It’s usually the first question that is asked “Does your heat pump keep the house warm in cold weather?” Yes it does.

Shortest post ever…

…but maybe a bit more detail would be helpful.

In cold weather the house loses more heat energy. For a back of the envelope calculation the heat energy lost is proportional to the difference between the inside and outside temperatures. We typically heat our main room to 20C in the evening, at 0C outside the house will lose twice as much heat energy as at 10C, at -5C it’s two and a half times more energy than at 10C. The result is whatever heating system you have it is going to have to work harder – use more gas, oil, electricity, whatever.

For a heat pump that means it’s going to use more electricity. Our system will use around 1.6 kW keeping the room at 20C when it’s 10C outside. So at -5C outside that’s 4kW just to keep up with the extra heat being lost by the house.

Unfortunately that’s not the end of the story. To deliver more heat into the house the heat pump needs to increase its flow temperature (or cycle less but well set up systems don’t cycle very much). Mitsubishi state “a 1% drop in flow temperature increases the Coefficient of Performance by 2%” which the other way around means increasing the flow temperature decreases the efficiency. Here’s a graph that shows the flow temperature changing with outside temperature.

You can see a flow temperature of around 35C on the evening of 6th Feb when it was around 5C outside. Compare that with 8th Feb am when it was -4C outside, flow temperature was over 40C. The 50C spikes are hot water heating – they aren’t relevant here. So heat pump would have been around 9C temp difference x 2% less efficient – 18%. It can’t be seen from the graph but compared to 10C outside, heat pump would be around 25% less efficient at -5C. The 4kW from earlier has now become 5kW.

And there’s more – the spikes on the graph are defrost mode in action. When it’s cold the heat pump collector ices up as water condenses on it – the ice is starting to form in the shot below.

To keep working efficiently the heat pump needs to defrost itself – it does this by using heat energy from the water in the heating system – during the defrost cycle it is not heating the house. Depending on outside temperature and humidity defrosting takes about 5 mins around once an hour. It’s another hit to efficiency – perhaps up to 10%. So 5kW at -5 C becomes 5.5 kW or well over three times the power consumption needed at 10C. This came as a bit of a shock the first time it happened!

Here’s the house power consumption over the same period as the graph above.

There’s lots of spikes but try to see through them. On the above zero evening of the 6th there’s a 2kW baseload, most of that is the heat pump. On the colder evening of the 7th consumption is up to nearly 4kW. Early morning on the 8th when it was -4C the consumption is somewhere around 5kW. It’s all broadly in line with the back of the envelope predictions earlier in this post.

What does this mean for heating costs? They go up when it’s cold – hardly news! The specifics will be covered in another post – in Our Home Electric with solar, storage and a dual rate electricity tariff the answer is anything but straightforward.

And finally, for heat pumps human cold isn’t that cold – there’s still plenty of heat energy in the air at -10C that a heat pump can use. The Centigrade scale is fixed to two convenient points – the freezing and boiling point of water (at normal atmospheric pressure). Heat energy is proportional to the temperature in degrees Kelvin. 0C is 273 degrees Kelvin. That massive temperature drop that is felt from 10C to -5C becomes 283K to 268K in degrees Kelvin and represents a fall in heat energy of only about 5%. Our perception of cold is (very) relevant to us but it barely changes the physics!