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How Mixergy cylinders work with a heat pump

By Mixergy Team Feb 13, 2024
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The traditional approach for hot water cylinders and heat pumps has been to use a large coil of pipe to achieve good heat transfer between the wet heating system loop and the stored hot water inside. But in recent years, several manufacturers, Mixergy included, have opted to use an externally mounted plate heat exchanger instead of a coil.

In this article, we explain how heat plate exchangers work and the efficiency they can achieve with a heat pump.


How conventional heat pump coil cylinders work

Most conventional heat pump systems transfer thermal energy through an indirect coil inside the hot water cylinder. The heat travels through the water in the system loop, which also circulates around the rest of the central heating system.

A probe inside the cylinder measures the hot water temperature, and whenever the temperature drops below the set point, the heat pump starts transferring energy to the coil.

How a conventional heat pump cylinder works

Most systems will prioritise the hot water cylinder by switching the diverter valve to call for heat.

For the system to work effectively, the hot water cylinder needs a very large coil to get as much surface area as possible between the coil and the water stored inside. This is so the thermal energy can travel easily with little resistance, keeping the temperature of the system loop as low as possible.

When the system loop temperature is low, the efficiency or COP (Coefficient of Performance) of the heat pump is at its best.

Households usually use about half of the capacity of a hot water cylinder each day, so by the end of the day, the top of the hot water cylinder might be around 50°C. The temperature in the rest of the cylinder drops considerably to around 10 to 20° C, depending on the time of year.

Conventional cylinder temperature variation

With about half of the cylinder coil in hot water, the heat energy is mainly transferred through the lower section of the coil, which is now immersed in colder water.

Heat energy moves less easily through the hotter part of the coil because the water temperature in that area of the cylinder is already high, so it is effectively locked out as the heat pump tries to heat the water in this partially heated state. The system loop temperature has to increase, reducing the overall COP of the system.

The efficiency of a plate heat exchanger

At Mixergy, we do things differently. Instead of having a coil inside the cylinder, we have a heat transfer module and heat pump control interface.

Mixergy heat pump cylinder

The heat transfer module comprises of a plate heat exchanger and circulation pump. This allows us to transfer energy from the bottom of the cylinder to the system loop.

The heat pump interface works with the heat pump’s temperature probe to determine when to heat the cylinder based on the hot water schedule programmed through the Mixergy app.

How Mixergy cylinders work with a heat pump

We take heat energy from the system loop via one side of the plate heat exchanger whilst pulling cold water from the bottom of the cylinder through the other side of the plate heat exchanger.

The advantage of this is that we achieve forced convection on both sides of our heat transfer surface, allowing us to achieve a low thermal resistance. This differs from a coil, where you have forced convection on the inside but only natural convection on the outside, which can be less effective.

How Mixergy cylinders work with a heat pump

The advantages of plate heat exchanges increase further when heating a cylinder that’s already partially heated. About half the hot water in the cylinder is used throughout the day when we heat through the plate heat exchanger, but we are always transferring energy from the coldest part of the cylinder. This means that we avoid the problems of coil lockout and continue heating with the best possible COP.

How Mixergy cylinders work with a heat pump

Plate heat exchangers can be added at any time

As homes each take their own path into the energy transition, our hot water storage solutions need to be retrofittable to multiple energy sources.

Using a plate heat exchanger means our cylinders can be retrofittable to a heat pump at any time in the cylinder’s lifetime. As all our cylinders are heat pump ready, households can start their journey to using a heat pump with Mixergy, regardless of when they install it.


All Mixergy smart hot water cylinders help you save energy by reducing your gas consumption. They are also future proof for use with any energy source, whether a heat pump, gas/oil boiler or solar. They also come with a 25-year warranty as standard.

Find out more here.



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