A heat pump may replace a gas or oil boiler in your property but practically everything about them is different. To deliver maximum efficiency and comfort a heat pump needs to be set up like a heat pump not a small boiler. Surely that’s obvious? Nope, our heat pump was installed to work exactly like a small boiler. It took a couple of engineer visits and a few emails to get it working sensibly.

So what changed?

Originally the boiler was set up to use ‘Flow Temperature Control’ with the flow temperature set to 45C. Not a good idea – explanation below

It was wired into the ‘smart’ thermostat that previously controlled the oil boiler. Thermostats only provide on/off control. Imagine if the accelerator in your car only had off or on – journeys would be really uncomfortable, the engine would get thrashed and you’d use loads of fuel. Guess what it’s pretty much exactly the same with heat pumps (and also with oil and gas boilers). Modern systems can do so much better with temperature sensors feeding actual room temperature data to the heat pump control system. The heat pump can modulate its output to provide just the right amount of heat – comfortable and efficient.

There was no climate compensation – when it’s less cold you don’t need so much heat energy to keep the house at the desired temperature. Knowing the outside temperature let’s the heat pump control its operation to match.

We persuaded the installer to replace the existing thermostat with a Mitsubishi wireless temperature sensor – this enables an ‘auto adaptation’ mode which uses the actual inside and outside temperatures to control the heat pump to achieve the desired room temperature.

The control system automatically varies the flow temperature to keep the room the sensor is in at the chosen temperature. The great thing about heat pumps is the lower the flow temperature the more efficient they are – Mitsubishi claim for every 1C reduction in flow temp the heat pump is 2% more efficient.

It works – the steady temperature is much more comfortable than traditional on/off heating achieves, we are using less power and the heat pump isn’t working as hard. Win, win, win.

Heat pump heating modes in action

Flow Temperature Control mode means the heat pump raises the flow temperature until the set temperature is reached. The heat pump maintains that flow temperature until the thermostat tells it to turn off. Our system was initially set to 45C. Here’s the house electricity consumption on a winter day.

Ignore the solid block overnight – that’s our EV charging. The 5kW (!) spikes are the heat pump turning on and off under thermostat control. This cycling is inefficient and the room temperature goes up and down. 5kW is a lot of power so the heat pump is either blasting away or turned off.

Climate compensation uses the external temperature to change the heat pump flow temperature according to the predicted heat demand – on cold days it increases the temp, on less cold days it reduces the flow temp. Here’s what that looks like in action.

The peak power consumption is reduced but there is still plenty of cycling. That’s probably because it takes a bit of trial and error to set the climate compensation curve correctly and we hadn’t done it. Too little climate compensation and you still get cycling, too much and the room doesn’t get warm enough. You can certainly get it better than shown here but why bother – temperature sensor/auto adaptation does it for you. Here it is in use.

That’s better – the heat pump is ticking over between around 1.5 and 2 kW and there’s way less cycling – the spike at 6pm is cooking. Less cycling means a more consistent room temperature which we find far more comfortable than the old oil-fired heating.