A DHW heat pump is a compact air-source heat pump factory-integrated with its own storage tank — typically 200–300 litres — dedicated solely to domestic hot water. Drawing heat from ambient or exhaust air, it delivers around 2.5–3.5 kWh of hot water heat per kWh of electricity under EN 16147 testing, roughly a third of the running cost of an electric immersion element.
How does a DHW heat pump work?
A DHW heat pump packages a small air-source refrigeration circuit on top of a hot water cylinder as one appliance: a fan draws air across the evaporator, the compressor lifts the captured heat to storage temperature, and a condenser wrapped around or immersed in the integrated tank heats the stored domestic hot water. It plugs into a normal socket-level electrical supply, needs no outdoor unit and no refrigerant pipework on site, and replaces an electric or gas water heater one-for-one.
Unlike a space-heating air-to-water heat pump, it serves only the hot water load — a few kWh per day — so the compressor is small and reheat is measured in hours, with a built-in electric element available for boost and anti-legionella duty.
What is the difference between ambient-air and exhaust-air models?
The difference is where the evaporator's air comes from, and it changes the economics of the installation:
| Configuration | Air source | Best fit |
|---|---|---|
| Indoor unducted | Room air (cellar, garage, utility) | Rooms with waste heat; cools and dehumidifies the space as a side effect |
| Ducted ambient | Outdoor air via inlet/outlet ducts | Heated homes where robbing room heat is unwanted |
| Exhaust-air | Building ventilation extract | Homes with mechanical extract ventilation — recovers heat that was leaving anyway |
An unducted unit in a boiler room or cellar quietly recycles waste heat and dries the air — useful beside a laundry. In a heated living space, though, it steals heat the heating system paid for, which is why ducting to outside or coupling to exhaust ventilation is preferred in efficient, airtight houses.
How efficient is a DHW heat pump?
Under EN 16147, the European test standard for dedicated hot water heat pumps, typical units reach COP values of roughly 2.5–3.5 — each kWh of electricity delivers two and a half to three and a half kWh of hot water heat, where an immersion heater delivers exactly one. On the EU energy label for water heaters this places DHW heat pumps in class A+ territory, the top of the water-heating scale, while conventional electric storage sits far below. The IEA identifies this multiple as the core case for heat pump water heating, and the technology rides a strong market: European heat pump sales reached 2.88 million units in 2025, up 13% year-on-year (EHPA). Real-world efficiency tracks the air source — warm cellar or exhaust air keeps winter performance high, outdoor ducting costs some COP in cold snaps.
When is a DHW heat pump the right choice?
Choose one when the job is hot water only: replacing an ageing electric cylinder, cutting the summer load of an oil or gas boiler, or pairing with PV to store surplus generation as heat at three times immersion effectiveness. It suits homes that are not ready for a full heating retrofit — no radiators change, no outdoor unit appears — and buildings with usable waste heat or extract ventilation. Solimpeks' Solido is this category: an R290 DHW heat pump with a 200 L or 300 L integrated tank, rated class A+. A full space-heating heat pump makes more sense when the boiler itself is due for replacement, and solar thermal competes directly where roof space and sunshine are generous — the trade-off is weighed in solar thermal or a heat pump for hot water only.
Frequently asked questions
How much can a DHW heat pump save compared with an immersion heater?
Roughly two-thirds of the electricity. At a COP of 3, the heat pump needs 1 kWh of power for hot water that an immersion element would make with 3 kWh. For a household using 3,000 kWh a year of water-heating energy, that is about 2,000 kWh saved annually.
Where should a DHW heat pump be installed?
Ideally in a frost-free utility space with spare heat — a cellar, garage or plant room — where its cooling and dehumidifying side effect is welcome. In heated living space it should be ducted to outdoor air, and in homes with mechanical extract ventilation an exhaust-air model recovers heat already being thrown away.
How long does a DHW heat pump take to reheat its tank?
Several hours rather than the minutes of a boiler — the compressor delivers modest power by design. Controllers schedule reheating outside peak draw times or into off-peak tariff and PV-surplus windows, and the built-in electric element covers exceptional demand.
Does a DHW heat pump handle legionella protection?
Yes. Units run periodic anti-legionella cycles, typically using the backup electric element to lift the stored water to 60 °C or above on a schedule, while normal operation stays at lower, more efficient set points.
Can a DHW heat pump use solar PV surplus?
Yes — it is one of the best PV loads in the house. Many units accept a signal (or simple timer) to run when export would otherwise occur, converting each surplus kWh into roughly three kWh worth of stored hot water heat compared with one from an immersion.