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Air Source Heat Pumps: The Basics Explained

a timber clad self build home with an air source heat pump
(Image credit: Allan Corfield Architects)

Air source heat pumps have been in the spotlight recently as having an important role to play in the move away from fossil fuel-powered homes. With government schemes, such as the Green Homes Grant, Renewable Heat Incentive (RHI) and the proposed Clean Heat Grant, bringing attention to this renewable technology, more people than ever are considering air source heat pumps for self builds and renovations. 

Air source heat pumps have been heralded as the most cost-effective residential heating technology of the future, with a recent study finding that costs are likely to be 50% lower than hydrogen power, another emerging renewable heat source. 

A Public Attitudes Tracker from the Department for Business, Energy & Industrial Strategy, released in February this year, found that 57% of people are now aware of air source heat pumps — an increase of over 20% from 2018. 

If air source heat pumps have come onto your radar as a potential investment for your home, this expert guide breaks down everything you need to know, from how these systems work, to how much they cost to install and run. 

(MORE: Get a quote for your heat pump)

a self build home with an air source heat pump

Built using a SIP system, this custom build by Allan Corfield Architects is heated using a Daiken air source heat pump, MVHR and 4kW solar PV array.   (Image credit: Allan Corfield Architects)

What is an Air Source Heat Pump?

Although an air source heat pump replaces a boiler as the home’s heat source, the way it works is very different.

An air source heat pump does not create heat. It simply moves it from one place to another through the vapour compression cycle (or refrigeration process) to make it more useable. Heat from the air gets absorbed into a fluid, which causes it to ‘boil’ and become a gas. 

The gas is then compressed, raising its temperature. The higher temperature is then transferred into the heating system. High temperatures require more work from the compressor and therefore result in lower system efficiencies. 

A 4kW Earth Save air-source heat pump was installed in this energy-efficient self build home on the Isle of Man

A 4kW Earth Save air-source heat pump was installed in this energy-efficient self build home on the Isle of Man. (Image credit: Jeremy Phillips)

Other important facts about air source heat pumps include:

  • Electricity is needed to power the pump. The efficiency, or the measure of the heat energy output per kW of electricity, is stated as the COP (Coefficient of Performance) or SCOP (or Seasonal Coefficient of Performance — the SCOP is the average COP over a defined period of time such as a year). For example, a SCOP of 3.2 means that for every 1kW of electricity, 3.2kW of heat is generated
  • The air source heat pump gets its energy from the surrounding air, so as the ambient air temperature drops, so does the efficiency. It is therefore key to understand the heat load of the property and the performance characteristics of the heat pump
  • For space heating, air source heat pumps work best with underfloor heating, but low-flow temperature radiators, such as oversized radiators, multi-finned aluminium radiators or fan convectors, will work as well.

Pros and Cons of an Air Source Heat Pump

What are the Advantages of Air Source Heat Pumps?

  • Unlike a gas boiler, an air source heat pump does not produce carbon when operating. While they do use electricity, ASHPs can be combined with solar PV panels for clean electricity 
  • Air source heat pumps have comparatively low running costs, especially when compared with off-grid fuels such as propane, oil or direct electric heating
  • Air source heat pumps also work well with underfloor heating and low temperature radiators

What are the Disadvantages of Air Source Heat Pumps?

  • They are not a ‘one-size-fits-all’ solution and may not be suitable for every home in the same way as gas boilers are. They're ideally paired with well-insulated, airtight homes. 
  • Electricity is still needed to power an air source heat pump. This will likely increase your electricity bills, but decrease other heating costs. 
  • It is essential to design and specify the system correctly and ideally the insulation, airtightness and emitters (typically underfloor heating or carefully sized radiators) of the property should be optimised, to allow you to get the most out of your air source heat pump.
  • The bigger the difference between the outside air and the target temperature (either the indoor room temperature or domestic hot water), the lower the efficiency.

Air source heat pump powering timber clad renovation with swimming pool

Once a brick-clad bungalow, the new swimming pool belonging to this converted home is heated by an air source heat pump. (Image credit: Alistair Nicholls)

Which is Better —  a Ground Source or Air Source Heat Pump?

There are a number of key differences to note between air source heat pumps and ground source heat pumps, including:

Efficiency

A typical claimed SCOP figure for an air source heat pump might be 3.2; the comparable figure for ground source heat pumps is more like 4, so for every 1kW of electricity, 4kW is generated. 

Thus ground source heat pumps appear to be slightly more efficient but as the compressor and the refrigerant is very similar in both systems you really have to check the real temperature of the heat source, namely the ground and the air.

Towards the end of the heating season (January onwards) the ground could be colder as the heat is extracted. If the air temperature is therefore warmer than the ground temperature then it can be argued that the ASHP could be more efficient. Ground conditions and geographical location are crucial when making this choice.

Installation

Ground source heat pumps require a large garden or piece of land to be installed, or are installed in deep boreholes. Both types of installation result in excavation costs. Air source heat pumps are not installed in this way. Instead the external condenser unit sits in a box on the outside wall and so is cheaper to install.

Incentives

The Renewable Heat Incentive offers a more generous return for owners of domestic ground source heat pumps — 21.16p as compared with 10.85p for domestic air source heat pumps. The maximum claim is 30,000kWh per year for ground source heat pumps and 20,000kWh for air source heat pumps.

Costs

How Much Does an Air Source Heat Pump Cost?

Expect to pay around £11,000 - £16,000, including installation, for a smaller single fan air source heat pump system which would suit homes up to around 200m². A larger system which would suit homes up to around 500m² will cost around £14,000 - £20,000

These figures are for guidance and a room by room heat loss calculation would need to be carried out prior to specifying the appropriate products (including installation). The air source heat pump unit itself can cost between £3,000 and £10,000. 

(MORE: Air Source Heat Pump Costs Explained)

Editor's Note: Homebuilding.co.uk partners with the UK's best heat pump specialists to match your requirements with their products and services. Simply answer a few questions on what you need from your heat pump and we’ll put you in touch with a suitable partner.

How Much Does an Air Source Heat Pump Cost to Run?

For a four bedroom home built to current Building Regulations‘ standard, expect running costs of between £540 - £800 per year for an air source heat pump, based on a SCOP of 3.2.

In comparison, a new(ish) gas boiler, with an efficiency of 90%, will cost between £550 - £900 a year to run.

Air source heat pump to oak frame self build

The underfloor heating in this oak frame self build (the frame for which was provided by Border Oak) is  powered by an air source heat pump, installed by Grange Heating. (Image credit: Jeremy Phillips)

What Grants are Available for Air Source Heat Pumps?

Renewable Heat Incentive

The running costs above do not take into account payments you will receive under the Renewable Heat Incentive (RHI) when investing in an air source heat pump. 

Under the RHI scheme, those with renewable heating technologies are paid back for the heat they generate for seven years — a real bonus for those considering investing in an air source heat pump. 

The domestic Renewable Heat Incentive scheme is still alive and well and has now been extended until March 2022.

The current payment (as set out in 1 July 2020) is 10.85p/kWh.

Green Homes Grant

Air source heat pumps are eligible for the Green Homes Grant (GHG) as a primary measure in properties with suitable insulation and airtightness levels. 

This government scheme offers vouchers to the value of two-thirds of the cost of a range of green home improvements. This is capped at £5,000 (or £10,000 for certain low income households). 

Though the Green Homes Grant may be utilised alongside RHI, the value of any vouchers issued under the former will be taken from subsequent RHI payments. 

One way to circumvent this issue is to use the GHG to fund improvements to your home's airtightness and insulation, where necessary, which will not affect future RHI payments on an air source heat pump installation. 

The Green Homes Grant is also currently slated to finish in March 2022. 

Specifying a Heat Pump

Can an Air Source Heat Pump Provide Both Heating and Hot Water?

One of the first decisions to make when buying an air source heat pump is whether it will provide space heating or domestic hot water — or both.

The key here is that the ‘flow temperature’ (the temperature of the water in the heating system) is lower in a heat pump than it is in a boiler:

  • Space heating will usually require a flow temperature of around 35°C to 45°C for underfloor heating or low-temperature radiators
  • Domestic hot water will, however, require a minimum flow temperature of 55°C.

On new homes that meet recent and current Building Regulations most air source heat pumps can do both, but this is not always the case.

Another solution is to use two heat pumps: one that is optimised for the space heating and another for domestic hot water.

The advantages of using the two heat pumps is that each unit is optimised for the required flow temperature and there is no priority system that causes the space heating circuit to ‘cool’ while the domestic hot water is being reheated.

an air source heat pump heated home

(Image credit: Panasonic Heating and Cooling Solutions)

The domestic hot water heat pump typically uses a different refrigerant that can produce higher flow temperatures but also requires a higher source temperature (10°C) to be efficient.

It tends to be lot smaller than a space heating heat pump and is usually built into the hot water cylinder. It draws its air from the room it is in, or the kitchen or bathroom (or all of them) or from the exhaust waste heat of a ducted mechanical ventilation system — and is known as an ‘exhaust air heat pump’ or a ‘micro heat pump’. 

It is crucial that this system is designed properly so as to not over ventilate the property and only uses heat from the air that would normally have been exhausted to atmosphere.

Examples include:

If you do not have a ducted ventilation system and don’t want to draw heat from inside the property, you could consider a different type of ‘micro’ heat pump such as a thermodynamic system. It's important to make sure that it is designed and specified properly though and is not being asked to do more than its design capability.

It is in effect an air source heat pump with an outdoor panel evaporator. The outdoor panel contains refrigerant and relies on air temperature and sunlight as a heat source. The panel is often mounted on a roof but can be wall-mounted. Bear in mind that it needs good exposure to sunlight and moving air, so it should ideally not be tucked away behind the garage or shed.

The micro heat pump only draws between 400W (watts) and 800W of electricity, and produces around 1,200W to 2,400W of heat (depending on the compressor and fan size and the air intake temperature), so if you have photovoltaic panels (PV) fitted to the property, the micro heat pump will also be optimised to use the on-house generation and possibly heat your water for free for much of the year.

A £200k Oak Frame Self Build

£14,000 of the total £200,000 build budget for this oak frame home in an off-mains gas area was allocated to underfloor heating and an air source heat pump. (Image credit: Simon Maxwell)

Higher Temperature Air Source Heat Pumps

In order to achieve higher temperatures, some manufacturers have built the two different refrigerant systems into one heat pump in a ‘cascade’ system that can create flow temperatures of up to 80°C.

These systems (such as the Daikin Altherma) are designed for hot water and should not really be used as a high temperature boiler replacement unless the lower efficiency has been carefully calculated to ensure that it is the best option for the property.

There are also other new technological advances that are worth noting such as compressors that allow the compressed vapour to be re-injected into the compressor to enhance the temperature. These systems can get flow temperatures of around 65°C.

The advantage of this system is that it reduces the complexity of the heat pump and therefore the cost. The operating pressures put a larger load on the compressor and push the tolerance of the refrigerant — examples include:

  • Mitsubishi Electric Ecodan
  • Panasonic
  • Heliotherm
  • Stiebel Eltron, amongst others.

Choosing Smart Controls for Your Air Source Heat Pump

Modern air source heat pump heating systems require specialist design and commissioning to achieve and maintain efficiency. In the age of the ‘app’ and smart heating controls, these systems can easily be tampered with, resulting in lower efficiency and high running costs.

As a result, some manufacturers have developed controls that can be monitored and maintained remotely. This is especially useful in second homes and rental properties, as well as for technophobes, as the systems can be reset and adjusted often without someone coming out to the property.

contemporary self build in Surrey that has an air source heat pump

Heating is rarely used in this contemporary self build in Surrey — the house, which has an EPC A rating, is heated with a Terra Therma air-source heat pump (Image credit: Jeremy Phillips)

In the event of a breakdown, the system can be checked, faults diagnosed and the correct spares sourced before incurring the expense of going to site. 

The engineering accessibility is often an after-sales add-on product, so check costs and requirements before ordering, but monitoring and metering systems can potentially attract an increased RHI payment if compliant components are installed.

In the absence of full remote control and monitoring, it is worth finding a controller that at least stores the operating data on a memory card so that it can be accessed for analysis and perhaps new settings and updates emailed to you for upload.

Installation

An air source heat pump should be installed and commissioned by an accredited Microgeneration Certification scheme (MCS) installer. An incorrectly commissioned air source heat pump may use a lot more electricity. 

Comparatively, installing an air source heat pump is a low-disruption process. A solid base for the air source heat pump should be constructed and, when retrofitting, all alterations to the fabric of the house and radiator systems should be completed before the installation date.

In this case, installation could take as little time as a single day. 

(MORE: Air Source Heat Pump Installation

Do Air Source Heat Pumps Need Much Maintenance?

No. Most maintenance by the homeowner is visual — checking that the outdoor unit is free from leaves and debris and the pipework is intact. 

Annual inspections by qualified engineers is also recommended. 

Like fridges, air source heat pumps do not tend to go wrong too often, and a good quality air source heat pump could last up to 20 years. 

Air Source Heat Pump Sourcebook 

The easiest route for installing an air source heat pump is to opt for a supply and install contract. 

For more information on air source heat pump models and certified installers, try the brands below: