An air-to-water heat pump pulls heat from outdoor air and puts it into water instead of air. That makes it a natural partner for radiant floor heating, which works with warm rather than hot water, the conditions where heat pumps run most efficiently. For Canadian homes with in-floor heating or a boiler, it's one of the clearest routes to low-carbon heat. You'll need a cold-climate model and good system design to handle deep cold, though.
Key takeaways
- Air-to-water heat pumps heat water for radiant floors, low-temperature radiators or fan coils, and many can also make domestic hot water.
- Radiant floors need water at roughly 30–45°C, which suits heat pumps far better than old cast-iron radiators designed for 70–80°C.
- Efficiency drops and maximum water temperature can fall in extreme cold, so design and backup matter.
- Installed costs are typically higher than for air-to-air systems, often $18,000–$35,000 or more depending on the home.
- Cooling is possible with fan coils, but radiant floors alone can't cool well without condensation risk.
How an air-to-water heat pump works
The outdoor unit works like any air-source heat pump. A refrigerant absorbs heat from outdoor air, a compressor raises its temperature, and the heat is released, here into water through a heat exchanger. The warm water then circulates through:
- In-floor tubing (slab or between joists)
- Low-temperature panel radiators
- Fan coils or air handlers with a hydronic coil
- An indirect tank for domestic hot water
There are two main configurations. Monobloc units keep all the refrigerant outdoors and pipe water (with antifreeze or freeze protection) into the house. Split units run refrigerant lines to an indoor hydronic module. Monobloc is simpler for installers without refrigerant work. Split avoids freezing concerns in the outdoor piping.
Why radiant floors are a great match
A heat pump's efficiency depends on the temperature gap between the heat source (outdoor air) and where the heat goes (the water). The cooler the water it needs to make, the higher its coefficient of performance (COP). (Our heat pump ratings guide explains COP.)
| Heat emitter | Typical supply water temp | Heat pump fit |
|---|---|---|
| Radiant slab floor | 30–40°C | Excellent |
| Radiant floor between joists (with plates) | 35–50°C | Good |
| Low-temp panel radiators (oversized) | 40–50°C | Good |
| Fan coils | 35–50°C | Good, and can cool |
| Cast-iron or baseboard hydronic (original design) | 60–80°C | Poor without upgrades |
A concrete slab with embedded tubing also stores heat, which smooths out defrost cycles and lets the heat pump run long, steady cycles.
Cold-weather performance
Air-to-water units face the same physics as other air-source heat pumps. At -20°C to -25°C:
- Output drops. Cold-climate models keep more capacity, but sizing has to account for your local design temperature.
- Maximum water temperature may fall. Some units can't reach 50°C or more in deep cold, which matters for radiators and hot water.
- COP falls, though with low-temperature floors it often stays well above 1.
Common solutions include an electric element or existing boiler as backup, a buffer tank to stabilize flows, and outdoor reset controls that lower water temperature in milder weather to boost efficiency. In very cold regions, a ground-source (geothermal) heat pump feeding the same floors avoids most of these limits. See our geothermal page for that comparison.
Domestic hot water
Many air-to-water systems include or support a hot water tank. Heat pumps can typically heat domestic water to about 50–55°C. A periodic boost from an electric element is common for legionella control, set according to local code and the manufacturer's guidance. Combining space heating and hot water in one system can replace a gas or oil boiler and tank entirely.
Costs and practical considerations
Approximate installed costs in Canada (2026):
- Replacing a boiler on an existing radiant floor: about $18,000–$30,000
- New construction with radiant slab: heat pump and mechanical room often $20,000–$35,000, plus the floor tubing
- Adding fan coils for cooling: several thousand dollars per zone
Things to discuss with a designer:
- Heat loss calculation, room by room. Essential for sizing and confirming floor output can meet demand.
- Emitter check. Can your existing floors or radiators deliver enough heat at lower water temperatures?
- Electrical capacity. Larger units and backup elements may need a load review. See our panel upgrade guide.
- Outdoor unit placement. Above snow level, with drainage for defrost water and short piping runs.
- Installer experience. Hydronic heat pump design is more specialized than mini-split installs. Ask for past projects.
The air-to-water market is smaller in Canada than in Europe, so equipment choice and experienced installers vary by region. Incentive programs change often. Check current offers by address with our rebate finder.
Can it cool my home?
Yes, but carefully. Running chilled water through radiant floors can cause condensation if the floor drops below the indoor dew point, so floor cooling is limited and needs humidity controls. Most homes that want real air conditioning add fan coils or a ducted hydronic air handler with condensate drainage.
Frequently asked questions
Can I connect an air-to-water heat pump to my existing radiators?
Sometimes. If your radiators were oversized or your home has since been insulated and air sealed, they may deliver enough heat at 45–55°C. A heat loss calculation and emitter check will tell you. Some homes add or swap a few radiators.
Is air-to-water more efficient than air-to-air?
Not inherently. With low-temperature radiant floors, seasonal efficiency is often similar or better. With high-temperature emitters it can be worse. Comfort with radiant floors is often excellent either way.
Do I still need my boiler?
Many owners keep an existing boiler as backup for the coldest days, at least at first. Others use an electric element instead. It depends on your climate, the heat pump size and your goals.
The bottom line
If your home has radiant floors or you're building new with in-floor heating, an air-to-water heat pump is worth a serious look. The pairing plays to a heat pump's strengths, and good design handles Canadian cold. Start with an energy-efficient envelope, as our whole-home upgrade sequence suggests, and work with an installer who knows hydronics.
Sundamentals is an independent, non-profit education platform. Program details and prices change often — check current programs for your address in our rebate finder.