Do I need a special cylinder for an air source heat pump?

Do I need a special cylinder for an air source heat pump? The answer is yes, if your air source heat pump is going to be heating a separate hot water cylinder through a coil, you’ll probably need one designed or sized to work with a heat pump. The thing is, heat pumps work at lower water temps than traditional boilers, so the cylinder needs a heat exchanger coil that’s big enough and effective enough to transfer heat into the hot water efficiently. But if you’re using one of those all-in-one heat pump water heaters, which have the cylinder and heat pump built in, then you don’t need a separate heat pump cylinder.

What makes a heat pump cylinder different?

At first glance, a heat pump cylinder looks pretty much like a normal indirect hot water cylinder. The difference is usually inside the cylinder.

A traditional boiler can chuck out nice hot water to a cylinder coil, and that makes for a nice big temperature difference between the hot heating water and the stored hot water – allowing loads of heat to transfer quickly.

An air source heat pump works best at lower water temps – which is not ideal for heat transfer. So, a heat pump cylinder has to have a bigger surface area coil to compensate.

This increased heat exchanger area lets more heat be transferred from the heat pump to the stored water.

For applications where a compact water heating solution is required, see our 100 litre wall mounted air source DHW heater.

For larger homes with increased hot water demand, explore the 200 litre residential DHW heat pump or 300 litre residential DHW heat pump.

For commercial and high-capacity applications, see the 400 litre commercial DHW heat pump water heater or 500 litre commercial DHW heat pump water heater.

Can I just use my existing hot water cylinder?

Possibly, but it will need to be checked very carefully.

If you’ve got an existing indirect cylinder that was designed for a gas or oil boiler, its coil might be too small for a heat pump. If that’s the case, connecting a heat pump to it could lead to rubbish heat transfer and long hot water recovery times.

The heat pump will probably have to pump out hotter water or run for ages to heat the cylinder up – both of which can be a bit of a drag on efficiency.

Rather than just checking if the cylinder has a coil, you need to know if its heat exchanger capacity and surface area will work with the heat pump’s output and operating temps.

The heat pump and cylinder folks should have all the data you need to match the two up.

Why is coil size so important anyway?

The heat exchanger coil is the link between the heat pump’s primary water and the hot water that is stored in the cylinder.

If the coil is too small, it can’t transfer the heat pump’s available output effectively.

For example, it’s not much use having a powerful heat pump if the cylinder coil can only transfer a fraction of that power at the operating temp.

That can lead to longer hot water waiting times and may even encourage the system to run at too high a temperature.

A properly matched cylinder lets the heat pump transfer its heat effectively while keeping the temperature lift low – which helps keep the CoP up.

Does cylinder size matter too?

Yes, it does. Coil performance is only part of what you need to consider when choosing a cylinder.

The cylinder needs enough storage capacity to cover the place’s hot water needs.

Heat pumps can be a bit slower at getting hot water out than boilers, so it’s especially important to store enough hot water for when people are in the shower or bath.

A household with loads of people taking showers might need a bigger cylinder than one with just a few people.

Just going for the biggest cylinder available is not necessarily the right answer either – bigger cylinders take up more space and can have a higher chance of heat loss, so make sure the size matches the place and the expected water use.

What about integrated heat pump water heaters?

An integrated heat pump water heater is a whole different animal from an air-to-water heat pump connected to a separate cylinder.

With an integrated unit, the compressor, evaporator and hot water tank are all part of one single appliance – it’s designed for making hot water.

Some designs use a refrigerant heat exchanger wrapped around the cylinder rather than a conventional primary coil.

In that case, you don’t need to choose a separate heat pump cylinder because the manufacturer has already matched the heat exchanger and storage vessel.

Do I still need an immersion heater?

Most heat pump cylinders will have provision for an electric immersion heater.

The immersion can provide backup hot water if the heat pump is not working and may also be used for rapid recovery or high-temperature cycles depending on the system.

Some of the newer high-temperature heat pumps – like some of the R290 ones – can make hot water at 60 degrees or above just using the compressor alone. Even then, having an immersion heater can give you a useful backup.

What to look for when choosing a heat pump cylinder

The cylinder should be chosen as part of the whole hot water system rather than just by its litreage.

Look at the coil surface area, heat exchanger performance, cylinder volume, heat loss, max operating pressure, recovery performance and immersion heater provision. For an unvented cylinder, you’ll need to think about the safety hardware and installation requirements too. The key takeaway is that an air source heat pump really needs a cylinder capable of doing the job of transferring heat well enough even at lower primary water temps. A properly matched heat pump cylinder ought to help boost recovery times & lets the heat pump run at lower temperatures, all of which means a more efficient overall domestic hot water system.