Air Source vs Ground Source Heat Pumps

Welcome to our guide covering the pros and cons of air source heat pumps vs ground source heat pumps.

Below we’ll cover:

How each unit works
Installation costs
Efficiency
Running costs
Grant eligibility (up to £7,500 on offer)
Which unit is best

If you’d prefer to jump right in and get a quote for your heat pump, you can use our clickable and free to use tool here.

How Air Source Heat Pumps Work

Air source heat pumps (ASHPs) are much more efficient than gas boilers. If we take the Ideal Logic Air as an example, it offers up to 534% efficiency when running at a flow temperature of 35°C. For every 1kWh of electricity it consumes, it's able to produce up to 5.34kWh of heat.

By comparison, leading gas boilers from brands such as Ideal Heating, Vaillant and Worcester offer just over 90% efficiency.

So, how are air source heat pumps so efficient?

Units are usually installed outside and incorporate a fan. This fan draws in air, which passes into a heat exchanger. Within the heat exchanger there is refrigerant which absorbs a small amount of heat from the air.

But, the real temperature increase comes from the compressor. By compressing the refrigerant its temperature increases dramatically, and this heat is then fed into your heating system.

Once heat has been released, refrigerant then travels through an expansion valve where it cools and the whole process starts again.

Both air source and ground source heat pumps hit high efficiency ratings thanks to them using a low flow temperature during operation. The higher the flow temperature of a heating unit, the lower its efficiency.

By working at flow temperatures as low as 35°C, air source heat pumps (such as the Ideal Logic Air) can hit efficiency ratings of over 500%.

How Ground Source Heat Pumps Work

Whilst similar in concept, ground source heat pumps (GSHPs) are certainly not the same as ASHPs.

During installation, your installer will fit a ground loop; a series of pipes that are buried underground. These pipes are filled with a blend of refrigerant (to absorb heat) and anti-freeze (to stop pipework freezing during winter months).

Boilers get all of their heat from the combustion of gas, which limits their efficiency. Just as an air source heat pump uses heat from the air, a ground source heat pump extracts heat from the ground. Typically, ground temperatures a couple of feet below the surface are 10–15 °C.

Ground source heat pumps work slightly better in winter than air source heat pumps. Credit: Nibe

Beyond the fact that a ground source heat pump extracts heat from the ground rather than the air, the process after this is similar to that of an air source heat pump.

Heat is absorbed by the refrigerant in the pipework. It's then transferred to the heat exchanger, where it's compressed. Compressing this refrigerant increases its temperature and that temperature is distributed to radiators or underfloor heating in your property.

After the refrigerant has completed a circuit of your heating system, it cools. The process of absorbing heat from the ground and compressing refrigerant to increase temperature, starts all over again.

Like air source heat pumps, ground source heat pumps achieve a high efficiency rating thanks to them working at low flow temperatures consistently. Depending on your system, flow temperature will be 30–50°C, compared to a gas boiler that tends to work at much higher flow temperatures of 60-75°C.

Air Source vs Ground Source Heat Pump Costs

Installation Costs

Air Source

Expect even a small 5kW air source heat pump to cost £12,500+ including installation, which is much cheaper than a ground source heat pump installation.

The gross price isn't the price you'll pay. Thanks to the UK government's Boiler Upgrade Scheme (BUS), most households qualify for a grant.

This grant entitles the average household to up to £7,500 in funding; meaning a small heat pump is likely to cost around £5,000. Obviously actual installation costs will depend on:

Whether or not you need radiators upgrading (heat pumps work best with large radiators)
The model and brand you choose
What size heat pump you need
Upgrades such as smart controls

Whilst you'll be able to get pumps fitted for a lot less, they're likely to be from low quality brands with an incredibly short warranty. As we detailed in our page covering air source heat pump reviews here, always stick to high quality brands with a long warranty.

These top brands include:

To get an exact price on your heat pump installation, use our clickable heat pump quotation tool here.

And for further reading, check out our guide to air source heat pump installation costs here.

Ground Source

Ground source heat pumps are typically much more expensive than air source heat pumps. The average homeowner will need to spend £15,000-£20,000 to have a suitable ground source pump fitted (compared to £10,000 for an air source).

However, like an air source heat pump installation, ground source units will benefit from the Boiler Upgrade Scheme. Most households in the UK will qualify for a £7,500 grant. But even with this grant, you’ll still be looking at an outlay of £7,500+.

But, why are ground source units more expensive than air source units? Put simply, ground source heat pumps are a lot more labour intensive.

Air source units require little to no groundworks, and generally hook up to an existing heating system. Your installer simply connects your outdoor unit to your interior pipework.

That's not the case with a ground source unit. Mass excavation is required in order for horizontal and vertical loops to be buried (which themselves take time to install).

The cost of design, equipment and labour for a ground source system adds up quickly, which is why they are generally 50-100% higher in price when compared to an air source unit.

Efficiency

Although its unit dependant, the efficiency and running costs of air source heat pumps vs ground source heat pumps is similar.

Ground source units certainly have the edge during winter months. And that’s simply because ground temperature doesn’t fluctuate as much as air temperature.

Ground temperature generally sits between 10–15 °C, and is likely to be around 10°C during winter months, and this is where a ground source unit draws its initial heat from.

By comparison, air source heat pumps draw heat from the air, meaning you’re at the mercy of outdoor temperatures. It’s common for outdoor temperatures during winter to hit 0 and below, so your air source heat pump will need to make up the difference to the 10 °C of ground temperature a ground source unit can utilise, by using electricity.

To make the comparison even more confusing, ground source units generally need to be slightly bigger than air source units.

If we take the Ideal Logic Air 10kW air source heat pump, we can see it can hit a Seasonal Coefficient of Performance (SCOP) of 5.19; it can work at up to 519% efficiency.

A popular ground source unit is the Vaillant flexoTHERM 15kW. This unit can hit up to 5.22 SCOP when running at 35°C, or, 522% efficiency.

So as you can see, the difference in efficiency (and therefore running costs) is negligible.

Running Costs

Estimating running costs for both air source and ground source heat pumps isn't exactly easy.

However, with efficiency ratings and electricity consumption of both units being similar, expect to pay anywhere from £800-£2,500 per year in electricity. Whilst this seems excessive, remember a chunk of this cost is offset against the money you save by not powering a gas boiler.

Factors that affect the running costs of both air source and ground source heat pumps include:

Whether or not you have underfloor heating fitted
The size of your radiators (larger radiators are better)
The flow temperature you set your heat pump to
Quality of your loft and wall insulation
Your heat pump's size

Use our clickable heat pump tool to determine the cost of installation and expected running costs.

UK Grant Eligibility

The government’s Boiler Upgrade Scheme allows UK households the ability to apply for up to £7,500 of funding for their heat pump.

Eligibility criteria is loose. Put simply, you need to own the property you're applying for and it must be located in England or Wales. Alongside a valid Energy Performance Certificate (EPC), you need to be replacing a heating system that burns fossil fuels (such as a gas boiler).

Use our free 30-second heat pump grant eligibility checker here to see if you qualify.

Air Source vs Ground Source - Which is Better?

Whilst there are benefits to ground source heat pumps, they’re negligible. You’ll get slightly higher efficiency during winter months (therefore, a slightly lower electricity bill) and a slightly longer lifespan.

However, most ground source installations are nearly double the cost of an air source unit, which is why air source heat pump installations are much more common.

Expect to pay around £5,000 for an air source heat pump after accounting for the £7,500 Boiler Upgrade Scheme grant. By comparison, you’ll see a bill in the region of £7,500+ for a comparable ground source heat pump.

You can get quotes on your air source heat pump using our 2-minute clickable tool here .

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