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Best air source heat pumps and set ups


Jo Alsop

Heating Hero

The Heating Hub

Are air source heat pumps the right replacement for your gas boiler, Jo Alsop reviews this much discussed technology


Are heat pumps our heating future? Which are the best heat pumps?

The best heat pumps are those that can operate the most efficiently, but so much of the performance hinges on the correct design and installation that even 'the best' heat pumps will operate poorly if they are incorrectly specified and set up,

In our ultimate guide to heat pumps we give you a broad understanding of heat pump technology, the industry, efficiency and suitability. The best heat pump for you will be one that is matched to your home and circumstances.


The UK Government has identified heat pumps as having a major part to play in decarbonising our homes and set a target for 600,000 installations per annum by 2028. The Committee for Climate Change project that a total of 5.5 million heat pumps will need to be fitted this decade of which 2.2 million will go into new build 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.


  1. How does a heat pump work?
  2. What are the types of heat pumps?
  3. Is a heat pump suitable for my home? Does my home have to be well insulated?
  4. Which are the best heat pumps?
  5. What changes will need to be made to my existing heating system?
  6. Monobloc v split heat pumps
  7. How much does a heat pump cost to install?
  8. Can I qualify for the RHI grant
  9. Understanding heat pump efficiency.
  10. How much does a heat pump cost to run? Will my energy bills go down?
  11. What size heat pump do I need?
  12. What temperature do they work at?
  13. Tables of all air source heat pumps by size and efficiency
  14. Which are the main heat pump brands?
  15. 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. 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 3-4kW hours of heat energy for your home.

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:

Converts latent heat in the air to high temperatures that is transferred to your radiators. The air-conditioning style unit is outside and inside is a hot water cylinder.

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:

At one time these were specialist product, by now most mainstream heat pumps can provide temperatures up to 70degC. See our Guide to high temperature heat pumps.

Hybrid heat pumps:

A limited number of manufacturers have developed hybrid products that combine a gas or oil boiler with a heat pump. In this set up, the heat pump raises the temperature of the water to say 55degC and the gas or oil boiler top ups the temperatures if necessary. This means that the heat pump runs efficiently and the gas or oil boiler is only on when absolutely necessary. Again high temperature heat pumps mean that hybrid systems are only useful if the home needs more than 15kW of power from the heat source, which is reasonably rare. 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. 

This question relates less to size and more to flow temperatures. At one time, heat pumps could only provide up to 55degC flow temperatures. This meant the home had to be well insulated or radiator sizes increased. Today 70degC flow temperatures are fairly standard which is the same temperature a gas boiler would operate.

Nevertheless, you are still well advised to carry out any insulation measures you can to reduce energy consumption altogether. An heat loss survey of your home will then be required to size each radiator to run at lower temperature. Radiator sizes will need to increase, but this can be limited by improving insulation measures first.

4. Which are the best heat pumps?

Which is the best heat pump is a subjective question. You can of course opt for better build quality heat pumps that run up to 70degC as standard at a high lab-tested efficiency and that will get you a lot of the way.

But, just 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.


Jo & Caroline - Heating Heroes

Help with renewables options

We give independent advice and specifications on all heat sources


5. 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.

Air source heat pumps ideally run at lower temperatures to operate at their label efficiencies. 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 if its at ground level. It can also go on a wall, down the garden or on a flat roof. Modern heat pumps are fairly quiet, but it is a consideration none the less. For 'split' systems (see below) there will be an internal unit which is usually wall hung.

6. 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 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.

7. How much does a heat pump cost to install?

An air to water heat pump costs between £2,500 and £7,500 fully fitted after the £7,500 grant is deducted. Costs depend on the size of the unit and how many radiators you decide to change.


Radiator balancing

More important than you think for efficiency


8. Grant subsidies

The Boiler Upgrade Scheme (BUS for short) pays a fixed grant of £7.5k for an air source heat pump. It does not cover hybrid systems. For more read our BUS blog.

9. 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 degC. But efficiencies drop when 1) external temperatures drop and 2) heating systems run at higher temperatures, for example 570degC.

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 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 3-4.

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.

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

ManufacturerNameModelSize kWSCOP
DaikinAltherma 3Split4kW4.48
HitachiYutaki SCombi split4kW4.93
HitachiYutaki MMono4kW4.60
PanasonicAquarea High Performance Bi-Bloc HSplit3kW4.95
PanasonicAquarea High Performance All in One HMono3kW4.95
ViessmannVitocal 200-AMono4kW4.40

5-6kW heat pumps

ManufacturerNameModelSize kWSCOP
DaikinAltherma 3Split6kW4.47
GrantAerona3 R32 6kW4.61
HitachiYutaki SCombi split6kW4.58
HitachiYutaki SSplit6kW4.60
HitachiYutaki SSplit6kW4.60
LGTherma VMono5kW4.45
LGTherma VSplit5kW4.65
PanasonicAquarea High Performance Bi BlockSplit5kW4.95
PanasonicAquarea High Performance All in One HMono5kW4.95
ViessmannVitocal 200-AMono4kW4.38



7-8kW heat pumps

ManufacturerNameModelSize kWSCOP
DaikinAltherma 3Split8kW4.56
FirebirdEnviroair 7.5kW4.18
HitachiYutaki SCombi split8kW4.25
HitachiYutaki SSplit8kW4.50
HitachiYutaki SSplit8kW4.25
LGTherma VMono8kW4.45
LGTherma VSplit8kW4.65
PanasonicAquarea High Performance Bi BlockSplit7kW4.95
PanasonicAquarea High Performance All in One HMono7kW4.95
ViessmannVitocal 200-AMono8kW4.46

9-10kW heat pumps

ManufacturerNameModelSize kWSCOP
GrantAerona3 R32 10kW5.19
LGTherma VMono9kW4.45
LGTherma VSplit9kW4.65
PanasonicAquarea High Performance Bi BlockSplit9kW4.83
PanasonicAquarea High Performance All in One HMono9kW4.83
ViessmannVitocal 200-AMono10kW4.47

11-13kW heat pumps

ManufacturerNameModelSize kWSCOP
FirebirdEnviroair 11kW4.73
GrantAerona3 R32 13kW5.40
HitachiYutaki SCombi split11kW4.80
HitachiYutaki SMono11kW4.69
HitachiYutaki SSplit11kW4.75
LGTherma VMono12kW4.45
LGTherma VSplit12kW4.65
PanasonicAquarea High Performance Bi BlockSplit12kW4.83
PanasonicAquarea High Performance All in One HMono12kW4.83
ViessmannVitocal 200-AMono13kW4.46

14-15kW heat pumps

ManufacturerNameModelSize kWSCOP
FirebirdEnviroair 14kW4.45
HitachiYutaki SCombi split14kW4.45
HitachiYutaki SSplit14kW4.45
HitachiYutaki SMono14kW4.60
LGTherma VMono14kW4.45
LGTherma VSplit14kW4.65

16-17kW heat pumps

ManufacturerNameModelsize kWSCOP
FirebirdEnviroair 16kW3.90
GrantAerona3 R32 17kW4.54
HitachiYutaki SCombi split16kW3.90
HitachiYutaki SMono16kW3.90
HitachiYutaki SSplit16kW3.90
LGTherma VMono16kW4.45
LGTherma VSplit16kW4.65
PanasonicAquarea High Performance Bi BlockSplit16kW4.83
PanasonicAquarea High Performance All in One HMono16kW4.83
ViessmannVitocal 200-AMono16kW4.46

12. What temperature do they run at?

Air source heat pumps can operate at -7degC as standard and some high quality models will operate even lower. The temperatures at which a heat pump can extract heat efficiently stops at -10-15degC with some higher end models still extracting heat as low at -22degC.

13. 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.

14. 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 up or stay the same, but much depends on the efficiency of your outgoing gas or oil boiler, the price of fuel and how well designed and set up your incoming heat pump system is. Whilst in the example below we see a small saving over oil, if the SCOP drops to 2.5 instead of 3.5, and this is not uncommon where the installation is poorly specified, then fuel costs rise to £1,500. For those on mains gas this will represent an increase in energy bills.

FuelFuel costEnergy requirement per annumRenewable element SCOP 3.5Paid-for energyAnnual fuel bill
Gas7p per kWh11,500 kWhNA11.500 kWh£805
Oil10p per kWh11,500 kWhNA11,500 kWh£1,150
Heat pump27p per kWh11,000 kWh (net of boiler inefficiency)8,000 kWh3,000 kWh£810

15. 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.

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