all regions; unlike air conditioners, heat pumps have no North/South split. Our SEER2 guide covers the regional AC rules and rating conversions.

HSPF2 is a BTU-per-watt-hour figure, so dividing by 3.412 gives an approximate seasonal COP: 7.5 HSPF2 is about 2.2, and 10 HSPF2 is about 2.9. In cold winters, HSPF2 and 5°F capacity deserve more weight than SEER2. New equipment also uses new refrigerants under the R-410A phase-down: mostly R-454B, with R-32 used by the Daikin family (Daikin, Goodman, Amana) and many mini splits. Operation and ratings are unchanged, but coils, line set practices and service tools must match the refrigerant.

Dual fuel: heat pump plus gas furnace

A dual fuel system pairs a heat pump condenser with a gas furnace that doubles as the indoor air handler. The heat pump handles cooling and all heating down to a set outdoor temperature, then the thermostat or an outdoor sensor switches to the furnace for the coldest hours. The two never run together: the evaporator coil sits downstream of the furnace heat exchanger, and running both would push hot air across the coil and overload the compressor.

The switchover point, or balance point, is typically set between 25°F and 40°F, either where the heat pump can no longer carry the house or where gas becomes cheaper per BTU than electricity in your area. Dual fuel is a strong fit for the northern half of the US where homes already have gas service and ductwork: you keep the security of gas heat while getting heat pump efficiency for most of the season. Browse matched dual fuel heat pump and furnace systems to see how they are paired.

Sizing, and when a heat pump is a good fit

Heat pumps are sized in tons of cooling (12,000 BTU/h per ton), but in a heating-dominated climate the heating load should drive the decision. The right method is a Manual J load calculation covering square footage, insulation, window area and orientation, air leakage and local design temperatures; square-footage rules of thumb are only a sanity check. Our guide to finding the right system size walks through the load factors.

• Cooling-dominated climates: size to the cooling load and confirm heating capacity at your design temperature, with strip heat as backup.
• Heating-dominated climates with a cold-climate inverter: size to the heating load using the manufacturer's capacity table at 5°F or 17°F; the inverter ramps down so it does not short-cycle in summer.
• Dual fuel: size the heat pump to the cooling load and let the furnace cover the gap below the balance point.
• Do not oversize. An oversized heat pump short-cycles, dehumidifies poorly and wears out faster.

A heat pump pays off fastest when it displaces an expensive fuel or when the central AC is due for replacement anyway: homes heated with electric resistance, propane or heating oil; homes with ductwork and an aging AC, since a heat pump condenser costs modestly more than an AC condenser and adds efficient heating; the South, Southwest and coastal West, where a standard inverter unit covers the whole heating season; and cold-climate homes with inexpensive natural gas, where dual fuel often beats both an all-electric heat pump and a furnace-only system on operating cost. Leaky, poorly insulated homes should fix the envelope first.

FAQ

Is an air source heat pump the same as a heat pump?

Almost always, yes. Nearly all residential heat pumps in the US are air source. The alternatives are ground source (geothermal) and water source units, which pull heat from the ground or a water body instead of the air.

Do air source heat pumps work in cold climates?

Modern cold-climate inverter models do. They retain much of their heating capacity at 5°F and operate at -15°F and below. Older single-stage units lose capacity quickly below freezing, which is why dual fuel or strip-heat backup is still common in the northern US.

Can a heat pump replace a boiler with radiators?

Only an air-to-water heat pump can feed radiators directly, and radiators sized for 180°F water need a high-temperature model or larger replacement radiators. Many US homeowners keep the boiler for backup and add an air-to-air heat pump for cooling and most of the heating.

Does a dual fuel system run the heat pump and furnace together?

No. The thermostat switches from the heat pump to the furnace at the set balance point, and only one heat source runs at a time.

Shop at HVAC MORE

HVAC MORE carries Goodman, ACiQ, MRCOOL, York, Guardian and Daikin air source heat pumps, from single-stage units to cold-climate inverters, as heat pump condensers, matched heat pump and air handler systems and dual fuel systems. Pair your own outdoor unit, coil and air handler in the system builder, and for a side-by-side look at the brands see our heat pump manufacturers guide.