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Using a Heat Pump

How Mixergy tanks behave with heat pump systems, including heating speed, partial heating and cleansing.

Heat pumps heat water differently from boilers or direct electric sources. They operate at lower temperatures and heat more gradually, which changes how the tank behaves compared to traditional systems. This is normal and expected. 

Understanding these differences will help explain why heating may take longer or appear to stop part-way through. 

Correct heat source selection 

If your hot water is heated by a heat pump, the heat source in the Mixergy app must be set to Heat Pump. 

This tells the tank to expect lower flow temperatures and to manage heating in a way that protects efficiency. If a different heat source is selected, the tank may default to the immersion heater or behave unpredictably. 

If you are unsure whether your system uses a heat pump for hot water, your installer or housing provider should confirm this before you change the setting. 

How heat pumps heat a Mixergy tank 

Heat pumps work best at lower temperatures and over longer heating periods. Unlike boilers, they do not rapidly raise water temperature. Because of this, a Mixergy tank connected to a heat pump: 

  • heats the whole tank, not 10% increments 
  • may still appear in the App and Gauge that it’s heating in 10% increments, but inside the water is heating from the bottom where the cold inlet pipe is 

Mixergy is designed to work with these characteristics and avoids forcing the heat pump to operate inefficiently. This approach improves the efficiency of the heat pump system by up to 24%.

Low flow temperature and partial heating 

One of the most common issues on heat pump systems is low flow temperature. If the heat pump is not supplying a high enough flow temperature: 

  • Heating may stall around 30–40% 
  • Boost may not progress 
  • Cleansing may take longer or stop early 

This does not usually indicate a fault with the tank. It means the heat pump is operating with it's configured limits. Your installer should check: 

  • Flow temperature settings 
  • Cylinder priority configuration 
  • Whether space heating is taking priority over hot water 

Why the tank may not heat fully 

On heat pump connected tanks systems, the tank may stop heating before reaching 100 percent for several reasons: 

  • The heat pump is optimising for efficiency 
  • Flow temperature is capped 
  • The system is prioritising space heating 
  • Installer settings limit cylinder demand 

These behaviours are usually intentional and designed to improve overall system performance. If full heating is regularly required, your installer may need to adjust system configuration. 

Cleansing behaviour with heat pumps 

Cleansing cycles can behave differently on heat-pump systems. Because heat pumps operate at lower temperatures: 

  • Cleansing may take longer 
  • The system may show a timeout 
  • The tank may retry cleansing later 

If the required temperature cannot be reached using the heat pump alone, the immersion heater may be used as a fallback. 

This is normal and part of the system’s safety design.