Learn more about how air source heat pumps work, the running and installation costs and whether this renewable heat source is the right choice for your home. 

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

Image: Viessman's Vitocal 300a air source heat pump offers a modern aesthetic for style-conscious homes. 

As a primary energy-saving measure under the Green Homes Grant scheme, many homeowners may be considering changing a gas boiler heating system for an air source heat pump.

These low-carbon heating systems are estimated to help lower your home’s carbon footprint by up to 50% and also qualify for further financial incentives from Government schemes. 

Find out more with this in-depth guide. 

How do air source heat pumps work? 

Grand Designs’ own Kevin McCloud once described the air source heat pump as a ‘reverse fridge’, which offers a frame of reference for how an air source heat pump operates. 

Heat is absorbed from the outdoor air at a lower temperature into a fluid, before being compressed to raise its temperature. This higher temperature heat can then be passed to the hot water and heating of your home. 

 Illustration of how an air source heat pump works - green homes grant - grand designs

Image: Adobe Stock 

Air source heat pumps can be one of two varieties: air-to-water heat pumps are the most common in the UK, transferring the heat into water which can then be used for traditional home heating elements; while air-to-air heat pumps work with a circulation system to heat the air itself. The latter is not included in the Renewable Heat Incentive scheme; however, the Green Homes Grant has not yet been clearly limited to air-to-water heat pumps. 

What are the benefits? 

prewett bizley bloomsbury house exterior - green homes grant - grand designs

Image: Retrofit specialist architects Prewett Bizley upgraded the thermal performance of this Georgian property in London, reducing the heat requirement by 90%, meaning they could replace conventional radiators with underfloor heating driven by an air source heat pump. Photo: Adam Parker 

The core benefits of an air source heat pump is a more energy efficient method of heating your home, and therefore lower heating bills and carbon emissions. According to heat pump supplier Nu-Heat, for every unit of energy an air source heat pump uses to extract heat from the air, it outputs three times as much into your home. 

There are savings to be made in your home's carbon footprint too, as even in replacing an A-rated gas boiler, you'll save between 2,150 to 2,250 kg of carbon a year, according to the Energy Saving Trust

Compared to ground source heat pumps, air source pumps are relatively easy to install, around two-thirds of the cost and don’t require a great amount of outdoor space. This makes it a superior option for retrofitting to the majority of homes. 

"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."

Read more: A guide to ground source heat pumps

Will an air source heat pump work for my home?

mitsubishi Ultra Quiet Ecodan air source heat pump - green homes grant - grand designs

Image: With market-leading low-noise technology, Mitsubishi's Ultra Quiet Ecodan air source heat pump virtually eliminates the need for considering noise as part of planning permission.

Air source heat pumps offer an alternative for traditional gas boilers, however, they’re best used in airtight, well-insulated homes. This is because they work best at producing heat at lower temperatures than boilers, so ensuring that your home is draught proof will up its efficiency.

"We need to look at a fabric first approach," explains Mark. "If you spend a lot of money on a heating source for your poorly insulated building, that's probably your wrong priority. If you do something in the future that does reduce the demand of space heating dramatically, you may have wasted that investment on an oversized heating system." 

An air source heat pump 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. However, with an optimum operating temperature of 7°C, this shouldn’t be too much of a concern for the majority of homes in England, where the Green Homes Grant applies.

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Because air source heat pumps work 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 air source heat pump. 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. 

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

How much does it cost?

Nu Heat NIBE Air Source Heat Pump on exterior of home - grand designs - green homes grant

Image: Heat pump installer Nu-Heat uses NIBE air source heat pump for heating and hot water, such as for this client home located in West Camel in Somerset. 

According to quote comparison site Green Match, the average price for installation of an air source heat pump 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 air source heat pump 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 air source heat pump. 

However, as a sustainable alternative, and with help Green Homes Grant and the RHI Scheme to recompense for the initial outlay of expense for installation, it’s an attractive option for those considering their home’s contribution to carbon emissions. 

Estimates from Green Match found that, for an average four bedroom house with an air source heat pump, annual heating costs start from £759 per year, with RHI payback of up to £1,600 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 Air Source Heat Pumps of 10.71p.

However, according to the Simple Energy Advice website, you will need to notify Ofgem if you claim vouchers under the Green Homes Grant, and these savings will be deducted from your Renewable Heat Incentive payments. 

At present, the benefits for going green with your home heating are only increasing, and with the likes of Good Energy’s UK-first heat pump tariff, which offers favourable rates for households with higher electricity usage with heat pumps installed, there’s the potential for even greater savings to be made. 

Is it eligible for the Green Homes Grant?

Air source heat pumps are categorised as a primary measure under the Green Homes Grant. As the cost of installation exceeds the £5,000 voucher value of the grant, homeowners will by and large be eligible to claim the full amount against the cost of installing an air source heat pump, while contributing the remainder of cost themselves. For low-income homeowners who are in receipt of relevant Government benefits, homes may be eligible for a grant up to £10,000, without further contribution, which may cover the entire cost of installation. 

With the Green Homes Grant paying back some of the initial outlay for installation, the extra income from the RHI Scheme will contribute to overall lower running costs throughout the first seven years of owning an air source heat pump. 

However, to qualify for installation of an air source heat pump, your home must have adequate insulation for low-carbon heating, as outlined above before vouchers will be issued.

The eligible products list is exactly the same as the for the Domestic Renewable Heat Incentive scheme; all products must be listed in the MCS product directory.

Read more: The Green Homes Grant: how does it work?

Installation and maintenance 

Vitocal 100 viessman air source heat pump - green homes grant - grand designs

Image: 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

An air source heat pump 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 air source heat pump to last for up to 20 years according to the Energy Saving Trust. When purchasing, 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. 


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