Heat pumps in extreme cold

I’m interested in the performance of heat pumps during extreme cold weather events.
My angle is in the pressure on the electricity grid and the reinforcements required.

I understand that we design for specific temperatures. Say 21 degrees inside when it is -3degrees outside. Let’s say this gives us a 10kW size. What happens when it is colder outside? Say -10deg for a week. Would a heat pump pretty much run flat out, and the temperature of the house would drop?

And is there a way to estimate COP based on different temperatures across a region or neighbourhood? I presume this is very dependent on the refrigerant used? Let’s say an estate of 1000 homes across Sheffield, in the future when they all have heat pumps installed, presumably from a range of gases. I can see from the heatpumpmonitor data that at 3degrees outside, good installers can get COPs between 3 and 5. But what about when temp drops to minus 10. Or what about an extreme winter like the minus 27 that was seen in Scotland once.

If the designed output of the system matches the energy required at a particular temperature, then the system will run flat out if it is told do do so. Another issue with a system designed that closely to match supply and load is that the house will not get back up to temperature if it is allowed to cool.

If the energy requirement exceeds the supply the house will cool down. However, long periods of very low temperatures are not that common in the UK. Unless they are very poorly insulated, buildings don’t cool down that quickly and most people can cope with internal temperatures a few degrees below normal for a few days. If it’s really going to be a problem you massively over size the heat pump.

Most systems are designed with an amount of extra capacity to allow for contingencies. This is exactly the same as any other heating system, but it’s generally not manifest as boilers are often massively over sized and the output doesn’t drop with temperature.

Many heat pumps list the CoP at various outside temperatures/flow temperatures in their literature. For instance these are Vaillant Arotherm plus heat pump CoPs at 35C flow temperature (one of the better ASHPs at low temperatures).

	Vaillant Arotherm+		
Outside temp 5kw 7kw 10kw 12kw
-3 3.7 3.8 4 4.2
2 4 4.3 4.6 4.7
7 4.6 4.9 5.3 5.5
10 5.3 5.6 5.7 5.9

A CoP of 5 at 3C OAT sounds remarkably optimistic.

Another issue is that of frosting. The humidity here has been approaching 100% for the last 2 months so defrosts have been taking a big toll on performance.

its just gone properly cold in the UK in the last couple of days, if you look at heatpumpmonitor.org you can see some genuine real-world performance for a large group of systems . COP of 3 at these temperature levels is more likely, not 5. some vendors do better than others. they all publish databooks for performance at different outside temperatures, thats what you need to consult for whats theoretically possible.

as per your original query, -10C for long period in uk is pretty unlikely. If that very rare thing did happen, what would happen in practice is that if the house now needed (say) 12kw to keep at 21C but the HP could not do 12kw, the house temp would drop until an equilibrium was reached. At a lower internal temperature the output of the radiators for the same heat pump water output temperature will increase (its the delta that matters) so it wouldn’t drop as far as you think. likely it will drop to the point where the occupants need to wear a jumper / mid layer instead of t shirt comfort. Not a big deal for most people for a few days. its not as though the heating is “off”. thermal mass of the house will slow down the effect.


As a slight aside, if you wanted to see examples of what happens to temps when there isn’t enough heat supplied you could see the extreme version of that where people are completely turning off their heat pumps:

That includes some graphs showing how the initial heat loss is notable but then the equilibrium is reached as mentioned.

However, in your scenario the delta would be significantly higher so the house would cool further than those examples.

In my personal experience of -16 °C for a few days the 14kW oil boiler struggled to keep the house warm. Our 14kW heat pump would struggle too. At least the icing up would be less likely because the humidity at that temperature is much lower. Clearly the solution is to improve our insulation rather than fitting an even bigger heat source. If we had lost electrical power the oil boiler would have failed so we’d be in the same (cold) situation as with a heat pump.

As you imply, having the entire country running their heating at full pelt would be stressful for the grid. Others have mentioned this and are working on that problem too. It might be interesting for you to read this consultation which refers to other heat sources such as district heating (which may be heat pump or other types):

Depends what the limiting factor is and user behaviour. Which or these is limiting?

  • emitter output to property?
  • heat pump output to emitters?

If you do 4 kW at 45C but the heat pump can do 6 kW then you’re emitter limited. If you bump up to 55C and it can still do 6 kW you might be happy still.

If it can do 8 kW at 45C but only 6 kW at 55C then you have a problem. You could crank UP the flow temperature…but only if you zone down part of the house and hammer 55C into the remainder (emitters can do this because they’re hotter) to try keep a cosy corner…or you could let it sit at 45C and what will be will be.

Probably a mix of both in many circumstances.

As room temperatures fall.you do get a useful bump in output. Mean water temp 40c and room 21C is 19C DT. Room 18c is 23C DT. So available output increases the chillier the room gets. Drop to 15C with jumpers and output really rockets.

Or you crack on air conditioning at COP3, the wood burner, the extra blankets, decide that your better half is suddenly much more attractive etc.

Not all could afford to run flat out anyhow and will likely self limit. Flat out “7kW” heat pump is say 3 kW for 72 units a day or £20/day+ so many of that 1000 will dress up or go stay with relatives etc.

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On the flip side, you probably won’t be driving your EVs everywhere either, so net the off the grid and it may not be quite as stressful as you first imagine.