Mixergy Hot Water Cylinder

How Mixergy works

A conventional cylinder heats the whole tank uniformly. Mixergy uses stratification deliberately. Hot water rises to the top, cooler water sits below, and the heating element works only in the hot zone. The level indicator on the unit and the app shows exactly how much hot water is available.

This gives two practical benefits. First, faster hot water availability: you can get usable hot water much more quickly because only the top portion needs to heat rather than the whole cylinder. Second, smart tariff integration: the app connects to smart electricity tariffs, loads the cylinder when electricity is cheapest, and only to the level it predicts you will need.

The heat pump problem

When you connect Mixergy to a heat pump, the original concept changes. Mixergy's top-down heating requires a fixed-temperature hot zone and a cold zone below. Heat pumps are most efficient when they can vary flow temperature gradually across the whole cylinder and respond to actual store temperature. The two approaches are in tension.

To accommodate heat pumps, Mixergy automatically switches from its top-down mode to whole-cylinder heating, essentially operating like a standard cylinder. The stratification advantage disappears.

There is also a controls issue. Mixergy uses a resistor switch rather than a temperature sensor pocket to communicate store temperature to the heat pump. This means the heat pump cannot read the actual cylinder temperature and modulate its output accordingly. Instead it is told either hot or cold, a fixed signal that forces the heat pump to run at a set flow temperature rather than adjusting dynamically. For maximum heat pump efficiency, the heat pump needs to read the store temperature and adjust its output in response. Losing that feedback limits efficiency.

Additionally, the anti-legionella cycle cannot be done by the heat pump on a Mixergy, because the heat pump cannot be given a specific target temperature instruction. The Mixergy immersion heater handles it instead, at 100% efficiency rather than the 300 to 400% efficiency a heat pump would achieve.

Efficiency comparison

A correctly set-up standard cylinder with a heat pump will typically achieve 400%+ COP for hot water. Our view, based on how the system operates, is that a well-specified standard cylinder designed for heat pump use will outperform Mixergy in most configurations. The efficiency gap is not enormous, and for a household primarily interested in smart tariff management, Mixergy's control software may provide savings that offset the hardware performance gap. But on the raw efficiency question, a standard cylinder wins.

Where Mixergy makes sense

Gas boiler or immersion primary heating is Mixergy's strongest use case. The top-down heating concept, smart tariff integration, and ability to blend boiler and immersion heating intelligently deliver real value. If you are on a gas boiler and want to cut bills using cheap overnight electricity, Mixergy is a thoughtful product.

If you are decarbonising gradually, optimising a gas setup now and planning a heat pump in a few years, a Mixergy can serve the current setup well. When the heat pump arrives, you will be operating it in whole-cylinder mode, which works, even if it is not the most efficient configuration.

What a better alternative looks like for heat pumps

A high-quality unvented cylinder designed for heat pump use, with a large coil, good stratification, correct tapping heights, and a temperature sensor pocket, will consistently outperform Mixergy when paired with a heat pump. The Heat Geek Super Cylinder is designed around exactly these principles and is independently monitored. The Joule Pinnacle and Cyclone are also well-regarded options.

If smart tariff management is important to you alongside heat pump heating, the most efficient approach is to use the heat pump's own smart grid-ready controls to set different store temperatures at different tariff rates, rather than relying on an immersion element for tariff-shifting.

Mixergy at a glance: which setup suits it best

A quick summary of where Mixergy performs well and where a standard cylinder is the stronger choice.