Air source heat pump: a buyer's guide

Air source heat pumps: a buyer’s guide

Find out whether your home is suitable for an ASHP with this comprehensive guide

By Hugh Metcalf, Rebecca Foster and Karen Styliandes |

This article explores how an air source heat pumps (ASHP) work, the likely running and installation costs and whether this renewable heat source is the right choice for your home. You may be considering changing a gas boiler heating system for an ASHP. According to EDF Energy, these low-carbon heating systems are estimated to help lower your home’s CO2 output by 23 tonnes over 10 years.

Energy suppliers British Gas and Octopus Energy are competing to provide heat pumps at lower costs as part of a commitment to scale up installation rates. This may lead to ASHP installation prices falling in the coming months. But the upfront cost is not all you have to consider when contemplating an ASHP to reduce your home’s carbon footprint while staying cosy and keeping running costs low.

 Aerona 3 Inverter Driven outside unit air source heat pump.

A base tray heating element helps prevent ice forming on this unit. Aerona 3 Inverter Driven, from £5,000 for a 17kW model from Grant.

How do air source heat pumps work?

Powered by electricity, ASHPs come in two types – air to air and air to water. The latter is more common in the UK and the subject of this guide. Air-to-water systems include a fan unit containing a special coil. Heat in the air is absorbed by fluid inside the coil, which passes through a compressor to increase its temperature.

This warmth is transferred to a water-based heating system, including radiators and/or underfloor heating (UFH), and the hot water supply.  Air-to-air heat pumps run like air-conditioning units in reverse. The circulation system heats the air, but does not supply hot water.

air source heat pump outside of modern home - green homes grant - grand designs

Viessman’s Vitocal 300a air source heat pump offers a modern aesthetic for style-conscious homes

What are the benefits?

Installing an ASHP is one way to reduce a home’s carbon footprint whether as part of a self-build or a retrofit project. The warmth generated is better retained when a home has good levels of airtightness and insulation. A thermally efficient house requires a pump with a smaller capacity to heat it effectively than a leaky building of a similar size.

It will also cost less and be cheaper to run. To get the best from retrofit ting an ASHP you may need to improve the fabric of the building. A heat pump installer will carry out a room-by-room heat-loss calculation before advising you. ASHPs are most energy-efficient when circulating water at as low a temperature as possible, ideally around 35°C to 40°C.


how an air source heat pump works

Photo: Adobe Stock

To keep a home cosy at this temperature requires radiators with a sufficiently big surface area or to have UFH, which releases gentle warmth over a large expanse. Installing an ASHP may involve changing an existing radiator set-up, and possibly the pipework, for compatible models or UFH, which will affect the project cost and the level of disruption.

There will need to be suitable outdoor space for the fan unit, somewhere inside to put a hot water storage cylinder, or indoor unit, and room to run all the pipework. For quiet operation, look for models that run at around 45dB or less, which is about the same as a loud fridge.

“One of the key considerations for low-carbon technology is how likely it is to go wrong,” says architect Mark Elton of retrofit specialists Cowan Architects. “Heat pumps have been around for a long time. They’re not much different to your fridge, and we know that fridges don’t tend to go wrong.”

This model runs at 38.5dB and is certified by Quiet Mark CS7400iAW, Worcester Bosch.

This model runs at 38.5dB and is certified by Quiet Mark. CS7400iAW, from £8,900 for the 5kW model, Worcester Bosch.

Providing it is well-maintained and serviced every year, an ASHP should last around 20 years. Some installers offer a maintenance contract, or it can be arranged directly with the manufacturer. Signs that an ASHP isn’t working properly include problems with radiators, UHF or hot water, excessive noise, a pump that cycles between on and off, and increased energy bills.

Most appliances have sophisticated controls and monitoring systems. If there’s anything amiss or the appliance is underperforming, it should display an error message that can be diagnosed and rectified by the installer. Before buying, check the length of the manufacturer’s warranty, which can be anywhere between two and seven years, and make sure there’s a minimum two-year workmanship guarantee.

available in six different kW models. LG Therma V Monobloc, 5.5kW model, ScottishPower

Designed to work at temperatures as low as -25°C, this pump is available in six different kW models. LG Therma V Monobloc, from £3,019 for the 5.5kW model, ScottishPower.

Will an air source heat pump work for my home?

An ASHP draws heat from the outside air, so when the outside temperature is lower, its efficiency levels decrease as it requires more power to increase the temperature of the trapped heat. If you lived in a very cold climate, the case would be better made for a ground source heat pump which draws relatively steady heat from the ground.

Because an ASHP works at lower temperatures, they’re a good match with underfloor heating and low-flow radiators. You may need to replace some, or all, of your radiators if upgrading to a heat pump.

There is also the option of a hybrid ASHP. This heat pump works in partnership with a gas boiler to meet any shortfall in efficiency on the part of the heat pump. While the majority of your hot water and heating needs would be met by the heat pump, however if heating demand is high and the outside temperature is not, the gas boiler kicks in to contribute to the heating mix.

mitsubishi Ultra Quiet Ecodan - green homes grant - grand designs

Mitsubishi’s Ultra Quiet Ecodan air source heat pump virtually eliminates the need for considering noise as part of planning permission.

However there are some potential drawbacks to an ASHP. Because of the lower temperature produced, it can affect the maximum tap water temperature your home can achieve. Some ASHP’s can still be noisy too, though technology on this front has improved in recent years.

Installation is relatively straightforward and can be completed in around two to three days by an engineer registered with the Microgeneration Certification Scheme (MCS) – once any associated improvement work is complete. The outdoor fan unit is of ten mounted on the floor close to an exterior wall or attached to it, but there must be sufficient clearance space around it.

Ideally, the unit will be a short distance from the water storage cylinder inside the building. Many ASHP installations come under permitted development rights (PDR), so planning permission is not necessary. But there are exceptions, so check with your local authority – the Planning Portal provides further detail. It must also satisfy Building Regulations requirements.

aroTHERM plus 5kW, Vaillant

An outdoor fan unit can be floor mounted. aroTHERM plus 5kW, from £3,500, excluding installation, Vaillant.

How much does it cost?

According to the 2021 guide of quote comparison site Green Match, the average price for installation of an ASHP is between £8,000 and £18,000 for an air-to-air source heat pump and £7,000 and £11,000 for an air-to-water source heat pump, dependent on the size of your home, whether it’s a new build or renovation and the pump brand, size and performance/efficiency.

In terms of running costs, an ASHP may actually increase your annual bills, compared to an A-rated gas boiler. According to figures from the Energy Saving Trust, you may spend £95-£100 extra a year to run an ASHP.

Estimates from Green Match found that, for an average four-bedroom house with an ASHP, annual heating costs start from £759 per year. This is an estimation, based on an air-to-water heat pump with 3.7 SCOP and a household with good quality insulation, an average UK electricity price of 14.37p/kWh and an RHI tariff for an ASHP of 10.71p.

Connect to this system through an app on your smart device. Daikin EDLA16D3V3 16kW, outside unit

Connect to this system through an app on your smart device. Daikin EDLA16D3V3 16kW, from £10,000, including fully designed replacement heating system, E.ON.

Installation and maintenance

An ASHP is usually made up of both an indoor and outdoor unit. This means you’ll need an outdoor space for the heat pump to be placed – one that has a good air flow is essential, while a sunny spot will help your heat pump run more efficiently when the sun is out. The indoor unit usually isn’t much bigger than a standard boiler system.

At present, in England and Scotland, installation of a heat pump is covered under Permitted Development Rights if it meets a series of criteria, which include keep heat pumps one metre away from property boundaries and ensuring placement minimises the effect on the appearance of the building. You can read more on the Planning Portal.

With regular maintenance and servicing, you can expect an ASHP to last for up to 20 years according to the Energy Saving Trust. Enquire about the length of warranty and workmanship warranty – the former is often 2-3 years, while the latter can be up to 10 years.

Vitocal 100 viessman grand designs ASHP

Viessman’s Vitocal 100-A air source heat pump is A+++ rated, and can achieve temperatures of 60°C at an outside temperature of -5°C

A pump’s efficiency is indicated by its co-efficient of performance (COP), which shows how much energy is extracted from the air compared to how much electricity powers the pump, so a COP of four indicates that for every 1kW of electricity, 4kW of heat is generated.

This information is available for every ASHP model. It ’s also important to determine the seasonal co-efficient of performance (SCOP) to calculate how effective an ASHP is on an annual basis. A heat pump installer makes this calculation, providing the result before beginning any work based on the specifics of your home and the ASHP system.

This gives you an idea of the likely annual running costs. The total cost of an ASHP installation depends on the model, and whether you need new radiators, UFH or thermal efficiency improvement work.

Eligible retrofit installations may benefit from the government’s Boiler Upgrade Scheme (BUS), available in England and Wales, providing grants of up to £5,000 to switch from an existing fossil fuel heating system to an ASHP. Check your eligibility on the government’s website.