An air source heat pump is a great way to reduce your carbon emissions and benefit from an efficient method of hot water and space heating, but what are the considerations for installing a heat pump in your house?
Since 2007 installed wind power capacity in the UK has grown from 2 GW to 21.7 GW in 2018. Solar PV capacity has grown over the same period from 1.5 GWp to 13 GWp. This huge growth in renewable energy together with a steady reduction in coal power generation has dramatically changed the carbon intensity of grid electricity. The carbon emission factor for UK grid electricity has decreased by almost a half from 0.467 kg CO2e in 2007 to 0.255 kg CO2e in 2019, and this rate of reduction is expected to continue with the UK one of the best areas for wind power in Europe. This means that using electricity for space heating is now a much more low carbon option than previously and technologies like heat pumps can offer huge carbon savings compared to a gas boiler.
There is a wide difference of opinion as to what locations are acceptable. Historically heat pumps have been used primarily for cooling purposes. As they were only used for short periods of the year during the summer, the efficiency wasn’t taken into consideration and they were installed pretty much anywhere they would fit. It’s common to see heat pumps for cooling stacked very close together on the wall, mounted on a flat roof or boxed in with an enclosure.
For a residential home however some of these locations can pose problems and as a heat pump providing space heating will be working all through the winter, the heat pump needs to work as efficiently as possible to ensure it maintains a high coefficient of performance and low running costs.
To understand good locations, it’s helpful to understand how the heat pump works best. A heat pump has a fan inside which draws the outside air in across a heat exchanger, it then removes heat from the air to be used inside the house for space heating or hot water and finally blows the cooled outside air away from the heat pump. If the heat pump is too constricted and enclosed it’s more difficult for the air to go through the fan. One of the dangers is when the cool air blown out is recirculated back into the heat pump, the coefficient of performance drops and the heat pump eventually cuts out due to freezing over.
For optimal efficiency, it’s best to allow a minimum of 30cm around the sides and rear of the heat pump and 1.5m of unobstructed space in front. There are some other restrictions due to planning requirements which also need to be taken into consideration, for example the heat pump needs to be 1m away from a neighbouring property. It may also need planning permission if situated in the front of the property, on a roof, or in a conservation area.
The simplest locations are at ground level in a rear garden, or at the side of the house. It could also be mounted on a roof but noise can travel through the structure so preferably not if there is a bedroom below. Finding a good spot can be the hardest part of knowing if the heat pump will work so it is worth getting advice at an early stage in the project.
A heat pump has a limited water heating flow temperature. It can get up to about 55 degrees Celsius instead of 80 degrees Celsius which you may be used to with a gas boiler. If you are planning to have underfloor heating this will work very well but with radiators care needs to be taken to ensure they are sized to work at lower temperatures. This might mean swapping radiators and getting a double fin K2 radiator instead of K1, or going for a larger size. A benefit of radiators is that they can be switched on and off quickly allowing you to fine control when heating is needed only during the main times the house is occupied for example in the morning and evening. This can help to keep heat energy consumption down.
The heat output from a typical single phase domestic heat pump is limited up to around 14kW. For most properties this will be sufficient; some houses may only need a 5kW heat pump. For a larger property which is uninsulated with leaky single glazed windows, for example, the heating demand could be so high in winter that the lower temperature heat pump system needs unfeasibly large radiators to keep the space warm. In this case you might consider a hybrid system with a gas boiler or making improvements to the fabric such as loft, wall or floor insulation, or replacing the windows with double or triple glazing and sealing up the leaks. There can be a lot of decisions to make and factors to consider so getting advice early on can highlight potential savings and help you to match the plans with your budget
An air source heat pump does require a fair amount of electricity to run and so you may need to upgrade the main electricity supply fuse. For a dwelling with single phase supply the fuse generally could be 60A or 100A. A typical 11kW heat pump which would be sufficient for a well-insulated 4 bedroom house could use 4kW of electricity during the winter. This represents 16A of fixed electrical load. In a loading calculation you add the fixed loads such as kettle, electric oven, and intermittent loads such as clothes iron, which quickly gets to above 60A. In comparison to an electric system, with 2kW direct electric radiators in every room, a heat pump would use between 2.5-3 times less electricity. The good news is that once notified most district network operators (DNOs) will carry out a 60-100A upgrade free of charge (the DNO is the company responsible for maintaining the electricity network supply cables in your region and not your energy provider who you actually pay for electricity).
Hot water use at a shower is typically 40-45 degrees Celsius which will be a mixture of the hot and cold supply via the shower’s thermostatic mixing valve. A well-designed air source heat pump should be able to get the tank up to 52-55 degrees Celsius which should be sufficient for a hot shower. It’s important to consider insulating the hot water pipes and checking the distance between the hot water cylinder and heat pump to make sure the system will work as intended.
Due to the lower flow temperatures a heat pump connected to a standard hot water cylinder will take longer to heat the hot water tank up to temperature. If you are able to buy a new hot water cylinder which is heat pump compatible this will have a larger coil inside which allows the heat pump to heat the hot water tank faster. If you are expanding the house with an extension for example you may want a bigger capacity hot water tank, providing opportunity to buy a heat pump compatible type.
A well-designed heat pump system should work at a coefficient of performance of 2.5-3. This means for every 1kWh of electricity used to run the system you will get 3kWh of energy for heating. The extra 2kWh of heat are transferred from outside via the refrigeration cycle. If we compare the heat pump to a gas boiler it uses a lot less input energy (as electricity) than a gas a boiler. The problem with the financial equation is that gas can be 3-4 times cheaper than electricity at 4p/kWh instead of 14p/kWh in the case of electricity. So all in all the lower energy use of the heat pump is offset by the cheaper cost of gas and both systems should have similar running costs.
The government are offering subsidies for homeowners who choose an air source heat pump via the Renewable Heat Incentive (RHI). Payments can range between £6,000 – £10,000 depending on the amount of renewable heat which will be provided by the air source heat pump. Why not find out if a heat pump would be an option for your home by contacting our expert team.
Kaspar Bradshaw, Project Engineer, Enhabit
Find out more about Enhabit’s Mechanical Services Design service.
To find out more join Kaspar’s Webinar & Q&A on Low carbon heating on Wednesday 6 May 2020 at 12.00-13.00 – REGISTER HERE
 https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2007. Conversion factors 2007 – Condensed set (for most users)