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Air-to-Water Heat Pumps in Ontario (2026): What They Are, Output at Minus 25, Buffer Tanks, Defrost, Backup, Hot Water, Cooling, Cost and Who Installs Them
Heating, Cooling & Ventilation
Air-to-Water Heat Pumps: The Heat Pump That Talks to a Radiant Floor, What It Costs, and What It Needs Around It
Most heat pumps in Ontario blow warm air through ducts or a wall head. An air-to-water unit takes heat out of the outdoor air and puts it into water, which goes to the floor manifolds instead of a duct. In a radiant house that is a natural match, because a slab wants a lot of low-temperature water and a heat pump is at its best making exactly that. The catch is that it is a system, not an appliance: the outdoor unit, a buffer tank, pumps, controls, a backup and a plan for hot water and cooling. It is priced per house and there is no honest square-foot number.
How It Works, and Why Water Temperature Is Everything
The common residential type is a monobloc: the whole refrigerant circuit is sealed inside the outdoor unit at the factory, and only water, or water with glycol, runs through the pipes into the house. Inside, the water goes to a buffer tank and from there to the manifolds. The refrigerant work happens at the factory, not in your basement, and that changes who you need on the job, which I come to below.
A heat pump works harder the bigger the gap between the outdoor air and the water it is making. A radiant slab in a tight house runs on water not much warmer than a bath, which keeps that gap small. Cast iron radiators and old baseboard want much hotter water, and a heat pump making hot water in a cold January is a horse pulling uphill: it gets there, slowly and expensively. That is why I only suggest air-to-water for a radiant floor or a low-temperature system, and why the radiant design has to be done with a low supply temperature in mind from the start.
Output at Minus 25, and the Backup
Simcoe County design temperatures run about minus 24 to minus 25 C. An air-source heat pump makes less heat the colder it gets outside, so the number that matters is not the big one on the brochure. The manufacturer publishes output tables by outdoor temperature and by water temperature. Ask the installer for the capacity at your design temperature and your design supply-water temperature, in writing, and put it beside the room-by-room CSA F280 heat loss.
The gap between the two is the backup. It is usually an electric element in the buffer tank or the unit, or, in a hybrid, a gas or propane boiler that takes over on the coldest nights. A tight ICF house helps here more than anything else: a 2,000 sq ft frame house in Simcoe might lose 40,000 to 60,000 BTU/h at design, and the same house in ICF built to our standard about half that. Half the load means a smaller unit, less backup and less time running the expensive heat.
Rule of thumb: never let anyone size an air-to-water unit from square footage. It is sized from a CSA F280 heat loss at your design temperature, and the backup covers whatever the table says it cannot.
Buffer Tank, Defrost, Hot Water and Cooling
The buffer tank is a tank of heating water between the heat pump and the floors. It stops the compressor short-cycling when only one small zone calls, a car driven only to the end of the driveway, and it gives the unit something to borrow from during defrost.
Defrost. In cold, damp weather the outdoor coil frosts up. The unit reverses for a few minutes and uses warm water from the system to melt it off. The meltwater has to drain away from the unit, and the unit has to sit up off the ground where snow and drift will not bury it. An outdoor unit sitting in its own ice pile is the most common thing I see wrong with these.
Hot water. Most systems heat the domestic water through a dedicated tank with a coil, with an element to finish the job on cold days. Keep the expectations honest: a heat pump makes hot water slowly, so the tank does the work at shower time.
Cooling. Many units can reverse and make chilled water, which goes to fan coils or a small air handler. Cooling through the floor itself is limited by condensation, and the radiant floor cooling page explains why. A radiant house still needs an HRV, and many still get a ductless head or two for the summer.
What It Costs
| Item | Range | Note |
|---|---|---|
| Air-to-water plant: monobloc outdoor unit, buffer tank, pumps, controls, backup | $15,000 to $35,000 and up | quoted per house, never per sq ft |
| Hybrid: heat pump plus a gas or propane boiler | $25,000 to $55,000 and up | quoted per house |
| Dedicated 240 V circuit and disconnect | $500 to $1,500 | ESA licensed contractor, permit |
| Panel upgrade to 200 A, if the service will not carry it | $2,500 to $4,500 | |
| Radiant tubing in a new slab | $6 to $12 per sq ft | separate from the plant |
| CSA F280 heat loss / full HVAC design and drawings | $395 / $695 | OntarioHeatLoss.ca |
Why Ontario prices are all over the map: on air-to-water, the spread is wide and the published numbers are thin. A Toronto contractor’s guide gives $15,000 to $35,000 and up, and nobody we checked publishes a tighter figure, because every system is different: the size of the outdoor unit, the buffer and hot water tanks, the backup, the controls, and whether cooling is part of it. A one-truck hydronic shop and a company with a showroom will quote the same house very differently. Budget the middle, $25,000, until a design is on paper, then get two or three quotes on the same written scope.
Worked through for a 2,000 sq ft ICF house with the slab tubing already priced: the plant at $25,000, a 240 V circuit at $1,000, and the full HVAC design and mechanical drawings at $695 from OntarioHeatLoss.ca. That is $26,695 before HST, $3,470 HST, $30,165 total. If the house is on a 100 A service, add $2,500 to $4,500 for the panel; the 100 or 200 amp page helps you find out.
Who Installs It, and the Rebate Question
It is three trades. The water side is hydronic piping. The electrical is done by an ESA licensed electrical contractor under an ESA permit. Any work on the refrigerant circuit, which on a monobloc should be rare, is done by someone with an Ozone Depletion Prevention card and a 313A or 313D certificate on the Skilled Trades Ontario register; the ODP card on its own does not allow work on the equipment. If a backup boiler is in the system, that part is gas work for a TSSA-registered contractor with a certified gas technician. Ask for every number before anyone starts.
On the rebate: the Home Renovation Savings heat pump stream pays for cold-climate air-source heat pumps on the NRCan qualified products list, by fuel and per ton of capacity at 8.3 C, with pre-approval through a participating contractor before installation. Whether your air-to-water model is on that list is something to check by model number, not assume. New builds do not qualify; a house occupied six months or less counts as new. If you qualify, the amount is on the program site, and check the program site before you sign.
Questions people ask
How much does an air-to-water heat pump cost in Ontario?
The plant, which is the outdoor unit, buffer tank, pumps, controls and backup, runs $15,000 to $35,000 and up, installed, before HST. A hybrid with a boiler is $25,000 to $55,000 and up. The floor tubing is separate. Every one is quoted per house.
Does an air-to-water heat pump work at minus 25 C?
It keeps running, with less output and lower efficiency the colder it gets. The output table at your design temperature and supply-water temperature, held against a CSA F280 heat loss, tells you how much backup you need.
Can it heat my existing radiators?
Hot-water radiators sized for a boiler usually want hotter water than a heat pump makes comfortably in January. It suits radiant floors and other low-temperature systems far better. A heat loss and a look at the radiators tells you whether yours can run cooler.
Can an air-to-water heat pump do air conditioning?
Many can make chilled water for fan coils or a small air handler. Cooling through the floor is limited by condensation. Many radiant houses use a ductless head or two instead.
Does it qualify for the Ontario heat pump rebate?
Only if that exact model is on the NRCan qualified list and the house and the contractor meet the program rules, with approval before installation. New builds do not qualify. Check the program site before you sign.
Put air-to-water beside the other four systems
The HVAC system cost comparison calculator prices the same house five ways, including air-to-water with radiant, and shows the installed cost and 15-year cost side by side. If you have quotes, the $99 review reads up to three of them and tells you what each one left out.
Sources and further reading
- Heating and Cooling With a Heat Pump: Natural Resources Canada: how air-source, ductless, ground-source and air-to-water heat pumps work in a Canadian climate, sizing, backup heat and balance points.
- NRCan Searchable Product List: where you check that the exact outdoor and indoor model numbers on a quote are a listed cold-climate heat pump.
- Home Renovation Savings: heat pumps: the heat pump rebate by existing heating fuel, the per-ton rule, pre-approval through a participating contractor and the new-build exclusion.
