Air-to-water heat pump
A heat pump that transfers heat from outside air into a water circuit serving radiators, underfloor loops, or a hot water cylinder. This is the common domestic arrangement in the United Kingdom and much of Europe, where it replaces a boiler rather than a ducted system.
Also called hydronic heat pump, monobloc heat pump
What is an air-to-water heat pump?
An air-to-water heat pump takes heat from outdoor air and puts it into a water circuit, so it heats radiators, underfloor pipes, and a hot water cylinder in place of a boiler.
Air-to-water heat pump: how it works
The outdoor unit extracts heat from air and transfers it through a plate heat exchanger into the heating water. Because the efficiency of the cycle falls as the water is pushed hotter, the design aims at the lowest flow temperature the emitters can work with rather than the boiler temperatures the house may have been built around.
Where you meet the term
- On a heat loss survey, where emitter sizing is recalculated because the system will run at a lower flow temperature than the boiler did.
- In a running cost complaint, where an immersion heater or a backup element has been doing work the heat pump was meant to do.
- In cylinder documentation, where a weekly high temperature cycle is specified for hot water hygiene.
How to check it
- Read the flow and return temperatures. Both are shown on the controller. The difference between them, and how high the flow runs, says more about efficiency than any single reading does. Owner check
- Establish whether weather compensation is active. A fixed flow temperature set at commissioning and never revisited is a common reason a correctly installed system costs more to run than expected. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| Design flow temperature, underfloor | Around 35 C | Low flow temperature suits the large surface area of an underfloor circuit |
| Design flow temperature, radiators | Around 45 to 55 C | Requires radiators sized for the lower temperature rather than boiler-era sizing |
| Boiler flow temperature for comparison | Around 60 to 80 C | The temperature most existing radiator systems were originally sized around |
United States and United Kingdom usage
- United Kingdom
- The standard domestic heat pump arrangement, installed under MCS or equivalent scheme rules and quoted with SCOP.
- Europe
- Widely installed with weather compensation as standard practice, rated under the ecodesign framework.
Air-to-water heat pump: what it is not
- It is not a like-for-like boiler swap. Emitters sized for boiler temperatures may need enlarging before the system can run at its efficient flow temperature.
- Monobloc and split are not the same. A monobloc holds the refrigerant circuit outdoors and carries water indoors, which changes both the freeze protection and the certification a service visit needs.
Air-to-water heat pump questions
What is the difference between monobloc and split?
A monobloc keeps the whole refrigerant circuit in the outdoor unit and sends water indoors. A split sends refrigerant to an indoor hydrobox, which puts the refrigerant joint inside the house and brings F-Gas duties with it.
Can it heat hot water as well as the rooms?
Yes, through a cylinder with a heat pump coil. The system switches between space heating and cylinder heating, and the cylinder runs a periodic higher temperature cycle for hygiene.
Why is my flow temperature so much lower than my old boiler?
Efficiency falls as the water is driven hotter, so a heat pump is set to deliver the lowest flow temperature the building needs. That is why weather compensation and correctly sized emitters matter.
Where this definition comes from
Written from government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Energy label and ecodesignThe European framework behind SCOP, seasonal space heating efficiency, and the product energy label.
- Heat Pump SystemsConsumer-facing explanation of air source, ductless, and geothermal heat pump operation and efficiency metrics.