Guide: heat pump
Air-to-water heat pump: what to know before you buy
How an air-to-water heat pump works, how efficient it is in new and old buildings, what applies to noise protection and permits, and how it differs from ground-source and air-to-air heat pumps.
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Photo: own installation
How an air-to-water heat pump works
The heat pump extracts heat from the outside air – even at sub-zero temperatures. A refrigerant circuit with evaporator, compressor and condenser raises this heat to a higher temperature and transfers it to the heating water. As with oil or gas heating, the water flows through radiators or underfloor heating and can also heat domestic hot water.
Electricity mainly drives the compressor. How much heat is produced per kilowatt hour of electricity is described by the seasonal performance factor (SPF, German: JAZ): an SPF of 3 means that, over a year, about 3 kWh of heat are produced from 1 kWh of electricity.
Efficiency: what field measurements show
The seasonal performance factor depends strongly on the flow temperature. Field measurements of more than 20 systems commissioned by EnergieSchweiz (heat pump test centre WPZ Buchs) show:
| Building (flow temperature at design point) | Air-to-water, heating | Brine-to-water (ground source), heating |
|---|---|---|
| New building (35 to 30 °C) | SPF 3.7 | SPF 5.7 |
| Renovated (45 to 40 °C) | SPF 3.3 | SPF 5.0 |
| Old building (55 to 50 °C) | SPF 2.9 | SPF 4.4 |
Air-to-water heat pump in an old building: flow temperature, radiators, underfloor heating
Often yes. What matters is the required flow temperature: according to a study by Energieforschung Stadt Zürich, an air-to-water heat pump is an option if a flow temperature of no more than 55 °C is sufficient at an outside temperature of −8 °C. Up to 65 °C feasibility should be checked; above that it is usually only used as a supplementary system.
Underfloor heating works with low temperatures and is ideal. Radiators also work if they are large enough; sometimes it helps to replace individual radiators, set the heating curve correctly or insulate windows and roof.
Outdoor or indoor installation?
Outdoor installation (monobloc or split): saves space in the house and is the most common choice when replacing heating. Choose the location so that neighbours' windows are as far away or as shielded as possible.
Indoor installation: air is drawn in and blown out through ducts – quieter for the surroundings, but needs space and suitable openings. According to the canton of Zurich, indoor installation as an additional precautionary measure is usually only proportionate in new buildings or if suitable openings already exist.
Heat pumps and noise protection: what applies?
Heat pumps are assessed under Annex 6 of the Noise Abatement Ordinance (LSV). New systems must comply with the planning values at the nearest window of a noise-sensitive room – in residential zones with sensitivity level II these are 55 dB(A) during the day and 45 dB(A) at night. The night is usually decisive.
Since 1 November 2024, the sound power level at an outside temperature of 2 °C has been decisive for air-to-water heat pumps. The proof is prepared according to Cercle Bruit enforcement guide 6.21, for simple cases with the online noise assessment of the Swiss heat pump association FWS. The precautionary principle also applies.
Permit or notification – depending on the canton
| Canton | Procedure (extract) |
|---|---|
| Zurich | Every heating replacement requires a permit (WTA application to the building authority), with a noise assessment for air-to-water heat pumps. |
| Aargau | Replacing the heat generator requires notification; depending on the project, a building application. |
| Lucerne | Replacing the heat generator in residential buildings requires notification (at least 20 days before work starts); depending on building work, a building permit. |
| Zug | Building notice with energy certificate to the municipality, in certain cases a building application. |
| Basel-Stadt | Small outdoor units (up to 2 m³, within the dimensions) without building permit but with notification; larger ones with building permit. |
| Basel-Landschaft | Outdoor air-to-water heat pumps require notification; building permit from 2 m³ volume. |
Details on applications and noise protection in Zurich: Heat pump planning & permits.
Air-to-water heat pump: costs and subsidies
In an example for a single-family home (previously about 2,400 litres of oil per year), EnergieSchweiz calculates CHF 42,000 for purchase and renovation for an air-to-water heat pump and CHF 60,000 for a ground-source heat pump – each before deducting the assumed subsidies. This is an example; the costs for your house depend on output, location, heat distribution and additional work.
Cantons subsidise replacing oil, gas or electric heating with a heat pump – in the canton of Zurich, for example, CHF 2,900 for an air-to-water heat pump up to 15 kWth (2026 programme). Up to about 15 kW many programmes require the heat pump system module (WPSM). Overview: Heat pump subsidies.
Air-to-water heat pump or ground source? Comparison with air-to-air
| Air-to-water | Ground source (brine-to-water) | Air-to-air | |
|---|---|---|---|
| Heat source | outside air | ground via borehole | outside air |
| Heat distribution | radiators, underfloor heating, hot water | radiators, underfloor heating, hot water | indoor units, no water distribution, no hot water |
| Efficiency | good; lower with high flow temperature | highest | depends on the unit |
| Permit | noise assessment | drilling permit; not allowed everywhere | depends on canton/municipality |
| Typical use | heating replacement in single- and multi-family homes | higher demand, quiet location, drilling possible | replacing electric heating, also cooling |
More on the air-to-air solution: Replace electric heating.
Maintenance
A heat pump is low-maintenance but not maintenance-free: keep the evaporator and condensate drain clean, keep leaves and snow away from the outdoor unit, check the heating curve and operating data. Systems with larger refrigerant charges are also subject to inspection and documentation obligations. More: Air conditioning & heat pump maintenance and Refrigerant over 3 kg.
Frequently asked questions
How efficient is an air-to-water heat pump in an old building?
In field measurements, air-to-water heat pumps with a flow temperature of 55 to 50 °C achieved an average SPF of 2.9 in heating mode, and 3.7 in new buildings with underfloor heating.
Does an air-to-water heat pump need a permit?
It depends on the canton: in Zurich every heating replacement requires a permit; in other cantons a notification or building notice is sometimes sufficient. A noise assessment is usual for air-to-water heat pumps.
How loud may the heat pump be?
The planning values of the Noise Abatement Ordinance at the nearest window of a noise-sensitive room are decisive – 45 dB(A) at night in sensitivity level II.
What does an air-to-water heat pump cost?
In an example for a single-family home, EnergieSchweiz cites CHF 42,000 for purchase and renovation before subsidies. A binding figure is only available with a quote for your property.
Can an air-to-water heat pump also cool?
Some units can provide light cooling via underfloor heating. Whether this makes sense for your system is something we clarify during planning.
Sources
- OST/WPZ – field measurement of heat pumps (PDF)
- Energieforschung Stadt Zürich – case studies on air-to-water heat pumps (PDF)
- EnergieSchweiz – heating replacement: expected costs
- Cercle Bruit – enforcement guide 6.21 air-to-water heat pumps (PDF)
- Canton of Zurich – noise protection for new installations
- Canton of Zurich – heating replacement
- Canton of Basel-Stadt – heating replacement
- Canton of Basel-Landschaft – heating replacement
- Canton of Lucerne – KEnG compact
- Canton of Zug – energy regulations
As of October 2026. No guarantee – the regulations and programmes of the federal government, canton and municipality are authoritative.
Plan your air-to-water heat pump
We check flow temperature, location and noise protection, prepare the quote and take care of applications and subsidies.