air/water heat pump

An air-to-water heat pump captures heat from the outside air and transfers it to a water circuit for heating (underfloor heating, radiators, fan coil units) and often domestic hot water. Its advantages include simple installation (no drilling or groundwater required), good seasonal efficiency, and quick setup for both renovations and new builds. However, its performance varies with the outside temperature: excellent during the shoulder seasons, but more demanding in very cold weather (for defrosting and supplemental heating), hence the importance of careful sizing and operation.

Air/water heat pump: definition and operation

A typical installation comprises an outdoor unit (evaporator, compressor, fan) and an indoor unit (plate heat exchanger, circulator, controller), connected to a hydraulic network. The heating curve adjusts the flow temperature according to the weather; a buffer tank limits cycling; 2/3-way valves distribute the flow between circuits (underfloor heating, radiators, domestic hot water).

Performance is measured via COP (instantaneous) and SCOP (seasonal). A bivalent point is often defined: below a certain outside temperature, a backup system (electric, existing boiler) takes over, either partially or completely. Key technical points include: defrosting by cycle reversal (comfort impacts to manage), electrical quality (protections, starting currents), acoustics (location, screens), hydraulics (minimum heat pump flow rate, target temperature difference), and suitable emitters (ideally low temperature). For domestic hot water, a tank with a suitable heat exchanger, an anti-legionella strategy, and a mixing valve at the outlet are recommended.

Advantages, limitations and points to consider regarding air/water heat pumps

Interests

• Rapid decarbonization as a replacement for an oil/gas boiler.
• Flexible installation (no drilling), relatively simple maintenance.
• Controlled OPEX during the mid-seasons thanks to a high SCOP.
• Compatible with underfloor heating and properly sized radiators.

Boundaries

• Yield decreases when it is very cold (COP ↓).
• Thawing may create brief dips in heating.
• Noise level and location to be addressed (neighborhood, vibrations).
• Additional funding may be required depending on climate and high-temperature DHW needs.

Points to consider

• Sizing: actual power at -7/-10 °C, relevant bivalent point.
• Hydraulics: buffer tank, balancing, minimum flow rate, sludge traps.
• Regulation: water law, outdoor sensor, anti-cycling, DHW priority.
• Electricity: protections, possible load shedding, power contracts.
• Acoustics: anti-vibration base, screens, distance from windows.
• Emitters: aim for low temperature (or review surfaces/instructions).
• Maintenance: clean heat exchangers, defrost checks, firmware updates.

Anecdote — “The frost didn’t kill Nantes”

In Nantes, a small office building wanted to switch off its gas heating system without major renovations. A 30 kW air-to-water heat pump was installed, along with a buffer tank, a weather-compensated control system, and radiators recalibrated to 50/40°C. An electric backup heater only activates below -3°C. During a three-day cold snap, the heat pump handled most of the heating load; the backup heater only ran for one hour in the morning to boost the system. The result: stable comfort, lower bills compared to gas, and happy neighbors—the outdoor unit was placed behind a sound barrier. The moral of the story: with a well-chosen dual-fuel system and clean hydraulics, the Atlantic air is more than sufficient.