Air-to-Water Heat Pumps for Radiators: Will Your Boiler System Work?

Air-to-Water Heat Pumps for Radiators: Will Your Boiler System Work?

By EnergyBS Editorial Team | November 13, 2026

A panel radiator sized for a boiler gives about 57% of its rated output when the water leaves the heat pump at 55 °C (131 °F), about 35% at 45 °C (113 °F) and about 16% at 35 °C (95 °F). So an air-to-water heat pump can replace a boiler on existing radiators only where each radiator is big enough for its room at a lower water temperature. A ten-minute check, room by room, tells you which ones are.

Those three percentages come from the rating formula radiator makers publish, not from a rule of thumb. Caleffi, which makes hydronic components, gives it in its idronics series as output = rated output × (actual temperature difference ÷ rated temperature difference) raised to the power 1.3. Zehnder, a radiator maker, uses the same 1.3 exponent in its own correction calculator. The exponent is the reason a small drop in water temperature costs so much heat: halve the temperature difference between water and room and output falls to about 41%, not 50%.

The second half of the decision is what the lower temperature buys. On one manufacturer's published data sheet, the same air-to-water unit at -7 °C outdoors turns 1 kWh of electricity into 2.10 kWh of heat at 55 °C water and 3.15 kWh at 35 °C. Every radiator you enlarge lets the whole system run cooler and cheaper. The rebate side is narrower than for ducted heat pumps: as of November 13, 2026, the clearest dedicated air-to-water lines are in British Columbia and parts of New York, and the U.S. federal HEEHR rebate no longer pays to replace a gas or oil boiler.

How much heat a radiator loses when the water is cooler

Radiators are rated at one test point. Zehnder publishes its outputs at a temperature difference (ΔT) of 50 K: mean water temperature 70 °C in a 20 °C room, which it describes as a flow of 80 °C and a return of 60 °C. That is the European basis used in the tables below. North American panel radiators are typically rated at 180 °F average water in a 68 °F room, a 112 °F difference, according to Caleffi. Either way, the rated number on the box assumes water far hotter than a heat pump likes to make.

The temperature difference that counts is the mean water temperature minus the room temperature. With a heat pump running 55 °C flow and 50 °C return in a 20 °C room, the mean is 52.5 °C and ΔT is 32.5 K. Caleffi adds one refinement: when the return water is close to room temperature (its test is the ratio of return-minus-room to supply-minus-room falling below 0.7), use the logarithmic mean instead of the simple average. That only changes the 35 °C row below, and only slightly.

Here is the formula worked for a radiator rated 2,000 W (6,824 Btu/h) at ΔT 50, in a 20 °C room, with a 5 K drop across the radiator.

To check the 55 °C row by hand: 32.5 ÷ 50 = 0.65; 0.65 raised to 1.3 is 0.571; 0.571 × 2,000 = 1,142 W. For the 35 °C row, the log mean is (35 − 30) ÷ ln(15 ÷ 10) = 5 ÷ 0.405 = 12.33 K, and (12.33 ÷ 50)^1.3 = 0.162.

The North American version runs the same way in Fahrenheit. A radiator rated 8,000 Btu/h at 180 °F average water in a 68 °F room, fed at 131 °F with a 10 °F drop (mean 126 °F, ΔT 58 °F), gives 8,000 × (58 ÷ 112)^1.3 = about 3,400 Btu/h, or 42.5% of its rating. At 113 °F supply (mean 108 °F, ΔT 40 °F) it gives about 2,100 Btu/h. The percentages are lower than the European column because a 180 °F rating starts hotter than a 70 °C one.

Caleffi's own example lands in the same place: a radiator rated 9,500 Btu/h, fed at 115 °F and leaving at 92 °F in a 65 °F room, puts out about 2,277 Btu/h, roughly a quarter of its rating. We re-ran their numbers and got the same answer, 37.3 °F log-mean difference and 2,277 Btu/h.

The oversizing ratio: one number per room

The table above answers "what does this radiator give?" The more useful question runs the other way: "how hot must the water be for this room?" Turn the formula around and it depends on one ratio per room:

Oversizing ratio = radiator rated output ÷ room heat loss at the design outdoor temperature

If the radiator in a room is rated at exactly the room's design heat loss, the ratio is 1.0 and the water has to reach rating conditions on the coldest day. If the radiator is twice the room's loss, the ratio is 2.0 and the water can be much cooler. The required temperature difference is the rated ΔT × (1 ÷ ratio) raised to the power 1 ÷ 1.3.

Read it as a set of thresholds. For 55 °C flow on the coldest day, every room needs a ratio of about 1.75 or more (European rating). For 45 °C flow, about 2.8. For 35 °C flow, about 6.2: radiators six times the room's heat loss, which is where underfloor heating or fan-assisted emitters take over.

Two inputs make or break the ratio, and both can be wrong on an old quote.

The radiator's rating. For a modern panel, the model code (type 11, 21, 22 or 33 plus height and length) leads to the maker's catalogue figure. Check whether the catalogue gives ΔT 50, ΔT 30 or the North American 180 °F point; Zehnder notes that it prints outputs at ΔT 30 on some data sheets, so a "1,000 W" label is not always comparable with another brand's "1,000 W". Old cast-iron column radiators in North American houses are rated in square feet of equivalent direct radiation (EDR) from the maker's or industry tables; the installer measures height, column count and section count and looks the rating up. Our steam radiator guide covers EDR in more detail.

The room's heat loss. It must be calculated room by room at your local design temperature, the way a Manual J or CSA F280 heat loss calculation does it. A whole-house number divided by floor area hides exactly the rooms that fail: the corner bedroom with two outside walls, the bathroom that people want warmer than the rest of the house, the room over the garage.

A six-room house, checked room by room

Here is a hypothetical house to show the method end to end. The inputs are stated so you can replace them with yours: design heat loss 7,500 W (about 25,600 Btu/h) at a design outdoor temperature of -7 °C, 20 °C indoors, and existing steel panel radiators with ratings taken at ΔT 50.

Three things come out of this table that a whole-house shortcut would miss.

First, the house as a whole looks fine. Its radiators add up to 15,800 W of rating against 13,130 W needed at 55 °C, an overall ratio of 2.11. Someone dividing total radiator rating by total heat loss would sign off on 55 °C. Yet the back bedroom would sit at 1,142 W against a 1,500 W loss on the design day, and the bathroom at 514 W against 600 W.

Second, the worst room sets the water temperature for every room. One undersized radiator in the back bedroom forces a 63 °C flow on the coldest day. The Systemair unit used for the COP figures below lists a maximum heating water setting of 65 °C, so this house sits close to the top of that unit's range before anything is changed.

Third, the fix is two radiators, not six. Replace the back bedroom panel with one rated about 3,600 W at ΔT 50 (ratio 2.4) and the bathroom with one rated about 1,500 W (ratio 2.5), and the worst room becomes the living room at about 50 °C flow. Going further, to 45 °C, would mean enlarging five of the six rooms, about 21,300 W of total rating against 15,800 W today. That is where the cost per degree climbs and where a homeowner should ask whether the efficiency gain repays it.

Low water temperature also changes the pipework question. A boiler system designed around a 20 K drop (80 °C out, 60 °C back) moves 7,500 W with about 5.4 litres per minute; a heat pump running a 5 K drop needs about 21.5 litres per minute for the same heat, four times the flow (flow in kg/s = watts ÷ (4,186 × ΔT)). In U.S. units the rule is gallons per minute = Btu/h ÷ (500 × temperature drop in °F): 25,600 Btu/h at a 20 °F drop is about 2.6 gpm, and at a 10 °F drop about 5.1 gpm. Small-bore or microbore branches that were fine for a boiler may not carry the higher flow without noise or a larger circulator, so the installer should check pipe sizes along with radiator sizes.

What a lower flow temperature is worth in electricity

Radiator output is half of the decision. The other half is how much less electricity the heat pump uses when the water is cooler. The cleanest way to see it is a single manufacturer's data sheet that tests the same unit at several water temperatures.

Systemair's engineering data for its SYSHP MINI EVO-P monobloc air-to-water range (R32 refrigerant) lists heating output, electrical input and COP at three outdoor temperatures and three leaving-water temperatures. The 7 kW and 9 kW models:

Converted into electricity per 1,000 kWh of heat delivered (1,000 ÷ COP), the 07 model needs:

On the coldest test day, dropping from 55 °C to 45 °C cuts the electricity for the same heat by about 16% (476 to 400 kWh), and dropping to 35 °C cuts it by a third. Over a season the gap is larger in relative terms: the same 07 model's seasonal COP for an average European climate is listed at 5.18 for 35 °C systems and 3.67 for 55 °C systems. That means a 55 °C system uses about 41% more electricity for the same annual heat than a 35 °C one (5.18 ÷ 3.67 = 1.41), all else equal.

Back in the six-room house, the two radiator swaps take the design-day flow from about 63 °C, above the sheet's highest test point, to about 50 °C, between the 45 °C and 55 °C columns. The sheet gives no 50 °C figure, so we do not invent one; the honest reading is that the design-day COP moves from below 2.10 (the 07 model's A-7/W55 value) to somewhere between 2.10 and 2.50 for the 07 model.

Two limits on those numbers matter for Canada and the northern U.S.

The coldest test point is -7 °C. Systemair lists an operating range down to -25 °C outdoors, but its performance table stops at -7 °C. If your design temperature is colder, as it is in much of Canada, the table does not cover your design day. The output column shows the trend: the 07 model's heat output at 55 °C falls from 8,200 W at 7 °C to 6,600 W at -7 °C. Ask for the maker's extended capacity table at your design temperature before sizing; if the unit cannot cover the house on the design day at the flow temperature your radiators need, the gap is made up by a backup element or by the old boiler kept as a second stage. Our cold-climate heat pump performance data page explains why capacity, not COP, is usually the binding figure in deep cold.

Design day is a few hours a year. A heat pump with weather compensation (outdoor reset) runs warmer water only when it is cold outside and cooler water the rest of the time. The design-day flow temperature from the room check is a ceiling, not the operating point for most of the winter. That is why the seasonal COPs above are higher than the -7 °C figures.

Systems that do not fit the radiator check

The ratio method assumes a two-pipe hot-water system with radiators or panels. A few common boiler setups need a different conversation.

One-pipe and two-pipe steam. Steam radiators are fed by steam, not water, and air-to-water heat pumps make warm water. Systemair's heating range tops out at 65 °C. Moving a steam house to an air-to-water heat pump means converting it to hot water, which means re-piping and often new radiators. The alternative for a steam house is an air-to-air heat pump, ducted or ductless.

Fin-tube baseboard. Copper-and-aluminium fin-tube baseboard is common in North American hot-water systems. It has its own output tables, and the 1.3 exponent used here is Caleffi's figure for panel radiators, not baseboard. Ask the baseboard maker for low-temperature ratings, or measure the length per room and have the installer use them. Baseboard lengths in older houses were set for boiler water, so expect the ratio test to be tight.

Gravity systems and very old large-bore pipe. Big cast-iron radiators on large pipes are often generously sized for a house that has since been insulated, which helps the ratio. But the system volume and flow paths are unusual, and the installer needs to work out how the heat pump's circulator interacts with the existing pipe.

Radiant floors and fan coils. These are the easy cases. Underfloor heating already runs near 35 °C, and fan-assisted emitters can be sized for 45 °C. Our hydronic radiant heating guide explains how floor heating works, and the radiant floor heating cost guide covers what it costs to add.

Domestic hot water from the boiler. If the boiler also heats an indirect tank, the heat pump will need to do the same, usually with a cylinder designed for heat pumps (a larger coil, because the water is cooler). Systemair lists a domestic hot water setting range of 20 to 60 °C on the heat pump alone, with a built-in electric backup heater able to reach 65 °C. Ask how the system handles the periodic high-temperature cycle many tank setups use, and what that costs in electricity.

What goes into an air-to-water quote

Installed prices for air-to-water conversions turn on the radiator and pipe work, and two houses with the same heat loss can need very different amounts of it. The way to read a quote is line by line, with the room check beside it.

The radiator line is the one the room check controls. In the six-room example, a 55 °C design means two new radiators; a 45 °C design means five, plus whatever pipe work the larger flows need. Ask for the quote both ways. The difference between the two totals is the price of the lower flow temperature, and the electricity tables above show what it buys back each winter.

Rebates come off after that. As a hypothetical illustration only: a British Columbia household at income Level 2 with a $25,000 quote for an air-to-water system replacing an oil boiler would have $7,000 deducted under the Energy Savings Program rates from July 6, 2026, leaving $18,000; if the model is on the federal eligible list, the two programs can be combined (neither program page says so; ask before counting on it) and the household is above the federal median-income line, the $2,000 National Heat Pump Rebate would bring it to $16,000. The $25,000 is an assumed number to show the arithmetic, not a price we have verified.

Rebates for air-to-water heat pumps in late 2026

Air-to-water units fall between categories in many programs. Some programs list them by name, some treat them as a type of "air-source heat pump", and some do not cover them at all. Confirm the line your system is filed under before you sign. All program details below were read on November 13, 2026.

A few points behind the table.

British Columbia pays the most, but only through the income-qualified program outside the north. The CleanBC Energy Savings Program's air-to-water line matches its central ducted line at every income level, so a BC boiler home that qualifies is not penalised for choosing water over air. Outside the northern region, the general fuel-switching rebates for homes leaving gas, oil or propane ended in April 2025. Our BC home energy rebates guide sets out the income levels and the pre-registration steps.

The federal Canadian rebate is new and its eligible list is the test. Environment and Climate Change Canada's page says the system must be able to heat the entire home and must be on the eligible product list by AHRI number, and that program parameters are still being finalized. It does not say whether air-to-water units are on the list. A homeowner buying now should ask the contractor to show the model on the list in writing, or treat the federal money as uncertain.

In New York, the utility decides the line. Long Island's program names air-to-water outright. In O&R's territory, the higher Category 2B line is linked to air-to-water in the sheet's equipment list but labelled as cold-climate air-source full-load heating in its incentive table, a $3,000 gap at the base rate. Our New York rebates guide covers the other utilities.

In the U.S., the federal side is mostly gone for boiler homes. DOE's Program Notice 26-2 removed fuel-switching upgrades from HEEHR, so replacing an oil or gas boiler with any heat pump no longer earns that rebate; a home already heated by an electric boiler is on the electric-to-electric side. State and utility programs are the remaining money, listed by state in our heat pump rebate guide.

Keep the boiler, or take it out?

Not every house has to get the radiators to 45 °C or even 55 °C on day one. There are three workable end states, and the room check tells you which ones are open.

The third row is a hybrid: the heat pump carries the mild and moderate days at low water temperature, and the boiler takes over below a set outdoor temperature when the radiators need hotter water. It keeps comfort while radiators are upgraded room by room over several years. It can also change rebate eligibility: whole-home lines in Canadian and state programs may treat a retained boiler differently, so confirm before deciding. Our dual fuel heat pump cost guide works through the forced-air version of the same choice, and the boiler replacement cost guide prices the path of putting in a new boiler instead.

The electrical side applies to every option. A monobloc air-to-water unit with a backup heater is a large 240 V load, and the heat pump electrical requirements guide shows how breaker and wire size follow from the nameplate.

Doing the check before anyone quotes

The room check needs nothing more than a tape measure, the radiator labels or model codes, and a room-by-room heat loss. Write one line per room: room name, design heat loss, radiator rating (with the test point it is quoted at), ratio. Find the lowest ratio. Look it up in the oversizing table above. That is your coldest-day flow temperature with the radiators as they are.

Then ask each installer for three things in writing: the room-by-room heat loss they used, the design flow temperature they are designing to, and the radiators they propose to change to reach it. A quote that names a heat pump model without a design flow temperature has skipped the step that decides how much it will cost to run. Quotes that agree on the flow temperature are comparable on price; quotes that do not are pricing different systems.

For more on heat pumps of every type, including ducted and ductless options for houses where the radiators cannot work, start at our heat pumps hub, and for every program we track by province and state, see the rebates page.

Frequently Asked Questions

Can I keep my cast-iron radiators with a heat pump?

Often, yes, if they are large for the room. If the house has been insulated or air-sealed since the radiators went in, its heat loss has fallen while the radiator ratings have not, which raises the oversizing ratio. The test is the same: radiator rating (from EDR tables for cast iron) divided by the room's present heat loss. A ratio of about 1.75 or higher on the European basis means 55 °C flow works on the design day.

What flow temperature should an air-to-water heat pump run at?

As low as your worst room allows. On one maker's data sheet, the same unit at -7 °C outside makes 2.10 units of heat per unit of electricity at 55 °C water and 2.50 at 45 °C. Weather compensation then lowers the water temperature further on milder days.

Is an air-to-water heat pump the same as a hydronic heat pump?

Yes, for a house with radiators the two names describe the same thing: an outdoor heat pump that heats water instead of air, connected to the existing hot-water pipes. Ground-source water-to-water units also heat water but draw their heat from the ground.

Do I need a buffer tank?

It depends on the system volume, the number of zones and the maker's minimum flow and volume requirements. If thermostatic valves on every radiator can all close at once, the heat pump may have nowhere to send water. Ask the installer to state the minimum system volume from the unit's manual and how the design meets it.

Will my rooms feel different?

Radiators running at 45 to 55 °C feel warm rather than hot to the touch, and they run for longer periods. Room temperature should be the same if the radiators pass the check. Rooms that fail the check will fall short on the coldest days until their radiators are enlarged.

Can the heat pump also give me air conditioning?

Many air-to-water units can make chilled water, and the Systemair sheet lists cooling ratings at 18 °C and 7 °C water. Radiators are not built for cooling, though: water below the room's dew point makes moisture condense on the radiator surface. Cooling needs fan coils or a separate air-to-air system.