Heat Pump Hot Water
How heat pump hot water systems work, and why they are the right choice for high-performance, all-electric homes.
How do heat pumps produce hot water?
A heat pump hot water system works like a reverse refrigerator. Instead of moving heat from inside a box to the outside air, it extracts heat from the surrounding outdoor air and transfers it into the water in the storage cylinder.
This process uses electricity to drive a compressor, but generates significantly more heat energy than the electrical energy consumed — typically 3–5 times more. This ratio is called the Coefficient of Performance (COP).
For every 1 kWh of electricity used, a heat pump might generate 3–5 kWh of heat energy. Compare this to a direct electric element, which generates exactly 1 kWh of heat per kWh of electricity — heat pumps are 3–5 times more efficient.
Why CO₂ heat pumps?
Most heat pumps use synthetic HFC refrigerants — which work well in moderate temperatures but lose efficiency in cold weather. CO₂ (also written as R744) is a natural refrigerant that maintains high efficiency at low outdoor temperatures, making it better suited to climates with cold winters.
CO₂ also has a Global Warming Potential (GWP) of 1 — compared to 1,000–2,000 for common HFC refrigerants. This makes CO₂ systems significantly better from an environmental perspective.
For high-performance homes and Passive House projects where environmental credentials matter, CO₂ heat pump hot water is the appropriate choice.
Integration with solar PV
Heat pump hot water systems are well-suited to pairing with solar PV. Because hot water demand is flexible (it can be heated at any time of day and stored), the system can be scheduled to run during peak solar generation periods — using cheap or free solar energy rather than grid electricity.
Smart controllers and solar diverter systems can automate this process, maximising solar self-consumption and minimising grid import.