Icon arrow black Heat Pumps

Using your Mixergy tank with a heat pump

By David White May 18, 2023
featured Image

All Mixergy tanks come with the ability to connect to a heat pump – whether you have a heat pump from the outset, or if you decide to change your energy source to a heat pump in the future.

It is a future-proof hot water tank that won’t require replacing, should your energy source change within its lifetime.

When using a heat pump, our technology works slightly differently, which we will explain in this article.

Mixergy connected to a heat pump

Mixergy provides a better solution for heat pumps than competitor solutions by using an externally mounted plate heat exchanger.

Once this heat pump module is fitted to the tank, the plate heat exchanger and pump assembly circulate the full contents of the tank to heat to a single temperature.

Comparative tests have shown that this gives you up to 20% more usable hot water vs. a conventional heat pump cylinder and a 5-10% CoP improvement on the heat pump by having a more efficient heat transfer through the plate heat exchanger.

Mixergy 120 with hp exchanger

Whilst conventional heat pump cylinders are very big to accommodate a large internal coil, Mixergy tanks are typically 20% smaller, as well as being available as a slimline option where space is tight.

Alongside using the heat-pump circuit, you can rapidly top up the tank using the immersion or via diverted solar PV power – this is great for providing short bursts of heat without ‘cycling’ the heat pump.

You’ve also got all the other Mixergy benefits of being a smart and connected device, with smart scheduling and optimisation of dynamic electricity tariffs.


We answer some of the most popular questions that we are asked when it comes to the functionality of how to use our Mixergy tank together with a heat pump.

Does the Mixergy tank work with any heat pump?

Yes, the Mixergy tank will work with any make/model of air source heat pump and most models of ground source heat pumps.

Please contact our technical support team if you would like to double-check compatibility with your heat pump before purchase.


How does the Mixergy tank work with a heat pump?

The Mixergy tank has an add-on module called the Mixergy Heat Pump Interface – this allows us to simulate a temperature reading that the heat pump interprets as coming from its own sensor.

This will then interpret when to heat and how much to heat rather than using a ‘real’ temperature reading taken at a nominal point 2/3rds of the way down the cylinder (which is how normal heat pump cylinders operate).

Mixergy heat pump interface

Why does Mixergy not operate its unique ‘top-down’ heating benefit when connected to a heat pump?

Heat pumps operate most efficiently when running for long periods with a consistent and small temperature delta between the primary flow and return (i.e. sending warm water out at 55°C and returning back to the heat pump at 50°C).

This allows the heat pump to avoid cycling the compressor, which is energy intensive. The greater the temperature delta between flow and return, the harder the heat pump has to work to warm it back up to its target temperature.

Mixergy and heat pump modules diagram

When doing top-down heating with the Mixergy tank, the tank needs the water that is entering at the top to be nice and hot. As a result, it will try to take as much energy as possible through its heat exchanger, making the temperature delta quite large. In this scenario, the heat pump would have to work harder to provide Mixergy’s signature top-down heating.

By using our externally mounted plate heat exchanger module, we can control and maintain a consistent and low-temperature delta during water heating events, which helps keep the heat pump in its most efficient operating zone for as long as possible.

The difference in the operating method has a considerable impact on CoP. For example, top-down heating might reduce the CoP to as low as 1.7, whereas our external plate heat exchanger method can boost CoP as high as possible!


I have a high-temperature heat pump, why can’t I do top-down heating?

Top-down heating can be achieved with a high-temperature heat pump, however, the heat pump has to work much harder to generate water at 70°C, which in turn lowers the CoP and overall system efficiency.

The aim of the game with a heat pump is to maximise CoP (the return on investment from energy used and heat generated).

To do this, you need to operate the heat pump at a much more conservative temperature for longer periods, rather than short intense bursts, which ultimately lowers your heat pump’s efficiency.

Mixerhy heat pump cylinder

Given that heat pumps are already ‘smart’ and can be programmed, does the Mixergy tank need to be in dumb mode (schedule control set to be off) to not interfere with heating?

No, the Mixergy tank has access to a range of data points that the heat pump does not. This demand-side data allows the Mixergy tank to determine when to heat and whether reheating/evening schedules are required. This helps optimise the utilisation of the heat pump, avoiding unnecessary heating.


If my heat pump is running constantly for my under-floor heating, how can I best utilise the Mixergy app?

The plumbing for heat pumps is such that hot water is a ‘priority’, therefore if your under-floor heating is running constantly, and then there is a need to heat hot water, the energy from the heat pump will be diverted to the hot water cylinder. The timing and decision as to when this might occur is determined by the Mixergy smart schedule within the Mixergy app.


If I set the heat pump to heat the tank to, say 45°C, can I separately program any surplus PV energy to heat the tank further, up to, say 60°C?

The Mixergy app allows you to set a target hot water temperature. If your heat pump is programmed to only supply hot water at 45°C and the set point for your cylinder is 60°C, the tank will use the immersion to ensure the target temperature is achieved.

Mixergy app settings

If you have the Mixergy Embedded Solar PV diverter, as opposed to a 3rd party diverter (Eddi, Solar iboost+ etc.), you will be able to set target temperatures for top-up heating using solar PV, target volumes (e.g. heat to 50% using solar PV), cut-in thresholds etc.

To learn more about the different Mixergy solar options, you can read more here.


What benefits can you unlock when combining a heat pump and solar PV?

  1. Maximum possible local consumption of locally generated solar PV, allowing you to undertake top-down heating from your locally generated solar PV. Effectively treating your Mixergy tank as a hot water battery.
  2. 20% more usable volume of the cylinder when compared to a conventional heat pump cylinder (you could drop a cylinder size to 250L with Mixergy, over a standard 300L).
  3. Increase your heat pump CoP by up to 10%.
  4. Optimised schedule control, ensuring that there are no unnecessary heating events.


I’ve heard about Mixergy’s new iHP tank, what is the difference?

The Mixergy iHP is our new integrated heat pump cylinder/tank (a hot water tank with a heat pump connected on top). It is a hot water tank that’s suitable when you want or plan to separate your water heating, from that of your space heating.

The iHP was developed with the below objectives in mind:

  1. To speed up the reheat performance by stratifying thermal energy during heating
  2. Improve the lifetime of the product
  3. Simplify manufacturing and distribution

This is achieved by our modularised Stratos heat pump head unit. The Stratos instantaneously heats hot water which is drawn from the bottom of the cylinder and returns it back to the top to achieve very quick reheat times. Alternatively, a flow path back to the bottom of the cylinder allows the system to achieve the highest possible COP on a full reheat cycle.

You can learn more about the Mixergy iHP here.


If you want, or plan to use a heat pump for your hot water, please discuss with your plumber or installer which Mixergy tank option is best suited for you. Or check out our ‘Find an Approved Installer‘ map to search for someone local to you.

You may also like

Real-World Performance of Mixergy and Heat Pumps

By Pete Armstrong Nov 29, 2023

Making the transition to a gas-free home: Start with Mixergy!

By Toby Morris Nov 28, 2023

The Heat Pump Interface V2: Building on Success

By Mixergy Team Nov 24, 2023

Introducing the Mixergy Cube X

By Mixergy Team Nov 22, 2023

Experience Sustainable Luxury: The UK’s First Fully Renewable Holiday Home

By Mixergy Team Nov 16, 2023

Shortlisted in CIBSE Building Performance Awards 2024

By Lynsey Jamieson Nov 15, 2023

Want to know more? Get in
touch to discuss your needs

Get in touch