An air-to-water heat pump (AWHP) extracts heat from outdoor air and delivers it to a water or glycol loop. In heating mode it replaces or supplements a boiler; most units can also reverse to produce chilled water in summer. Because many commercial buildings already distribute heat with hot water, AWHPs are one of the most direct routes to electrifying an existing hydronic plant.
The technology is mature in Europe and Asia and increasingly common in Canada. Its success in a given building depends mostly on two things: the water temperature the building needs, and how cold it gets outside.
How they work
An AWHP is a packaged refrigeration circuit with an outdoor air coil, a compressor, an expansion device and a water-side heat exchanger. In heating mode, refrigerant evaporates in the air coil, absorbing heat from outdoor air even at sub-zero temperatures, then condenses in the water heat exchanger, releasing heat to the loop.
Units range from small modules to large multi-compressor machines. Many projects use several modular units in parallel, which provides redundancy and good part-load efficiency. Some designs are four-pipe or multi-function units that can produce hot and chilled water simultaneously.
Refrigerants
Common refrigerants include R-410A, R-32, R-454B, R-513A, R-290 (propane) and R-744 (CO2). Canada's HFC phase-down under the Ozone-depleting Substances and Halocarbon Alternatives Regulations is pushing manufacturers toward lower global warming potential options. CO2 units can produce much higher water temperatures and suit domestic hot water and some high-temperature heating loads, but they perform best with a large temperature rise and low return water temperature. Refrigerant classification and charge also affect where a unit may be placed under CSA B52.
Supply water temperature is the key constraint
The efficiency of any heat pump falls as the temperature lift between source and supply increases. An AWHP making 40°C (104°F) water on a mild day may achieve a COP well above 3. The same unit making 60°C (140°F) water at minus 20°C (minus 4°F) outdoor air may struggle to exceed a COP of 2, and may not reach that temperature at all.
| Supply water temperature | Typical suitability |
|---|---|
| 30 to 45°C (86 to 113°F) | Radiant floors, low-temperature fan coils, DOAS coils; good AWHP efficiency |
| 45 to 55°C (113 to 131°F) | Upsized coils and radiation; workable for most modern AWHPs |
| 55 to 65°C (131 to 149°F) | Possible with some units, reduced efficiency and capacity in cold weather |
| Above 65°C (149°F) | Generally requires CO2 units, cascade systems or boiler assistance |
Many older Canadian buildings were designed for 82°C (180°F) supply water with an 11°C (20°F) temperature drop. Running those systems at 50°C can reduce the output of fin-tube radiation and coils substantially. Before committing to an AWHP, confirm what temperature the building really needs.
Testing an existing system
- Review BAS trends of boiler supply temperature and terminal valve positions during the coldest recent weeks.
- Implement or lower the outdoor air reset curve in stages, watching for comfort complaints and valves at 100 percent open.
- Identify the zones that limit low-temperature operation; these are often perimeter radiation in older areas or heating coils in air handlers serving high outdoor air fractions.
- Evaluate fixes for those zones only: coil replacement, added radiation, envelope improvements or energy recovery on ventilation.
- Rebalance the hydronic system for the new flows and temperature differences.
The design of hot water systems is covered further in heating hot water system design.
Sizing in cold climates
AWHP heating capacity drops as outdoor temperature falls, while building heat loss rises. Manufacturer data must be read at the actual design conditions: the local design temperature from the NBC climatic tables, the required supply water temperature, and the glycol concentration if used. Defrost cycles also reduce delivered capacity in humid conditions near and below freezing.
Common sizing approaches:
- Monovalent: AWHPs sized for the full design load. Large, expensive, and inefficient at the coldest hours on the Prairies, but avoids combustion entirely.
- Bivalent parallel: AWHPs carry the load down to a balance point, with a boiler or electric boiler supplementing below it. Heat pumps often deliver the large majority of annual heating energy even when sized well below peak load, because most heating hours are at moderate temperatures.
- Bivalent alternate: AWHPs operate down to a cutoff temperature, then the backup plant takes over completely.
Selecting the balance point is an economic and carbon decision informed by an hourly bin analysis or energy model.
Freeze protection
Outdoor units contain water heat exchangers that can freeze during a power failure or pump failure. Options include glycol in the AWHP loop with a heat exchanger to the building loop, glycol throughout the system, or freeze protection valves and heat tracing. Glycol reduces heat transfer and increases pumping energy, so concentration should be chosen for actual risk. See glycol systems.
Integration with existing boilers
Most commercial AWHP projects keep at least some boiler capacity. Integration details that matter:
- Piping arrangement: return water should reach the heat pump first, since it operates more efficiently at lower entering temperatures. The boiler then lifts the supply temperature when needed.
- Buffer tanks: help prevent short cycling and provide volume for defrost.
- Control sequencing: the BAS should lead with heat pumps and stage boilers only when heat pumps cannot meet setpoint. A frequent fault is boilers firing first or heat pumps locking out at unnecessarily warm temperatures.
- Flow rates: AWHPs often require minimum flow and a defined temperature difference. Constant-flow primary loops or decoupled arrangements may be required.
Placement and site issues
AWHPs need free airflow around coils, clearance above snow, and drainage for defrost meltwater. Roof installations need structural review for weight and vibration isolation. Fan and compressor noise must be checked against local bylaws, particularly at night when heating demand is high.
Frequently asked questions
Can an AWHP also provide cooling?
Most units are reversible and can produce chilled water. Some four-pipe units serve heating and cooling loops at the same time, which is useful in buildings with interior zones that need cooling all year.
What COP should I expect?
It depends on outdoor and water temperatures. Seasonal performance is more meaningful than a single rated point. An hourly analysis using local weather and the building's reset schedule gives a realistic seasonal figure.
Are AWHPs a good fit for very cold sites?
They can be, but usually in a bivalent arrangement with backup heat. For sites with land available, a ground-source system may offer more stable winter performance.
Key takeaways
- AWHPs are a direct route to electrifying buildings that already use hot water heating.
- Efficiency and capacity depend heavily on supply water temperature and outdoor temperature.
- Test whether the building can run at lower water temperatures before selecting equipment.
- Bivalent designs with a backup boiler are common and can still deliver most of the annual heating energy.
- Piping, buffer tanks, freeze protection and control sequencing determine real-world performance.
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