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Mixergy + Heat Pumps FAQ’s

By David White Jan 26, 2021
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Mixergy are in the fortunate position that we are exposed to a wide range of installers from a wide range of backgrounds. Some are from a heat-pump background, others are just starting their journey into the new realm of domestic heat pump installations. Over the past few months, we have been gathering questions posed by installers and summarising the most common here:


1. How does the Mixergy external PHE alter the cylinder specification/size?

The external plate heat exchanger assembly circulates the full cylinders contents through the PHE to heat the cylinder contents to an even temperature throughout. This removes the thermal gradient effect typically caused when using a large internal coil traditionally found in regular heat pump cylinders. The ability to evenly heat the contents and not having to factor additional capacity for non-usable volume, means we can install a cylinder that is usually 1 size smaller e.g. drop from a 300L to a Mixergy 250L.


2. Does the Mixergy heat-pump tank come in a slimline size?

Yes – the use of the externally mounted plate heat exchanger (PHE) assembly removes the need for a high surface area coil inside. This means we can offer slimline heat pump cylinders that are perfect for retrofitting into tight airing cupboards!


3. Why doesn’t the Mixergy heat pump cylinder utilise ‘top-up’/volumetric heating in heat-pump mode?

This is primarily due to technical limitations of heat pump technology. Heat pumps are designed to efficiently produce low-temperature heat and suffer significant performance penalties when asked to produce higher temperature flows in the >65˚C region. As a result, our heat pump cylinders are designed with a large heat exchanger, which keeps the heat pump’s flow temperature as low as possible during water heating (see graph below). Due to the fact that top-up technology requires heat transfer into already heated water in the 50-65˚C range, it necessitates a maintained primary flow (heat pump/boiler) temperature of at least 70˚C to effectively transfer heat into the cylinder. This means that for heat pumps, top-up is often not the best solution for heating water – even though it allows for less hot water storage losses from partial charging, this is offset by the performance penalty of running the heat pump at an elevated temperature for longer.

4. What performance gains can be achieved by using the Mixergy heat pump cylinder instead of a regular heat pump cylinder?

Mixergy’s laboratory tests have shown that our external plate heat exchanger assembly delivers 10-20% more usable hot water than a similar sized conventional cylinder. This is possible due to the consistent temperature profile throughout the cylinder that avoids the thermal gradient effect.

Coefficient of Performance of the heat pumps has been demonstrated to increase by 6-10% when combined to a Mixergy cylinder. This is due to the cylinders ability to retain a flat delta across the PHE on the primary loop, giving the heat-pump best possible conditions to operate.

5. How does the Mixergy tank connect to the heat pump to call for heat if there is no stat pocket?

The team at Mixergy have designed a special purpose voltage relay, which we affectionately call the ‘resistor switch’. This allows heat pumps that use a cylinder temperature sensor, to be connected to the resistor switch, so that the Mixergy tank can generate a signal that the heat pumps controller infers as a temperature (hot, cold or any value in between). This is important because it means the Mixergy tank can use its advanced sensor strip that runs the full length of the cylinder to inform heating decisions, rather than a single stat pocket located 2/3rds of the way down the cylinder.


6. Why does the Mixergy heat pump cylinder occasionally use the immersion heater to heat?

In cold weather conditions, some heat pumps are incapable of producing flow temperatures high enough to produce hot water at 50˚C or above. To achieve this temperature in the hot water cylinder, it becomes necessary to switch to the immersion heater. If the Mixergy cylinder detects that the heat pump’s maximum flow temperature has been reached before a charge is completed, it will automatically switch to the immersion heater to complete the charge. If this behaviour is undesirable, then it is recommended to reduce the desired hot water temperature to a level that the heat pump is able to satisfy without assistance. Heat pump target temperature can be set by the user in the Mixergy app.


7. My proposed heat pump has a stated flow temperature of 65˚C, why can’t I use a regular Mixergy indirect cylinder?

In many cases, the stated maximum flow temperature of a heat pump can be a misleading figure as it often comes with three caveats:

  1. This elevated temperature may only be achievable for a short time before the compressor hits an over-pressure condition. For example, the higher temperature output is usually limited for quick bursts of 15 minutes or less – not enough time to heat the tank.
  2. The maximum stated temperature is only achievable during the warmer months of the year as the ambient air surrounding the heat pump increases in temperature.
  3. Performance (COP) is often worse at an elevated temperature, making use of an indirect cylinder less efficient.

Using the Mixergy heat pump configuration will provide best overall and year-round performance.


8. Can I connect solar PV to the Mixergy heat pump tank too?

You can connect solar PV to the Mixergy heat-pump cylinder either through the use of our own diverter or a third part offering (currently only available on slimline units). The Slimline Mixergy tank is compatible with all 3rd party solar PV diverters including: Myenergi EddySolar iboost+Immersun and more.


9. Can I still use the Mixergy tanks immersion for top-up and connecting to smart tariffs?

Within the Mixergy app you can choose your preferred energy source at different times of day, so if you want to top up your hot water with the immersion whilst the heat-pump is space heating or because your energy provider has dropped electricity prices for 30mins overnight, you can simply schedule this through the app.


10. Where can I find the heat pump installation guide?

The installation guide and ‘resistor switch’ guide can be found here.


11. Can I install a Mixergy cylinder with a Vaillant/Daikin/Mitsubishi/Kensa heat pump?

Yes, Mixergy is compatible with all those listed. If you are unsure and want to speak to the team, please reach out on 01865 884 343 or Get in Touch.

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