Domestic Hot Water Heat Pump (Booster)

A standalone heat pump for hot water only — using ambient air, for example from the basement, as its source.

How does a domestic hot water heat pump work?

A domestic hot water heat pump, also called a hot water booster heat pump, is a stand-alone heat pump that produces hot water only. Its heat source is the ambient air in the room where it is installed, usually the basement or plant room. A small refrigeration circuit extracts heat from this air and raises it to 50 to 55 °C in the integrated cylinder of 300 to 500 litres. Efficiency is described by the seasonal performance factor (SPF), the ratio of heat produced to electricity consumed over a year; it is typically 2.5 to 3.5. In summer, the hot water heat pump replaces the boiler entirely, which would otherwise have to run for hot water alone at a poor part-load efficiency. A side effect: the basement air is cooled and dehumidified. Combined with a photovoltaic system, the heat pump can be run deliberately during surplus generation and the cylinder used as a thermal battery.

Which buildings benefit from a domestic hot water heat pump?

Typical properties are detached houses, small multi-family buildings and commercial premises with moderate hot water demand. The prerequisites are a frost-free installation room with at least 20 m³ of air volume and sufficient air circulation so the heat pump can extract enough heat, plus an electrical connection. Alternatively, the air can be drawn through ducts from an adjacent room. The switch is particularly economical where hot water has so far been produced by an old oil or gas boiler in summer mode or by an electric immersion heater. If an efficient gas boiler is only a few years old, the saving is smaller.

What does a domestic hot water heat pump actually deliver?

Final energy demand for hot water falls by 50 to 65 %. The saving is derived from hot water demand and the difference between the reciprocals of the old efficiency and the SPF of the heat pump. A detached house with 2,500 kWh/a hot water demand and an old oil boiler saves around 3,400 kWh/a of final energy and roughly 0.9 t CO2, while the heat pump itself needs only around 780 kWh of electricity. Investment is €2,500 to €4,500 for a unit with a 300-litre cylinder. With a service life of 18 years, the measure pays back in 8 to 14 years, and correspondingly faster with PV self-consumption.

Energy carrier

Erdgas, Heizöl, Strom → Strom

Savings potential

50-65 %

Worked example

A detached house with four occupants has a hot water demand of 2,500 kWh per year. So far, an old oil boiler has produced the hot water, achieving an efficiency of only 0.60 in summer mode because it cycles frequently and standby losses are high. A domestic hot water heat pump with a 300-litre cylinder is installed in the frost-free basement, achieving a seasonal performance factor of 3.2. The saving is calculated as E_saved = 2,500 × (1/0.60 − 1/3.2) = 2,500 × (1.667 − 0.313) = 2,500 × 1.354 = 3,385 kWh/a of final energy. The new electricity consumption of the heat pump is 2,500 / 3.2 = 781 kWh per year. On balance, the house saves around 3,400 kWh of final energy and roughly 0.9 t CO2 per year, and the oil boiler stays completely off in summer. A separate electricity meter and cylinder temperature sensors make the real SPF verifiable.

Investment & payback

2.500-4.500 € (300 l Speicher inkl. WP) · Service life: 18 a · Payback: 8-14 a

Applicability

Requirements

Typical buildings

Frequently asked questions

Is a domestic hot water heat pump worth it?

A domestic hot water heat pump is worthwhile above all where hot water has so far been produced by an old oil or gas boiler in summer mode or by an electric immersion heater. Final energy demand for hot water falls by 50 to 65 %, and payback is 8 to 14 years. With a photovoltaic system, payback shortens because the heat pump uses surplus solar electricity.

How much electricity does a domestic hot water heat pump use per year?

A four-person household with a hot water demand of 2,500 kWh per year needs around 780 kWh of electricity annually at a seasonal performance factor of 3.2. Depending on the unit and installation room, the SPF ranges from 2.5 to 3.8. An electric immersion heater would use more than three times as much electricity for the same amount of hot water.

How much does a domestic hot water heat pump cost installed?

A domestic hot water heat pump with an integrated 300-litre cylinder costs €2,500 to €4,500 including installation. The service life is around 18 years and the payback period 8 to 14 years. Since only an electricity and water connection are needed, installation costs are low. Subsidies for heating system replacement can reduce the investment further.

What does the installation room need for a domestic hot water heat pump?

The installation room must be frost-free and offer at least 20 m³ of air volume with sufficient air circulation so the heat pump can extract enough heat from the air. Basements, plant rooms or utility rooms are suitable. The room cools by a few degrees during operation and is dehumidified. For smaller rooms, the air can be drawn through ducts from an adjacent room.

Which standard is used to test the efficiency of domestic hot water heat pumps?

ÖNORM EN 16147 specifies the test procedure for heat pumps for domestic hot water production, including the coefficient of performance (COP) at defined tapping profiles. This value allows different units to be compared under laboratory conditions. The real seasonal performance factor in operation additionally depends on the installation room temperature, cylinder temperature and user behaviour, and is typically 2.5 to 3.5.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Optimal: Stromzähler BWP separat, Speichertemperaturen oben/unten, Kaltwasserzähler. Damit ist JAZ ermittelbar (Wärmemenge = m × cp × ΔT / Δt).

All measures