Can I use an Existing Hot Water Cylinder with a Heat Pump?

Can I use an Existing Hot Water Cylinder with a Heat Pump? While you can sometimes use an existing hot water cylinder with an air source heat pump, the cylinder and its heat exchanger usually need to be up to the task, which isn’t always the case. Lots of cylinders originally designed for gas or oil boilers have relatively small coils that just aren’t suitable for the lower operating temperatures and heating output of a heat pump. Many times replacing the existing cylinder with a heat pump cylinder designed with the heat pump in mind is the better option.

Why an Existing Cylinder Might Not Be Suited

The main issue is usually the coil itself inside the cylinder.

Traditional boilers can supply relatively high-temperature water to the coil because there’s a big temperature difference between the primary heating water and the stored domestic hot water. This still allows a relatively small coil to transfer the heat effectively.

An air source heat pump, on the other hand, is most efficient at lower flow temperatures. This means there’s less temperature difference available to drive the heat through the coil.

Heat pump cylinders have larger coils with a bigger heat transfer surface area, which is why they tend to perform better.

What Happens if the Cylinder Coil is Too Small?

If the coil is too small, the heat pump won’t be able to transfer heat into the stored water fast enough. For example, the heat pump might be capable of delivering a certain amount of heat, but the existing coil might not be able to transfer all that heat at the intended flow temperature.

This can lead to longer hot water recovery times, higher heat pump flow temperatures, a lower COP (Coefficient of Performance) and longer compressor running times. The system may still produce hot water, but that doesn’t necessarily mean it’s operating efficiently.

When selecting a heat pump water heater, both storage capacity and expected hot water usage need to be considered. Our compact option is the 100 litre wall mounted domestic air source water heater.

For larger household requirements, consider the 200 litre domestic air source hot water system or 300 litre domestic air source hot water system.

For higher-volume applications, explore the 400 litre commercial air source hot water system and 500 litre commercial air source hot water system.

How Do You Know if Your Existing Cylinder is Suitable?

The best place to start is with the cylinder’s technical data. Ideally, you need to know the coil surface area and heat exchanger performance, rather than just the cylinder capacity.

A 250-litre cylinder could have a big heat exchanger designed for a heat pump or a tiny coil designed for a boiler. Storage capacity alone tells you very little about whether it’s suitable.

The coil needs to be matched to the heat pump’s output and the expected primary water temperatures.

If the cylinder manufacturer provides heat exchanger performance figures at different flow temperatures, these can be particularly useful when assessing compatibility.

Does the Age of the Cylinder Matter?

Age by itself doesn’t determine whether it needs replacing. An older cylinder may have a suitable coil, but you should also consider its insulation, condition and how much heat it loses when standing still.

Modern hot water cylinders generally have good insulation. Reducing standing heat loss means less energy is required to keep the stored domestic hot water at the right temperature.

If an existing cylinder is already quite old, replacing it at the same time as the heat pump can make a lot more sense than adapting the new system around an old inefficient cylinder.

Do I Need a Larger Cylinder When Changing to a Heat Pump?

Not automatically, although cylinder capacity should be reviewed.

Heat pumps often have a slower hot water recovery time than a high-output boiler. It can be useful to have enough stored hot water to cover periods of peak demand rather than relying on very rapid reheating.

The right capacity depends on how many people live in the house, how many bathrooms and showers there are, and the household’s pattern of hot water use.

Oversizing should be avoided, too. A larger cylinder takes up more space, costs more and may have greater total standing heat losses.

What About the Immersion Heater?

If the existing cylinder has an immersion heater, this can provide backup, but it shouldn’t be relied on to compensate for an unsuitable heat exchanger.

An immersion heater uses direct electric resistance heating. At the point of use, approximately 1 kWh of electricity produces 1 kWh of heat.

A heat pump with a good COP, on the other hand, could provide around 3 kWh of useful heat from 1 kWh of electrical input.

Relying on the immersion heater because the cylinder can’t recover quickly enough can undermine much of the energy-saving benefit of installing a heat pump.

What if I Have an Unvented Cylinder?

An existing unvented hot water cylinder may be suitable, but the same questions about coil performance, capacity and heat loss still apply.

The cylinder and associated safety equipment also need to remain suitable for the installation, including the expansion arrangement, temperature and pressure protection, discharge pipework and other required safety controls.

Changing the heat source doesn’t get rid of these requirements.

Should I Keep My Existing Cylinder or Replace it?

Keep the existing cylinder if it’s in good condition, has enough storage capacity and its heat exchanger can transfer the required heat pump output at the intended operating temperatures. If the coil is too small, recovery is poor or the cylinder has high standing heat losses, a purpose-designed heat pump cylinder is usually the better choice. The key here is whether an existing cylinder can be paired with an air source heat pump, but the real issue is this: will it actually work pretty well together? What you need to know is – will the heat exchanger, cylinder capacity and output of the heat pump match up nicely? – because when they do, it can make a world of difference to getting your hot water back on track, keeping your electricity bills low and the performance of the system on the up.