Heat Pumps

Using a heat pump for hot water

Hot water

Assuming that it has been sized and installed correctly, a heat pump can supply more than enough heat for both space heating and hot water. However, there are a few things to be aware of if you’d like to fully reap the benefits and maintain the heat pump’s high efficiency.

Cooler hot water reserve

One of the things that makes a heat pump so much more efficient than other types of heating is its reduced operating temperatures. Domestic heaters that are powered by oil, gas and electricity heat water up to around 70-80°C, whereas heat pumps may only be efficient up to 55°C. Therefore, increasing the surface area of your emitter system (larger radiators and/or underfloor heating) and minimising the heat loss wherever possible with proper insulation makes such a difference.

Will the water in my bath and shower be colder?

Although the heat pump’s temperature is roughly 20°C cooler than other domestic heaters, 30 seconds in a shower at 55°C would still leave you with severe burns, so for those who love their showers to be scolding hot, you won’t be disappointed. The only difference would be that you’d have to turn up the heat a little, as you’d need a greater volume of hot water than before.

Upsizing your hot water cylinder

As the name suggests, a hot water cylinder (or hot water tank) is a reserve of hot water that stands ready for immediate use i.e. when you run a tap, shower or bath. Typically, heat from a high-temperature boiler passes into the cylinder via heat exchanger coils. The problem arises when the boiler is replaced with a low temperature-efficient heat pump. The lower differential (the temperature difference between the flow and return) means that less heat will pass through the coils – the heat pump will think it’s doing its job without delivering enough heat, so the temperature in the cylinder won’t reach the desired level.

The solution to this is to replace your current cylinder with one that is designed for a heat pump. Just like with your emitter system, the main difference with heat pump-specific cylinders is that they have a much larger coil capacity, so there’s an increased surface area for the heat to transfer through the coils and warm the water in the cylinder.

Hot water priority

If there’s a demand for both hot water and space heating at the same time, the heat pump will assume that the hot water is urgent, and leave your space heating until after the hot water cylinder reaches the desired temperature.

Concerns about Legionnaires’ disease

Legionella is a type of bacteria that grows in warm waters. If left unseen to, legionella can cause headaches, coughs and even pneumonia over time. The bacteria never usually cause a problem with common boiler systems since the temperature of the water is too high, but the reduced temperature of heat pump systems make for more favourable conditions. The simple answer to this is to raise the temperature of the heat pump to above 65°C once a day, as hot water at 60°C or above kills legionella.

If for some reason your heat pump can’t raise the temperature to above 60°C, then a secondary heater is required such as an immersion heater, which most cylinders come with as standard.


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