Heat pump efficiency and environmental impact
The main measure of a heat pump’s efficiency is called the coefficient of performance (COP). It measures how many units of heat you get out per unit of energy you put in.
A 100% efficient direct electric heater, which does no heat pump magic, has a COP of one. A 90% efficient gas boiler has a COP of 0.9.
Standard ground source heat pumps average COPs of around four, and air source about three.
However, the COP varies depending on the weather. So, when you’re comparing individual pumps you want to look at the average COPs they manage over a heating season. This is either given as the Seasonal Coefficient of Performance (SCOP) or the Seasonal Performance Factor (SPF).
Carbon footprint savings from heat pumps
As the grid decarbonises over your heat pump’s lifetime, emissions will fall towards zero. But even now, the annual carbon savings are good.
The current carbon footprint of a unit of UK electricity is on average about 1.3 times that of natural gas. You can do a rough estimate of the emissions savings you can get from heat pumps from this and their SCOP – if it is three, emissions would be less than half of those from the most efficient gas boiler.
Estimates of immediate emission cuts from studies are around 70% for standard heat pumps, 50-60% for high temperature heat pumps, between 18 and 55% for hybrid systems (it depends on the set up and how you use them), and about 20% for gas-fired heat pumps.
To put it in absolute figures, the Energy Savings Trust estimates that air source heat pumps save more than two tonnes of CO 2e per year on the most efficient gas boilers, right now. The average UK carbon footprint per person, including all imported goods, is about twelve.
Energy labels for heat pumps
All heat pumps have energy labels. Although they may be changing now we have left the EU, the familiar ErP label is still being used.
The same scale is used for all heating technologies, meaning that heat pumps by definition fall into the top three classes: A+, A++ or A+++. The rating is based on the SCOP.
When heat pumps are required to deliver a hotter output their efficiency drops. Heat pumps have two ratings for space heating, for low temperature delivery at 35°C, and medium temperature delivery at 55°C.
There are several other numbers on the label as well, including a decibel noise rating, and a hot water rating if they produce hot water, although heat pumps are normally all in Class A for that. To achieve higher efficiency, they need to be combined with solar heating.
The quantity of hot water will also be illustrated by a tap symbol indicating sizes (tap profiles) from 3XS to XXL.
Refrigerants in heat pumps
Heat pumps use refrigerants, and the most common traditionally has been a hydrofluorocarbon (HFC) called R410A, which has a Global Warming Potential (GWP) of 2088, meaning that it has 2088 times the warming effect of CO2.
Although it should be possible to recover the refrigerant at the end of life, there may be some leakage.
R410A is now being phased out under EU F-Gas regulation. Some heat pump manufacturers, such as Mitsubishi, are moving to another HFC called R32, which has a GWP of 675, although that too is going to start being phased out soon.
Others such as Vaillant are switching to propane, which has a GWP of 3, or to ammonia or CO2.
But some of these have their own problems, such as being unacceptably toxic or significantly less efficient, meaning that more electricity is used. And while the grid is still half based on fossil fuels, this can result in a higher carbon footprint than the ones with high GWPs (although once the grid is fully decarbonised, we won’t need to worry about that).
According to the Climate Change Committee: “if existing standards are maintained, the greenhouse gas savings of a switch to heat pumps are orders of magnitude higher than the additional F-gas [HFC] emissions.”
Given the complexity and that the climate impact is small, we decided not to rate companies on the basis of the refrigerants they use in their heat pumps.