Air-to-Water Heat Pumps: What They Are and When One Can Replace a Boiler

If your house is heated by water — a radiant floor, panel radiators, fin-tube baseboard along the walls — and you are looking at the boiler and wondering what replaces it, an air-to-water heat pump is the electric option that keeps your pipes. It pulls heat out of outdoor air the way a ductless mini-split does, but instead of blowing warm air into one room it heats the water that already runs through your house.

Whether that works in your house comes down to one number: the temperature of the water your emitters need. A bare radiant slab can heat a room with 80–110 °F water. Cast-iron radiators were designed for 160–200 °F. The current models cited on this page reach only the lower part of that span — roughly 122 to 158 °F — and their efficiency drops as the target rises, then drops again as the outdoor air gets colder. If the water temperature your emitters need is too high for efficient air-source operation, a ground-loop water-to-water heat pump is the better answer.

What an air-to-water heat pump is

Heat pump names describe two things in order: where the heat comes from, then where it goes. An air-to-water heat pump takes heat from outdoor air on the source side and delivers it to a water or water/glycol hydronic circuit on the load side. That is the definition Caleffi uses in idronics 27, its 2020 design journal issue on this equipment, and it is all the name means.

The same naming covers the rest of the family. An air-to-air heat pump delivers to indoor air through a ducted air handler or a ductless mini-split head; that is the machine most people picture, rated under AHRI 1600. A water-to-water heat pump takes heat from a water-source circuit — groundwater, a closed geothermal loop, surface water or a district loop — and delivers hydronic water; a water-to-air heat pump takes heat from the same kind of source and delivers ducted air. Those two are what most of this site covers, and their standard, ISO 13256, is split into Part 1 and Part 2 along exactly that load-side line. Lay it out as a grid, source on one axis and load on the other, and our guide to ground-source vs air-source heat pumps covers the source axis while our guide to water-to-water geothermal heat pumps covers the ground-source machine that feeds a hydronic system. The air-to-water heat pump is the fourth cell: air source, hydronic load. It shares its outdoor half with a mini-split and its indoor half with a geothermal water-to-water unit.

Mechanically, the difference from a mini-split is one component. A mini-split transfers refrigerant heat directly to indoor air through a refrigerant-to-air coil. An air-to-water machine adds a refrigerant-to-water heat exchanger, and from there pumps distribute heated or chilled water to the emitters, as the Minnesota Department of Commerce's 2023 field report on air-to-water heat pumps describes it.

Monobloc vs split: where the refrigerant stops

Air-to-water heat pumps come in two physical arrangements, and the difference is which fluid crosses your exterior wall. In a monobloc, the compressor, refrigerant circuit, air coil, and refrigerant-to-water exchanger are all in the outdoor unit; refrigerant never leaves that box, and hydronic piping crosses the wall. In a conventional split, refrigerant lines cross the envelope to an indoor unit, often called a hydrobox, that holds the refrigerant-to-water exchanger, so the water circuit can stay indoors. Those are the Minnesota CARD report's definitions. Some products blur them: Nordic's split ATW puts the compressor indoors and only the air coil, fans, and expansion devices outside, per its August 2025 ATW-65 submittal.

The distinction matters for freeze protection. A monobloc's outdoor water piping needs manufacturer-approved protection, normally inhibited propylene glycol or another strategy the manufacturer specifies. A split whose whole water circuit stays in conditioned space may run plain water, but any exposed hydronic piping still has to be protected. Both Caleffi and the Minnesota report make this point. The glycol concentration comes from the equipment and fluid manufacturers' freeze-point, viscosity, and pump-correction tables, not from a general rule.

How supply water temperature sets COP and capacity

A heat pump's efficiency is expressed as COP, coefficient of performance: units of heat delivered per unit of electricity consumed. For an air-to-water heat pump, two things push that number down, and both are about temperature. Hotter leaving water means a larger temperature lift for the refrigerant circuit, and a larger lift normally means a lower COP. Colder outdoor air lowers the source temperature, which lowers COP, and also lowers available capacity, so the machine delivers fewer Btu per hour at the moment the house needs the most. NREL's 2024 Heat Pump Basics and Caleffi's idronics 27 describe the same relationship between temperature lift, COP and available capacity.

Manufacturer submittals show both effects on real machines. Nordic's ATW-65, a two-stage R-454B unit, is rated in its August 2025 submittal at four combinations of outdoor air and leaving water:

Source: Nordic submittal 003055SPC-01, August 18, 2025. One model; other equipment will rate differently.

At 47 °F outdoors, raising the target from 105 °F to 120 °F takes COP from 4.28 to 3.43. At 105 °F water, dropping from 47 °F to 17 °F outdoors takes COP from 4.28 to 2.40 and cuts capacity from 51,600 to 34,400 Btu/h, a loss of one-third. Cold day and hot water together put this unit at COP 2.01.

The high-temperature end of the market makes the same point more sharply. Daikin's Altherma 3 H HT is built to reach boiler-like temperatures, and the brochure for its 36-kBtu model, released with the product's April 24, 2026 North American launch, lists a maximum leaving-water temperature of 158 °F. The same brochure shows what that costs. At 44.6 °F outdoor air making 95 °F water, the unit delivers 34,733 Btu/h at COP 4.86. At the same outdoor temperature making 158 °F water: 28,885 Btu/h at COP 1.99. Drop the outdoor air to 5 °F: 31,354 Btu/h at COP 2.50 for 95 °F water, and 27,570 Btu/h at COP 1.50 for 158 °F water.

This is also why nominal "tonnage" is the wrong way to size one. A proper selection requires the manufacturer's capacity at your outdoor design temperature and your required leaving-water temperature together.

Find your emitters on this table

So what water temperature does your house need? That depends on what is emitting the heat into the rooms. Caleffi's idronics 19 gives engineering supply-temperature ranges for the common hydronic emitters.

Temperature ranges: Caleffi idronics 19 (2016). Fit assessments follow Caleffi idronics 27 (2020) and idronics 25 (2019).

Radiant floor

Radiant floor is generally the best fit, and the best case is a bare or lightly covered slab that carries the room's design heat loss with water near the bottom of that 80–110 °F range. Nordic rates the ATW-65 at COP 4.28 at A47/W105 and Daikin rates the 36-kBtu Altherma 3 H HT at COP 4.86 at A44.6/W95 — different air and water conditions, so these illustrate low-temperature operation rather than a head-to-head comparison. The required temperature is not fixed by the emitter type alone; Uponor's 2023 Complete Design Assistance Manual makes it depend on load, tube spacing, floor construction, floor covering, and flow. A slab under thick carpet needs hotter water than a bare one. Our guide to geothermal radiant floor heating covers the emitter side.

Panel radiators

Flat steel panel radiators work well when deliberately oversized, or selected to meet design load at approximately 120 °F or lower, per Caleffi's idronics 27. The catch is existing panels: a radiator sized to deliver its rated output at 160 °F delivers far less at 120 °F, and if the house needs the upper end of the range to stay warm, the heat pump has to make hotter water and the efficiency penalty follows.

Fan coils and hydronic air handlers

A fan coil moves air across a hot-water coil, and it is compatible as long as it was selected from the manufacturer's output tables at the entering-water temperature the heat pump will actually supply. Legacy coils rated only at boiler temperatures may be undersized for 110 °F or 120 °F water. New fan coils sized for low-temperature water are a common way to add emitter capacity.

Fin-tube baseboard and cast-iron radiators

These are the hardest retrofit. They were designed around 160–200 °F supply water. The models cited on this page top out well below the upper half of that band — roughly 120 °F for the Nordic ATW ratings shown, 149 °F heating-medium for the Viessmann Vitocal 100-AW, 158 °F for the Daikin Altherma 3 H HT — so a system genuinely needing 180–200 °F water is not a drop-in for any of them, and even the ones that reach 158 °F pay the efficiency penalty the Daikin figures show. The path forward is a room-by-room heat loss calculation and an emitter output calculation at the proposed water temperature. Two things can change the answer: weatherization, which lowers the heat loss so existing emitters can carry it at lower water temperature, and added emitter area, meaning more baseboard, larger panels, or supplementary fan coils.

Cold weather: capacity, defrost, and the balance point

The Nordic table shows the first cold-weather problem: capacity falls as the air gets colder, at the same time the house's heat loss rises. The second problem is frost. In cool, humid weather the outdoor evaporator runs colder than the outdoor air, so frost can build even when the air itself is above freezing — Caleffi puts the threshold at roughly 50 °F and below, given the right humidity — and the heat pump clears it with a defrost cycle that temporarily redirects refrigerant energy to warm the coil instead of heating your water. Per Caleffi, defrost reduces the seasonal heat delivered, and during the cycle the system may need stored hydronic heat or auxiliary heat to prevent a comfort dip. Nordic's ATW-65 submittal describes a demand-based "intelligent defrost" that runs when frost actually builds rather than on a timer; that is a control claim from one manufacturer, not a quantified seasonal-efficiency guarantee.

Operating limits are model-specific. The Nordic ATW-65 stops its compressor below −7 °F. The Daikin Altherma 3 H HT lists heating operation down to −18 °F. Those describe two products from their own literature, not the category, and the unit you are quoted will have its own limit on its own submittal.

The concept that ties this together is the thermal balance point: the outdoor temperature at which the building's heat loss equals the heat pump's available capacity at the water temperature the emitters need. Above it the heat pump carries the house alone. Below it, something else supplies the shortfall: electric resistance elements in the indoor unit, the existing boiler kept as backup, or another auxiliary source. Where your balance point falls depends on your climate, your heat loss, your emitters, and the specific machine.

The U.S. Department of Energy's Cold Climate Heat Pump Challenge covers residential centrally ducted, electric-only equipment, per the PNNL and ASHRAE paper describing its metrics. Its results apply to ducted air-to-air machines and do not certify hydronic air-to-water output.

Domestic hot water

Most homes that heat with a boiler also make hot water with it, so a boiler replacement raises the question of showers. How an air-to-water heat pump handles domestic hot water is model-dependent. A common configuration heats a separate indirect tank through a hydronic coil rather than circulating potable water through the heat pump itself, per Caleffi: the heat pump heats system water, the system water heats the tank through a coil, the tank supplies the taps.

Three manufacturers' current literature shows the range. Daikin offers 40-, 50-, and 80-gallon indirect tanks for the Altherma 3 H HT with a 3-kW electric booster; the tank's listed maximum water temperature is 167 °F, while the heat pump's listed maximum leaving-water temperature is 158 °F. Nordic's standard ATW literature describes domestic hot water preheating through an indirect tank; its high-temperature ATWC variant can heat domestic water to final temperature with an optional double-wall condenser, per the 2021 ATWC manual. Viessmann's 2026 Vitocal 100-AW data lists a 149 °F maximum heating-medium temperature and indoor-unit electric heaters of 6 or 9 kW depending on model. For rating purposes, equipment built only to heat potable water is excluded from the air-to-water standard below entirely. Dedicated commercial heat-pump water heaters are covered instead by AHRI 1300.

Buffer tanks

A buffer tank is an insulated tank of system water between the heat pump and the emitters, and many air-to-water installations include one. Caleffi lists its jobs: it adds thermal mass, preserves minimum flow and minimum compressor run time when small zones close their valves, reduces short-cycling, and can hydraulically separate the heat pump's circulator from the emitter circulators. A heat pump that starts and stops every few minutes because one small zone is calling wears faster and runs less efficiently.

Whether you need one, and how big, is model-specific. Nordic's ATWC manual requires at least one buffer tank for space heating and cooling and specifies a minimum of 8 US gallons per ton, with larger sizes recommended to reduce starts and lengthen run time. That is Nordic's rule for Nordic's equipment. The Minnesota CARD report lists what determines the need on any system: inverter turndown, active zone volume, piping volume, emitter mass, defrost strategy, and control method. A current packaged example is Viessmann's 20-US-gallon Vitocell 100-E buffer, supplied for its Vitocal 100-AW.

How air-to-water heat pumps are rated

Air-to-water heat pumps sit in an odd spot in the rating world, because the two standards a homeowner is most likely to have heard of are the wrong ones. AHRI 1600 covers unitary air conditioners and air-source heat pumps below 65,000 Btu/h, meaning ducted systems and mini-splits, and is not the primary reference for hydronic air-to-water output. ISO 13256, the standard behind geothermal ratings, is for water-source equipment.

In North America the governing standard is ANSI/AHRI 550/590-2023, which explicitly covers air-to-water heat pumps within its scope of water-chilling and heat-pump water-heating packages; the SI counterpart is AHRI 551/591, with a 2026 edition listed by AHRI. Its heating rating points are "high heating" at 47 °F dry-bulb and 43 °F wet-bulb outdoor air, "low heating" at 17 °F dry-bulb and 15 °F wet-bulb, each rated at leaving-water categories of 105 °F, 120 °F, and 140 °F. That is why the Nordic submittal is laid out at 47 °F and 17 °F with 105 °F and 120 °F water. When you compare two air-to-water units, compare them at the same outdoor point and the same leaving-water category, or you are not comparing anything.

Europe, where this equipment has a much longer market history, uses a different pair. EN 14511:2022 governs declared point capacity and COP; EN 14825:2022 governs part-load testing and seasonal performance, expressed as SCOP. The common declared conditions come from EU Delegated Regulation 811/2013: A7/W35, meaning 7 °C dry-bulb and 6 °C wet-bulb outdoor air with 30 °C entering and 35 °C leaving water, and a medium-temperature condition of 47 °C entering and 55 °C leaving. W35 is 95 °F water, which is why the Daikin brochure rates at 95 °F.

The distinction that matters most on a European-style spec sheet is COP versus SCOP. COP is one operating point: one outdoor temperature, one water temperature. SCOP is seasonal, built from climate-temperature bins, part-load performance, cycling degradation, supplementary heat, and applicable standby and off-mode energy. Never put one product's COP next to another's SCOP. Our guide to geothermal efficiency ratings explains how to read a certified rating and what the rating conditions mean; the same method applies directly to an air-to-water submittal.

Air-to-water vs geothermal water-to-water

The alternative for a hydronic house is the machine this site covers most: a water-to-water geothermal heat pump, which delivers the same hot water to the same emitters but draws its heat from a ground loop. Air-to-water avoids the ground heat exchanger; geothermal provides steadier cold-weather source temperatures.

On efficiency, no universal percentage difference is defensible. NREL's guidance is to compare equipment at the same load-water temperature and realistic source conditions. Air-source COP varies sharply with the weather, while a ground loop provides a far more stable entering-source temperature and usually a smaller winter temperature lift, because the ground in January is much warmer than the air in January. The air-to-water heat pump loses source temperature and capacity exactly when the load peaks. A ground-source unit does not have that problem.

Daikin's published figures illustrate the difference, though the two models are rated under different standards. At the same 95 °F hydronic load, Daikin's water-to-water Altherma 3 WS reports COP 5.33 at W10/W35 (10 °C source water, 35 °C leaving water) in its 2022 European catalog — a single fixed test condition, and one whose source loop can be fed by the ground, surface water, or a district system rather than a borehole specifically. Daikin's air-source Altherma 3 H HT reports COP 4.86 at 44.6 °F outdoor air and COP 2.50 at 5 °F, both making 95 °F water. In a geothermal application the source temperature is more stable than outdoor air rather than fixed — NREL describes ground temperature as relatively stable, and real entering-water temperature still drifts over a season — but the air-source number falls by nearly half between a mild day and a cold one. The caveat: these are different machines rated under different standards, so this illustrates source-temperature stability and cannot be turned into a savings percentage.

On installation, the difference is the ground heat exchanger. Air-to-water needs no drilling, no trenching, no groundwater permitting, and no yard space for a loop field, which is the whole reason a homeowner with a radiant floor and a small lot would choose it. Geothermal is the stronger answer where extreme-cold efficiency, stable capacity, summer heat rejection, and long operating hours justify the ground heat exchanger; NREL and Nordic's own system literature frame the trade the same way. If you cannot or will not put a loop in the ground, and your house has or can be given a low-temperature hydronic distribution system, air-to-water is the practical choice. If your climate is severe and the heating season long, the ground-source heat pump earns its excavation. A homeowner whose lot, geology or budget makes a ground loop impractical should not be talked into one.

What installation involves, and what cost depends on

An air-to-water installation is a hydronic job first and an electrical job second, and its scope follows from the decisions above: a room-by-room heat loss and emitter-output check at the proposed water temperature; equipment selection at design conditions rather than nominal tonnage; the monobloc-or-split decision and its freeze protection; a buffer tank if the model and zoning call for one; the domestic hot water method; auxiliary heat and the electrical service to run it, which on current products ranges from Daikin's 3-kW tank booster to Viessmann's 6- or 9-kW indoor heaters; and emitter changes where the heat loss calculation says the existing ones fall short.

Each of those is a line on the quote, and the spread between a simple job and a hard one is wide. A bare-slab radiant house with a monobloc, a small buffer, and the existing water heater left in place is one project; a cast-iron-radiator house that needs new emitters in three rooms, an indirect tank, and a panel upgrade for backup elements is a different project with the same heat pump. For the ground-source side, our geothermal installation cost guide breaks down where the money goes on a loop-field project. Incentives for either technology change frequently and vary by state and utility; verify what currently applies before counting on any of it, and treat nothing here as tax advice.

The Minnesota CARD report adds a warning that affects cost and outcome alike: North American availability remains narrower than in Europe, with fewer trained hydronic-and-heat-pump contractors, fewer locally stocked equipment combinations, and more model-specific design responsibility landing on the installer. The technology is well established in Europe and Asia and has historically had far less attention in North America.

Who sells them in the U.S. in 2026

The North American market is real but thinner than Europe's. As of September 2026, the manufacturers with confirmed official North American residential air-to-water products are:

• Daikin — Altherma 3 H HT, launched in North America on April 24, 2026; the high-temperature R-32 unit whose figures appear throughout this article.
• Viessmann — Vitocal 100-AW, with U.S. startup and programming training on the Viessmann Academy calendar in August and September 2026, confirming active North American technical support.
• SpacePak — Solstice line.
• Nordic / Maritime Geothermal — ATW and ATWC series, the Canadian manufacturer whose submittals supplied the two-stage figures above.
• Arctic Heat Pumps — air-to-water products with published specifications.

Enertech still publishes its Advantage air-to-water product and ratings but labels its next generation "Coming 2027," so its 2026 ordering status is something to verify with a distributor. Because the market is still small, model choice and contractor choice are tied together: freeze protection, buffer sizing, defrost behavior, low-temperature limit, and hot-water method are all specific to the machine, and the installer's familiarity with it is part of what you are buying.

Frequently asked questions

What is an air-to-water heat pump?

A heat pump that takes heat from outdoor air and delivers it to a hydronic heating system: radiant floor, panel radiators, baseboard, or fan coils. It is the same outdoor technology as a ductless mini-split, with a refrigerant-to-water heat exchanger in place of the indoor air coil.

Can an air-to-water heat pump replace my boiler?

It depends on the water temperature your emitters need. Radiant slabs at 80–110 °F are the natural fit; panel radiators sized to work at roughly 120 °F or lower work well. Fin-tube baseboard and cast-iron radiators designed for 160–200 °F are the hardest case. The high-temperature models cited here reach roughly 149–158 °F, not 200 °F, and they pay a steep efficiency penalty at the top of their range (Daikin's Altherma 3 H HT 36-kBtu model drops from COP 4.86 at 95 °F water to 1.99 at 158 °F on a 44.6 °F day). Systems that truly need 180–200 °F water generally require emitter upgrades, load reduction, or supplemental heat. A room-by-room heat loss calculation at the proposed water temperature answers the question for your house.

Do air-to-water heat pumps work in cold climates?

Some current units are rated to operate below 0 °F, but that is a permitted operating limit rather than a promise of good output: capacity and COP both fall as the air gets colder, and defrost cycles take some of the delivered heat. Limits are model-specific: the Nordic ATW-65 stops its compressor below −7 °F; the Daikin Altherma 3 H HT lists heating to −18 °F. Below the thermal balance point, auxiliary heat carries the difference. In a severe climate with a long heating season, a ground-source system's stable source temperature is its main advantage.

Is an air-to-water heat pump cheaper than geothermal?

Structurally it avoids the ground heat exchanger: no drilling, trenching, groundwater permitting, or loop field. What the equipment side costs depends on the emitter evaluation, monobloc-versus-split, buffer tank, hot water, and backup-heat electrical items above, which vary widely from house to house. Our geothermal installation cost guide covers the loop-field side.