The first question we are asked about our heat pump is always ‘Does it keep your house warm enough?’ The answer is yes – let’s take a look at the details.

The red line in the graph above is the target temperature we have set for our main room. The heat pump control system tries to maintain that temperature 24 hours a day – that doesn’t mean the heat pump is working 24 hours a day as we will see in a moment. We set an overnight temp of 17C, at 6am the target temp is 18C which is fine during the day when we are more active. Mid-afternoon the target temperature increases to 20C – comfortable for the less active evening typically spent watching TV, reading and listening to music.

The solid block is the actual room temperature. You can see it falling overnight, rising in the morning – the huge peak is because the sun was shining and our SE facing windows catch a lot of heat. Mid-morning the sun went in and the room starts to cool before the evening lift to 20C. By 5pm the room temperature was 19.5C and it maintained a constant 20C for more than 3 hours before it started to cool overnight.

This constant room temperature is a characteristic of heat pump heating we really like – the room just feels comfortable. With our old oil system we had to have the thermostat set higher so the room didn’t cool too much when the boiler was off – we frequently felt cool when the heating was off and too warm when the heating was on – rarely just right. The heat pump is always on – sending just the right amount of heat to maintain the desired temperature.

You can also see that the heat pump control system takes quite a ‘relaxed’ attitude to maintaining the target temperature – look at the time taken to get to 20C after the target temperature changed at 3pm. The graph makes things look worse than perhaps they actually are as the sensor resolution is only 0.5C – the reality is the temperature was slowly increasing all the time.

We can see more details by looking at the outside temperature and the heat pump flow temperature – let’s add them into the picture

The orange line that peaks up to 35C is the water flow temperature going to the radiators. The green line is the outside temperature from the heat pump sensor – only has 1C resolution.

You can see the heat pump swings into action around 5:30 am as the room temp falls to 17C. It keeps heating until around 9:30 when it turns off. Over that time the heat pump control system has been changing the flow temperature to better match the heat supplied with the heat needed. Same in the evening – the heat pump kicks in around 3:15 and works continually throughout the evening – adjusting the flow temp to maintain a constant room temperature at the temperature outside falls.

It wasn’t a particularly cold day but the heat pump wasn’t working that hard either. Here’s the house power consumption throughout the same day – this is from the Tesla Powerwall app.

The two periods the heat pump was actively heating are clear – can’t separate the exact heat pump energy consumption but estimate it is in the range 1.8kW to around 1.5kW. The heat pump is on a 7kW circuit and we’ve seen it take up to 5kW heating water on a cold day – flow temp of 50C – so there’s plenty more heat available for a really cold day.

Also interesting to note the power consumption falling in the morning as the control system reduces the flow temperature – nice!

Will post some details from a cold day when we get one.