What is a heat pump hot water cylinder?

What is a heat pump hot water cylinder? A heat pump hotwater cylinder is a clever storage cylinder designed to get on with air source heatpumps and help produce domestic hot water. Unlike a conventional boiler cylinder, a dedicated heat pump cylinder will usually have a bigger heat exchanger coil because heat pumps typically don’t operate at the same high temperatures as boilers. This means the bigger coil has more surface to transfer heat into the stored water, which helps hot water recovery and lets the heat pump work more efficiently.

How does a heat pump hot water cylinder work?

With a standard air-to-water heat pump system, the heat pump heats water that circulates through a coil inside the hot water cylinder.

Heat passes from the primary water in the coil into the domestic water stored around it. The two water circuits remain just for themselves – separate.

It’s basically the same principle used with a traditional indirect cylinder connected to a gas or oil boiler. The main difference is that an air source heat pump generally operates at a lower flow temperature, so the cylinder’s heat exchanger needs to be custom-designed.

For smaller homes and apartments, our 100 litre wall mounted heat pump hot water system offers the lowest storage capacity within our range.

For greater residential hot water requirements, we offer the 200 litre heat pump hot water system for homes and 300 litre heat pump hot water system for homes.

For larger-scale requirements, see the 400 litre commercial heat pump hot water system or 500 litre commercial heat pump hot water system.

Why does a heat pump cylinder need a bigger coil?

Heat transfer works better when there’s a big temperature difference between the primary heating water and the water stored inside the cylinder.

A boiler can supply a cylinder with fairly high-temperature primary water, creating a large temperature difference and allowing a conventional coil to work effectively.

A heat pump works most efficiently at lower temperatures, which reduces the temperature difference available for transferring heat into the cylinder.

A bigger heat exchanger surface area helps make up for this.

If the coil is too small, the heat pump may struggle to put all its output into the cylinder. This can increase recovery time and may even force the heat pump to operate at a higher flow temperature, which reduces its Coefficient of Performance (COP).

Is a heat pump cylinder the same as a normal hot water cylinder?

No way.

They might look almost identical from the outside, but the internal heat exchanger can be pretty different.

A cylinder originally designed for a boiler may have a smaller coil that’s absolutely fine when supplied with high-temperature water but totally unsuitable for a lower-temperature heatpump system.

Which is why just slapping an air source heat pump onto an existing cylinder doesn’t guarantee good performance.

The cylinder needs to be matched to the heat pump output, primary flow temperature and required hot water recovery time.

What size heat pump cylinder do I need?

The right cylinder capacity depends mainly on the property’s hot water demand.

You’ll need to think about the number of occupants, number of bathrooms, shower usage, baths and periods when several people will need hot water all at once.

Cylinder sizing is particularly important with heatpumps because hot water recovery often takes longer than with a high-output boiler.

Having enough stored hot water lets the cylinder handle periods of peak demand without running hot water taps constantly, relying on an immersion heater or high-temperature boost mode.

But bigger isn’t automatically better. An unnecessarily big cylinder costs more, takes up extra space and may have higher total standing heat losses.

Is a heat pump cylinder always unvented?

No, although unvented hot water cylinders are commonly used with air source heatpumps in UK homes.

An unvented cylinder stores mains-pressure hot water, which lets it supply showers and taps without needing a traditional cold water storage cistern in the loft.

Unvented cylinders need the right safety and pressure control equipment and have to be installed in accordance with the relevant Building Regulations and manufacturer’s instructions.

In England, this includes the requirements relating to hot water storage systems under Approved Document G, including G3.

Does a heat pump cylinder have an immersion heater?

Most heat pump cylinders have provision for at least one electric immersion heater.

The immersion heater can give you hot water when the heat pump is down for some reason. Depending on the system, it may also help with rapid recovery or a high-temperature hygiene cycle.

During normal operation, however, the heat pump should ideally provide the bulk of the energy used to heat the water.

Frequent immersion heater use can make your electricity bills go up because direct electric resistance heating doesn’t provide the same efficiency advantages as the heat pump refrigeration cycle.

Is this different from an integrated heat pump water heater?

Yes, and it’s useful to know the distinction.

A heat pump hot water cylinder typically refers to a cylinder designed to be connected to a separate air source heat pump.

An integrated heat pump water heater combines the hot water cylinder, compressor, evaporator, fan and refrigerant circuit into one single appliance. It’s specifically designed to produce domestic hot water rather than also providing space heating.

With an integrated system, the heat pump and cylinder are already matched by the manufacturer.

What should you be looking for when choosing a heat pump cylinder?

**A good heat pump hot water cylinder should provide enough stored water for the property while also having a heat exchanger that can transfer the heat pump’s output efficiently.** Check out the cylinder capacity, coil surface area, heat exchanger rating, recovery performance, standing heat loss, insulation, immersion heater provision and max operating pressure, rather than just settling on a tank that’s large enough.

Matching the cylinder to the air source heat pump is crucial. It really can boost hot water recovery, slash unnecessary immersion heater use and lets the heat pump run at lower flow temperatures – where it usually operates best and is generally most efficient.