What is a dedicated DHW heat pump?

A dedicated domestic hot water (DHW) heat pump is a self-contained unit that heats water using an integrated heat pump compressor. It extracts heat from surrounding air (typically a garage or utility room) and delivers it to a built-in or connected cylinder. Units are typically 1.5-3 kW and can be retrofit without major plumbing changes.

What COP does a DHW heat pump achieve?

DHW heat pumps typically achieve COP 2.0-3.0 when heating water to 55C. COP is lower than space-heating heat pumps because of the higher target temperature. COP is better in warmer locations (plant rooms) and when heating to lower temperatures (45-50C with thermostatic mixing valves).

How much hot water does a DHW heat pump produce?

A typical 2 kW DHW heat pump can reheat a 200-litre cylinder from cold in approximately 4-6 hours. For daily use (partial reheat), recovery time is typically 2-3 hours. This is adequate for most households but may require scheduling for large families.

Where should a DHW heat pump be installed?

An unheated space like a garage is ideal — the heat pump extracts heat from the air, cooling and dehumidifying the space. A utility room or plant room also works. The space should be at least 20 m3 and well-ventilated. The unit produces cool, dry exhaust air which can be beneficial in summer.

Is a DHW heat pump better than an immersion heater?

Yes. An immersion heater has a COP of 1.0 (100% efficient). A DHW heat pump achieves COP 2.0-3.0, meaning it uses 50-66% less electricity for the same amount of hot water. Annual savings are typically £150-300 for a family of 3-4. It also provides dehumidification in the installed space.

TradeCalculator

Hot Water Heat Pump Calculator

Size a dedicated DHW heat pump from daily litres demand. Calculate cylinder pairing, COP at hot water temperatures, and running costs.

Number of Occupants

Daily Hot Water (optional)

Leave blank for estimate

Installation Location

Warmer ambient = better COP

Current System

How We Calculate This

This calculator sizes a dedicated DHW heat pump based on your daily hot water consumption and compares running costs with your current water heating system.

Sizing method

Daily demand: ~50 litres per person at 55C
Energy needed: Litres x 4.186 x delta-T / 3600 = kWh
HP capacity: Sized for 4-hour full reheat
COP: Adjusted for target temperature and ambient conditions

Frequently Asked Questions

Related Calculators

Air Source Heat Pump Sizing Calculator

Size air source heat pumps from whole house heat loss with defrost derating.

Ground Source Heat Pump Calculator

Calculate ground source heat pump borehole depth or trench pipe loops per kW.

Heat Pump COP Calculator

Calculate heat pump COP and seasonal COP with running cost comparison vs gas.

Heat Pump Radiator Calculator

Re-size radiators for heat pump 45C flow with de-rating factor.

Solar PV Array Calculator

Calculate solar PV array size, orientation yield factor and annual kWh by UK region.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.