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Air Source Heat Pumps: A Beginner’s Guide

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)

Air source heat pumps have been in the spotlight this year. The launch of the Green Homes Grant in September will mean that homeowners can potentially claim vouchers of up to £5,000 to offset the cost of a new air source heat pump.

What’s more, while the original government Feed-in Tariff (FiT) scheme is no longer offered to homeowners installing solar panels and other electricity-generating renewable technologies, the domestic Renewable Heat Incentive (RHI) scheme is still alive and well and has now been extended till March 2021.

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. 

Other benefits of an air source heat pump include:

  • comparatively low running costs, 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.

The disadvantages of air source heat pumps are:

  • 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.

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.

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 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. 

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. 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. 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.

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)

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. 

Putting the costs into real context is not easy but the following costs (also including installation) can be used for guidance:

  • Combi-boiler: £2,000 - £4,000
  • System Boiler with cylinder: £5,000 - £9,000
  • Ground source heat pump: £16,000 - £25,000
  • Biomass boiler: £14,000 - £19,000

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

The real cost comparison comes when you compare running costs, which are based on the heat demand and the system losses of your home:

  • 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.
  • A newish gas boiler, with an efficiency of 90%, will cost between £550 - £900 a year to run.

The above does not take into account payments you will receive under the Renewable Heat Incentive when investing in an air source heat pump. The current payment (as of 1 July 2020) is 10.85p/kWh.

The RHI is however designed to be an incentive to help repay the uplift in the capital cost of the equipment and is not intended to be an income generator as the FiT scheme was.

Air source heat pump on timber clad self build

The self builder behind this new home in Scotland took a fabric-first approach — the house (built by Caber House) was designed to meet Passivhaus standard. An air source heat pump (far left) provides hot water. (Image credit: David Barbourr)

How do Air Source Heat Pumps Compare with Ground Source Heat Pumps?

There are a number of key differences, including:

The 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 compressore 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.

The 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.

The 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.

Air source heat pump powering timber clad renovation with swimming pool

It's hard to believe that this property was previously a brick-clad bungalow — a new first floor extension and timber cladding have transformed this home. The new swimming pool is heated by an air source heat pump (Image credit: Alistair Nicholls)

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.

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 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.

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)

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.

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.

Useful Air Source Heat Pump Contacts