Ground Source Heat Pumps Pros and Cons

This month’s article discusses ground source heat pumps, how they work and what’s involved installation wise. The heat energy stored in the ground as little as 1.5 metres down does not vary more than 4 to 6 degrees from season to season. This gives them a distinct advantage over air source which might see temperature variations of 30 degrees or more from summer to winter.

The majority of ground source systems are the indirect type and extract the heat from the ground via lengths of plastic pipes filled with glycol (anti-freeze). The pipes can be laid in shallow trenches or in boreholes up to 200m deep. The direct type has refrigerant in the ground loops and is far less popular though extremely efficient > 600%. There has been very little uptake on this type of installation as there is no government RHI available for them at the moment. The added benefits of a direct system is that it requires less pipe, the downside is it requires F-gas qualified engineers to install it correctly making it a more expensive.

Frequently Asked Questions

1. A ground source will run at -20 degrees ambient air temperature – is this true?

Yes as the ground temperature would be nowhere near the air temperature and would only have a minimal loss of efficiency in the colder weather.

2. How much land is required?

For a 10 kw an area of 20 metres by 50 metres is needed to space 1.5m deep trenches with a width of 1m. For a borehole system a 10m square would be adequate for the same 10kw heat pump, the holes being a min of 6 metres apart.

3. How do installation costs compare with air-source?

A ground source system can typically be 3 to 4 times more expensive to install than an air source. A borehole can cost from 4 to 6 thousand pounds to drill and a trench system could cost 3 thousand pounds or more depending on the size of the excavations.

4. Are ground source more efficient?

Yes especially when their S.C.O.P. is used to gauge their efficiency.

5. What are the best makes to buy?

You should seek out a company that sells modulating ground source heat pumps which have inverter driven compressors. They are more efficient and will have a longer lifespan.

6. How noisy are they?

Most ground source are relatively quiet but it is not recommended that you site them inside your property. A dedicated plant room or back of your garage is a good place to fit them.

7. How expensive are they to maintain?

They can be a bit pricey to maintain as the anti-freeze has to be checked and is recommended to be changed completely every 6 years. A typical annual service could cost between £200 and £300.