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Plate heat exchanger vs coil – What installers need to know

Pete Armstrong, Mixergy CEO, explores the differences between a traditional heat pump cylinder with a coil and a Mixergy heat pump cylinder with an external plate heat exchanger, and the effect of each heating method on the overall system Coefficient of Performance (COP).

A hot water cylinder is like a battery for thermal energy. The way that thermal energy is transferred into a hot water cylinder can have a significant impact on the overall system efficiency. The traditional approach has been to use a very large coil of pipe in heat pump cylinders to achieve good heat transfer between the wet heating system loop and the stored hot water inside. However, there are some very clear scientific reasons why Mixergy has opted to go down the route of using an external plate heat exchanger instead of a coil for its range of heat pump-ready cylinders.

The CoP of any heat pump system, when delivering hot water, is defined as the amount of thermal energy transferred to the hot water stored in the cylinder for every unit of electrical energy consumed by the heat pump.

Conventional heat pump cylinder with a coil

Firstly, let’s look at a traditional cylinder and how the heat from the coil is transferred to the water inside the cylinder.

The overall heat pump system might include a buffer cylinder, 3-port zone valve, and various space heating emitters (or radiators), along with a typical hot water cylinder, where heat is extracted from the central heating loop via a heat transfer coil.

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 that 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) of the heat pump is at its best.

Households typically use about half of the capacity of a hot water cylinder each day. So, when the hot water cylinder starts heating early in the morning before the household wakes up, the water in the top half of the cylinder might be around 50 degrees. Below this point, the temperature drops considerably to around 10 to 20 degrees C, depending on the time of year.

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 immersed in colder water. When heating a partially heated cylinder in this way, a lower COP is achieved when compared to heating for the first time when all the water is stone cold.

The reason for this is that the thermal energy (heat) moves less easily through the hotter part of the coil because the temperature of the water in that area of the cylinder is already high, so the coil 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. Some heat pumps will reduce their heat output, but this means that the hot water cylinder is very slow to reheat. This can mean that the system prioritises hot water instead of space heating, which can cause major comfort problems on a cold day.

In summary for a traditional heat pump cylinder with a coil:

  • A large coil delivers good performance when heating the cylinder for the first time from cold.
  • However, when reheating a partially heated cylinder, the overall COP (or efficiency) is worse.
  • Some heat pumps slow down the heating when they detect this high thermal resistance but this makes the heating process slower and risks causing comfort issues on a cold day.

Heat pump cylinder with plate heat exchanger

Instead of having a coil inside the cylinder, the Mixergy heat pump cylinder has a heat transfer module and heat pump control interface. The heat transfer module comprises a plate heat exchanger and circulation pump. This transfers heat 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 household’s hot water schedule, and programmed through the Mixergy app.

The heat energy is taken from the system loop via the plate heat exchanger on one side whilst pulling cold water through the plate heat exchanger on the other from the bottom of the cylinder. The advantage of this forced convection occurs on both sides of the heat transfer surface so that a low thermal resistance is achieved. This is different to a coil, where there is forced convection on the inside, but only natural convection on the outside, which is not as effective. The coil has to be very large to compensate, reducing the volume of water that can be stored in the cylinder. With a plate heat exchanger, a greater volume of hot water is generated more efficiently, whilst also achieving a lower heat transfer resistance to get the best COP.

The advantages of plate heat exchangers increase further when heating a cylinder that is already partially heated. As in the case of a cylinder with a coil, about half the hot water in the cylinder is used throughout the day. However, when reheating through the plate heat exchanger, only water beneath the thermocline (the layer between the hot water above and the cooler water below) is being heated. The problems of coil lock-out are avoided and all the thermal energy from the heat pump is focused on heating the coldest area of the cylinder, which ensures the fastest heating, with the best possible COP.

Mixergy’s approach to heating hot water with heat pumps delivers:

  • Enhanced heat transfer to always obtain the best COP, irrespective of how hot the water is in the cylinder.
  • The complete volume of the cylinder can be heated, meaning more litres of hot water from the same footprint.
  • Mixergy’s unique sensing arrangement, controls and app mean you get visibility of how much hot water you have at all times.
  • Using the app, you can take advantage of smart tariff integration.
  • Installing a Mixergy cylinder now future-proofs the heating system, as the heat pump plate heat exchanger can be retrofitted at a later date.

For more information about the differences between coils and plate heat exchangers please check out the blog: Plate heat exchanger vs Coil: the great debate! • Mixergy