Why my heat pump performed worse this winter

Over the first winter my heat pump (space heating only) gave me an average COP (efficiency) of 3.1 which was higher than I had been told to expect. I was moderately pleased. This winter it was only 2.9 which was a great disappointment.

The difference is due to the weather. The mean COP for one year is not necessarily a guide to the next. The mean temperature is part of it but colder days require proportionately more heat, which biases the average COP. So I have done my best to work out what the average COP should be over time. I was surprised at the year to year variation.

How big should your heat pump be?

Most people have combi-boilers and the size of the boiler required is often dictated by the need to heat hot water for baths and showers quickly rather than space heating. With heat pumps, you normally have a cylinder for hot water and it is the space heating requirement that dictates the size you need. So how big should it be? Hearsay tells me that even with boilers, sales-people love to sell you systems that are bigger than you need. My heat pump is oversized by at least 50%.

My heat pump is much bigger than it needs to be. I have tried to make it less intrusive with some stick-on leaf patterns. Still, at least I have renewable heating :-)

Oversizing does matter, both for boilers and heat pumps.

• Large systems cost more - not as much as you might expect, but still more.
• Running at small fractions of full capacity reduces efficiency (usually).
• Large systems are physically large and, in the case of heat pumps, have a greater impact on the landscape (see picture above).

In this post I discuss a very simple rule of thumb for heat pump size - thanks to Michael de Podesta - and some reasons it gives an underestimate in my case. Also I show you how to cross check your installer's estimate using the EPC certificate for your home. I wish I had done this!

Performance of my heat pump

Last year we installed a heat pump at home and we now have almost an entire heating season of data for it. Here I present some results. Our overall mean COP (efficiency) is a little better than we were told to expect. During the heating season we have made a number of changes: increasing the night time setback temperature and reducing the configured flow temperature, with impacts on heat demand, electricity consumption or both. It is tempting to focus on efficiency alone but, being practical, it is energy use that we want to minimise. For example you can expect increasing setback temperature to increase efficiency but also heat demand - which wins?