Our ultimate guide to Air Source Heat Pumps
Are heat pumps our heating future?
The UK Government has identified heat pumps as having a major part to play in decarbonising our homes. For the vast majority of us this will mean air source heat pumps. Heat pumps work best in highly insulated homes, but they are suitable for most of our older housing stock as well, providing they are specified and setup correctly. In our ultimate guide we explore this renewable technology and consider them as a direct replacement to gas and oil boilers.
- How does a heat pump work?
- What are the types of heat pumps?
- Is a heat pump suitable for my home? Does my home have to be well insulated?
- What changes will need to be made to my existing heating system?
- Monobloc v split heat pumps
- How much does a heat pump cost to install?
- Can I qualify for the RHI and Green Homes Grant subsidies
- Understanding heat pump efficiency.
- How much does a heat pump cost to run? Will my energy bills go down?
- Which are the best heat pumps?
- What size heat pump do I need?
- Tables of all air source heat pumps by size and efficiency
- Which are the main heat pump brands?
- How do I find an installer?
1. How does a heat pump work?
All heat pumps take latent heat from the air which is present even in cold temperatures, compress it and in doing so increase its temperature to around 45-55 degrees (some go to 65-80 degrees). This is high enough to run an underfloor heating system in a well-insulated property. All heat pumps need electricity to run and use an electric immersion to top up temperatures for hot water. As a rough guide, for every one kW hour of electricity needed by the heat pump to run/heat, it generates two-four kW hours ‘free’ from the air or ground.
2. What are the types of heat pump?
The main types of heat pumps are: air to water heat pumps, air to air heat pumps, ground source heat pumps, high temperature heat pumps and hybrid heat pumps:
Air to water heat pumps:
Take latent heat from the air and push it through a single compressor to raise the temperature to 45-55 degrees. Air to water heat pumps use a chemical ‘refrigerant’ to transfer the heat to the heating system. The compressor unit is outside and inside is a cylinder full of heating system water that is circulated around radiators or underfloor heating.
Air to air heat pumps:
Work in the same way as air to water heat pumps but use blown warm heat units internally instead of radiators.
High temperature heat pumps:
Work in the same way as air to water heat pumps but use two compressors to raise the temperature of the refrigerant to 80 degrees. This makes them suitable for retrofitting to conventional heating systems that run at higher temperatures - for more see our Guide to high temperature heat pumps.
Hybrid heat pumps:
A limited number of manufacturers have developer hybrid products that combine a gas or oil boiler with a heat pump. These allow for the heat pump to raise the temperature of the heating or hot water to 45-55 degrees and the gas or oil boiler to top up the temperatures if necessary. This means that the heat pump runs efficiently and the gas or oil boiler is only on when absolutely necessary. For more see our Guide to hybrid heating options.
3. Is a heat pump suitable for my home? Does my home have to be well insulated?
Contrary to popular belief, it is possible to fit an air source heat pump in the vast majority of UK homes. The average heat requirement of UK homes is just 6-8kW on a very cold day, much less the rest of the year. Heat pumps range between 3.5kW-16kW as standard so can comfortably heat older, larger and even poorly insulated properties. Nevertheless, you are still well advised to carry out any insulation measures you can to reduce energy consumption altogether.
4. What changes will need to be made to my existing heating system?
The key to air source heat pumps successfully heating your home is getting the heating system design correct. By this we mean sizing the radiators correctly or fitting underfloor heating where possible.
Air source heat pumps need to run at lower temperatures to operate at their label efficiencies, ideally 45 degrees or lower. Gas heating systems run at much high temperatures and our radiators have been sized accordingly. In order for our heating systems to operate at lower temperatures we need to fit much larger radiators in order to increase the surface area for heat transmission or improve the insulation of our properties so that we can keep our existing radiators.
The other main consideration is space. We have got used to combi boilers in kitchen cupboards. Heat pumps consist of a large outdoor unit and a hot water cylinder as a minimum.
The external unit must go on a hard stand and you need to be sensitive to location for yourself and your neighbours in terms of noise. Modern heat pumps are fairly quiet, but it is a consideration none the less. Internally there will be an accumulator tank for heating as a minimum (and can combine hot water production). For 'split' systems (see below) there will be an internal unit which is usually wall hung.
5. Monobloc v split heat pumps
A heat pump is made up of a compressor, which squeezes the air and raises its temperature, and a refrigerant circuit which transfers the heat to liquid before transferring it on again to the heating system (radiators/UFH) via a heat exchanger. A monobloc heat pump keeps all the components of the heat pump in a large outside unit. A split system keeps the compressor on the outside and houses the refrigerant circuit on the unit in a separate unit. The advantage of the monobloc is that it takes up less space internally and the refrigerant liquid comes sealed in the unit.
6. How much does a heat pump cost to install?
An air to water heat pump costs between £3,000 and £6,000 just for the unit, depending on the size, plus the cost of the hot water cylinder plus installation costs. High temperature heat pumps start around £5,000. If you already have suitable radiators or underfloor heating, you can expect to pay between £7,000 - £12,000 for your installation, but that is reduced significantly with the RHI and Green Homes Grant subsidies.
7. RHI and Green Homes Grant subsidies
The Renewable Heat Incentive (RHI) scheme has been extended and will remain open to applications until 31 March 2022. The RHI works by paying you a tariff of 10.85 pence for every kilowatt hour (kWh) you use in ‘renewable energy’ to heat your home. The payment is based on the kWh figure given in your Energy Performance Certificate (EPC).
As the RHI will only pay for the renewable energy element, the payment will be based on a proportion of the kWh figure stated in your EPC. For example, if your EPC states you use 15,000kWh for heating and hot water and - based on the efficiency of your heat pump -10,000kWh will be deemed to come from renewable sources, your RHI payment will be 10,000 x 10.85 = £1,085 every year for 7 years = £7,595.
This is capped at 20,000 kWh of qualifying renewable heat (30,000 kWh in total of renewable heat and non-renewable heat) or £9,100 over 7 years.
Homeowners and landlords are also able to apply for the Green Homes Grant (GHG). The GHG comes in the form of a voucher of up to £5,000 that can be used to cover 2/3rds of the cost of a qualifying energy saving measure. This includes air source heat pumps and other renewable technologies. Vouchers are valid for 3 months and must be used in that time. The scheme is set to expire on 31 March 22.
You can apply for both the RHI and GHG. You must apply for the GHG first and notify Ofgem that you have received the grant. The grant will be deducted from your RHI payments, but the benefit is that you receive a lot of your upfront cost back much sooner, as the RHI takes 7 years to pay out.
With both grants, the net cost of the installation should be much closer to the cost of a gas or oil boiler, however you will have to wait to get your money back.
8. Understanding efficiency – COP and SCOP
COP stands for Coefficient of Performance and it explains how efficiently a heat pump can operate. ‘Can’ is important here, as the COP relates to the best it can do based on the external temperature and the temperature of your heating system water.
Many heat pump manufacturers quote their very best COP, which is based on mild outside temperatures and a very low temperature heating system, say 35 degrees. But efficiencies drop when 1) external temperatures drop and 2) heating systems run at slightly higher temperatures, for example 55 degrees.
In these optimum conditions, the COP might be 5, so for every one unit of electricity used by the heat pump, the heat pump will generate 5 ‘free’ units from the air. In reality it is lower.
SCOP standard for Seasonal Coefficient of Performance. This is much more accurate view of the overall efficiency of the heat pump across the year, when sometimes it is operating optimally and sometimes it is working much harder and consuming more electricity. SCOP are commonly stated as being 3.5-4.5.
Again this is based on good conditions. If your heating system has not been correctly designed the heat pump will have to work harder to heat your home and efficiencies may drop to 2 or less.
Correct heating system design is the key to an efficient heat pump, i.e. that your radiators and UFH are sized correctly and set up correctly. Whilst you can do little about external temperatures, you do control what happens in your home so it is vital to get that right by using an installer that knows how to design heating systems for low temperature heat sources.
9. Which are the best heat pumps?
There is not a great deal between most good quality heat pumps. In the tables below we have ordered the main brands available in the UK and shown their SCOP so you can compare efficiency potential. NB these SCOP figures are based on 35 degree heating temperatures and will reduce if your heating system runs at 45 degrees or 55 degrees. In general the SCOPs are 3.3 at the lower end and nearly 5 at the upper end.
BUT REMEMBER, as we say for gas boilers, your heat pump is only as good as the installer that fits it. A heat pump with a higher efficiency potential will only work at that efficiency potential or close to if your installer has specified your whole system correctly.
10. What size heat pump do I need?
The average UK home needs just 6-8kW of heat on a cold day in February. Heat pump capacities range from 3.5-16kW as standard so can heat most UK homes.
What size heat pump you need will be based on the heat loss of your home. We provide a guide to sizing your heat requirement here: Guide to boiler sizing. Your heat requirement will be the same irrespective of whether you have a gas boiler, heat pump or oil boiler for example.
11. Tables of heat pumps by size and efficiency
3-4kW heat pumps
|Hitachi||Yutaki S||Combi split||4kW||4.93|
|Panasonic||Aquarea High Performance Bi-Bloc H||Split||3kW||4.95|
|Panasonic||Aquarea High Performance All in One H||Mono||3kW||4.95|
5-6kW heat pumps
|Hitachi||Yutaki S||Combi split||6kW||4.58|
|Panasonic||Aquarea High Performance Bi Block||Split||5kW||4.95|
|Panasonic||Aquarea High Performance All in One H||Mono||5kW||4.95|
7-8kW heat pumps
|Hitachi||Yutaki S||Combi split||8kW||4.25|
|Panasonic||Aquarea High Performance Bi Block||Split||7kW||4.95|
|Panasonic||Aquarea High Performance All in One H||Mono||7kW||4.95|
9-10kW heat pumps
|Panasonic||Aquarea High Performance Bi Block||Split||9kW||4.83|
|Panasonic||Aquarea High Performance All in One H||Mono||9kW||4.83|
11-13kW heat pumps
|Hitachi||Yutaki S||Combi split||11kW||4.80|
|Panasonic||Aquarea High Performance Bi Block||Split||12kW||4.83|
|Panasonic||Aquarea High Performance All in One H||Mono||12kW||4.83|
14-15kW heat pumps
|Hitachi||Yutaki S||Combi split||14kW||4.45|
16-17kW heat pumps
|Hitachi||Yutaki S||Combi split||16kW||3.90|
|Panasonic||Aquarea High Performance Bi Block||Split||16kW||4.83|
|Panasonic||Aquarea High Performance All in One H||Mono||16kW||4.83|
11. Which are the main heat pump brands?
The heat pump market is made up of a number of gas/oil boiler manufacturers who have diversified into heat pumps – namely Grant, Vaillant, Viessmann and Firebird – and usually Japanese air conditioning manufacturers that have made air source heat pumps for a long time, including Daikin, Mitsubishi and Hitachi. All have UK offices and operations.
12. How much does a heat pump cost to run? Will my energy bills go down?
Based on the current cost of gas, oil and electricity, your energy could go down by around 50% if you are off grid. But much depends on the efficiency of your outgoing gas or oil boiler and how well designed and set up your incoming heat pump system is. Whilst in the example below we see a 50% saving over oil, if the SCOP drops to 1.5 instead of 3.5, and this is not uncommon where the installation is poorly specified, then fuel costs rise to £750. For those on mains gas this will represent an increase in energy bills.
|Fuel||Fuel cost||Energy requirement per annum||Renewable element SCOP 3.5||Paid-for energy||Annual fuel bill|
|Gas||3.5p per kWh||15,000 kWh||NA||15,000 kWh||£525|
|Oil||6p per kWh||15,000 kWh||NA||15,000 kWh||£900|
|Heat pump||12.5p per kWh||15,000 kWh||11,667 kWh||3,333 kWh||
13. How do I find an installer?
There are lots of heat pump installers across the UK. In all cases they must be MCS registered. The standard of installation is patchy as we are still getting to grips with the technology and demand is rising. We have some of the best heating engineers in the UK on our Elite Installer network. You can contact us if you would like to explore a heat pump for your home.