Air Source Heat Pumps Vs. Ground Source heat Pumps

Now the Renewable Heat incentive has launched, we  are going to take a look at the key differences between an air source heat pump and a ground source heat pump, to help you make a better informed investment decision!

A bit of background on heat pumps

Heat pumps are a fantastic way to create hot water in the home; they absorb heat from either the air or the ground (hence air source and ground source heat pump!) into a fluid. This fluid then gets compressed which drives up the temperature even further and a heat exchanger then transfers the heat from the liquid to water travelling around your heating system.

Air Source Heat pumpElectricity is traditionally considered the more expensive way to heat the home – one kWh of electricity costs 15p, while one kWh of gas costs only 4p. The thing with heat pumps though, is that unlike a traditional electric radiator that turns that 1kWh into one unit of useful heat, a heat pump converts 1kWh into 3.5 or more units of useful heat.

Therefore, electrically run heat pumps are actually a really efficient way to heat the home. However, because the extent of the hot water temperature produced by them is not as much as with a gas boiler, you need to ensure that your home is sufficiently insulated in order for the heat pump’s influence to become maximised.

However, if you are satisfied that your property is sufficiently insulated then the next question is, which to go for – a ground source heat pump or an air source heat pump?

Air source heat pump versus ground source heat pump 

As mentioned previously, air source heat pumps absorb their heat from the air, while ground source heat pumps absorb it from the ground. In the following sections we will provide a comparison of the two technologies.

Cost of heat pumps

While the scientific basis between the two technologies is the same, the cost of having them installed is not! The cost of an air source heat pump (without considering underfloor heating) is approximately £7,500. A Ground source heat pump on the other hand is £23,000, with much of the additional cost going on bore hole that needs to be drilled – this could cost as much as £9,000 for a couple of (albeit very deep) holes in your back garden. If you have the space, you can lay the ground source heat pump piping not vertically down, but horizontally within a field, but this obviously requires a lot of space, which brings me nicely on to the next point.

Land Requirements of heat pumps

Air source heat pump units look very much like air conditioning units that you may have seen on office buildings. The size of the heat pump will be determined by your installer based on your homes heat demand – the bigger the home (+ the less insulated), the more heat it is going to require – so the larger the air source heat pump unit.

A typical unit might be a metre high by a meter wide with a depth of about half a meter. This will obviously vary greatly between models, but will give you something to work with!

Ground Source Heat Pump pipeworkA ground source heat pump requires buried pipes in the garden, so the first consideration is whether or not you have a garden to lay them! If you want to put the pipes in vertically, bore holing machinery will need to be able to access the garden. In some cases even this might be a showstopper. Although, as said previously, while laying the pipework horizontally takes up a larger surface area of land, it is a cheaper method.

Efficiency of Air Source heat pumps vs. Ground Source Heat Pumps

There is one clear winner here – ground source heat pumps.

Firstly, consider what time of year you use most hot water in your home – the middle of the winter. It is then that the radiators are on at full whack and you have more hot baths and showers.

Secondly consider how a heat pump works – it takes heat from either the air or the ground, which warms a ‘working fluid’, which in turn gets compressed maximising the potential temperature.

Now, the higher the temperature of the air / ground, the easier the heat pump needs to work to get up it up to the required temperature. So in the summer, an air source heat pump will take in warm air (although not always in the UK!), and the heat pump will have to do very little to heat the water to the required temperature.

Conversely, consider the winter months, when there is a far higher demand for hot water. At this time of year, the air temperature is far lower, so the air source heat pump will need to work far harder to reach the required temperature.

With a GSHP, the pipes are running buried in the ground, where you don’t get the same temperature fluctuations as you do above ground – the ground is fixed at about 100C. This means that during the winter, when the air temperature is cooler and the ASHP will need to be working really hard, a comparative GSHP can provide the same hot water requirements off fewer units of electricity.

The Coefficient of Performance of Heat Pumps

The coefficient of performance (CoP) is the ratio of heat delivered by the heat pump versus the electricity supplied to the compressor, so the bigger the CoP the better. For example, a heat pump with a CoP of 3 means that for every 3kWh of heat output, 1kWh of electricity is used to drive the heat pump.

Now as I have mentioned previously, the heat pump will need to work harder when it is colder, and less hard when it is warmer, so the CoP varies on a daily basis. Therefore, in order to compare different heat pumps it is best to look at the Seasonal Performance Factor, which is defined as the ratio of heat delivered by the heat pump versus the electricity supplied to the compressor over the heating season.

The Seasonal Performance Factor of Heat Pumps

For an air source heat pump you can assume a typical seasonal performance factor of 2.5 – 2.8. This takes into account that during the winter, the CoP will fall well below the 2.5 level, but during the summer it might be as high as 4.

For a ground source heat pump you can expect a typical season performance factor of about 4. That means that across the heating season, the GSHP will be much more efficient.

The financial reward of investing in a heat pump – the RHI

The Renewable Heat Incentive has now launched and has been introduced to reward early adopters of heat pumps with generous financial subsidies. The RHI for heat pumps are calculated by taking into account the heat demand for the property and the seasonal performance factor for that particular heat pump.

The RHI formula is calculated as per below:

1. Eligible Heat Demand = Total Annual Energy Demand x (1 – 1 / SPF)

2. Annual RHI = Eligible Heat Demand x RHI Tariff

However the RHI tariff for the GSHP is 18.8p / kWh, while for the ASHP it is just 7.3p / kWh. This is due to the up front cost of an ASHP, £7,500 and a GSHP, £23,000.

They have calculated that so by and large the RHI (at the initial rates – they will be reviewed downward on a quarterly basis) will cover the cost of the installation.

Final Thoughts – Ground Source versus Air Source Heat pumps

In summary, heat pumps are a very worthwhile financial investment (especially when you consider the renewable heat incentive), particularly if you don’t have access to mains gas.

Whether you opt for a ground source or air source heat pump is dependant on your situation, but to be honest as the technology improves there is very little in it – I guess it really depends on whether you can afford the additional expense of the GSHP for the increased efficiency – bearing in mind that the RHI at its current level should largely cover the install cost of both of the technologies.

Installing heat pumps

Are you thinking about getting a heat pump? We have scoured the country for the best tradespeople, so that we can make sure we only recommend those we really trust. You can find one of these tradespeople on our easy to use local installer map.


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