Domestic hot water from a heat pump: why cylinder volume alone is not enough for selection

The proposal specifies a 300-litre hot water cylinder. The client assumes that this will be enough and that the choice of heat pump has already settled the question of capacity.

In reality, stored water volume, recovery rate and heat transfer between the heat pump and the cylinder are separate parameters. A large cylinder with an unsuitable heat exchanger can take a long time to heat up, even with a high-output unit.

Calculating more than litres

Assume that 300 litres of water is heated from +10 to +55 °C. The thermal energy required, excluding losses, is:

E=300×0,001163×(55-10)≈15,7 kWh

If the average heat output actually transferred to the water during this process is 6 kW:

t=15,7/6≈2,62 h

This is the idealised heating time for the entire specified volume. The actual process depends on the initial conditions, water draw-off, losses and changes in output. The value of +55 °C is used here for an energy calculation example, not as a universal recommendation for storing potable water.

Why a suitable heat exchanger is needed

A heat pump may operate with a smaller temperature difference between the heat transfer fluid and the water in the cylinder than a boiler operating at high temperature. The heat exchanger must have sufficient capacity to transfer the required output. Caleffi specifically highlights its selection for indirect water heating with a heat pump. (Caleffi — Design Details for Air-to-Water Heat Pump)

I would not size the coil surface area solely using a ‘m² per kilowatt’ rule. We need heat transfer performance data at our operating temperatures and flow rates. Manufacturers offer storage solutions specifically designed for heat pumps, but their capacity and compatibility still need to be checked for the application. (NIBE — PUB storage tanks)

Mixed water volume is no guarantee either

With ideal mixing of 300 litres of water at +55 °C with water at +10 °C to obtain +40 °C, the mathematical equivalent is:

V₍40₎=300×(55-10) / (40-10)=450 l

However, the actual usable volume depends on temperature stratification in the cylinder, sensor position, draw-off design and cold water mixing. So 450 litres is the result of an energy balance, not the guaranteed rated hot water capacity of a particular cylinder.

For a hotel or sports facility, the demand profile also matters: a large, short peak may require different storage from the same daily consumption spread evenly over time.

What happens to space heating while the hot water cylinder is heating up

In many configurations, domestic hot water takes priority and heat supply to the space heating system is temporarily interrupted. This sequence is described, for example, in the NIBE S1155 documentation. It is a feature of a particular system architecture, not of all heat pumps. (NIBE — S1155 system solutions: cascades and domestic hot water)

We therefore check the acceptable interruption duration, the building’s thermal inertia and whether another unit can be brought into operation. For a large building, it can sometimes make sense to assign space heating and domestic hot water to separate heat sources.

Hygiene cannot be adjusted to suit a target COP

The design must coordinate the temperature regime, measures to prevent stagnation, recirculation operation and scald protection. Standard solutions may include thermal disinfection functions and supplementary electric heating. These should not be disabled to achieve an attractive efficiency figure. (NIBE — air-to-water solutions and domestic hot water production) (HSE — hot water system risks and control measures)

The client is not buying a cylinder of a particular volume, but hot water availability: the required quantity, temperature and recovery time, without an unacceptable impact on space heating.