Welcome to our short guide to Ground Source Heat Pumps (GSHP), highlighting details you won’t get from the manufacturer’s sales material. In our experience, they are important factors to consider before taking the plunge. PGSHP share the same issues as Air Source Heat Pumps (ASHP)with regards to radiators, use of UFH and hot water production highlighted in the previous article. However it also provides notable benefits to ASHP, such as, the elimination of an external noise source and also minimises the negative effects of external air temperature on the heat pump technology.
As highlighted in our last blog, the instantaneous ASHP performance varies with external temperature and heating flow temperature, the further apart these become the lower the performance of the ASHP will be. But with GSHP, because you’re using the external ground which annually is a relatively consistent temperature (8-12°C), there will be an efficiency uplift in Seasonal Co-efficient of Performance (SCOP) points.
Types of closed-loop ground source:
- Slinky or Meander: slinkies are a length of coiled plastic pipe buried in wide trenches using a digger. Meander installations are similar to slinkies but the pipe is not coiled and is simply run back and forth in a similar pattern to underfloor heating pipework.
- Boreholes: drilled vertically down. Boreholes tend to be 60-100m deep and usually consist of a single looped pipe inserted within the bore-hole.
- Water Arrays: If the property is next to a lake then it is possible to use the lake water as the energy source. This is generally the most cost-effective means of installing arrays as digging is kept to a minimum. Usually, slinky or meander pipework is laid in the bottom of the body of water.
So, consider whether your garden is suitable for a ground loop. It doesn’t have to be particularly big, but the ground needs to be suitable for digging a trench or a borehole and accessible to digging machinery, so consider access. Boreholes may (or may not) be costlier but will dramatically reduce the mess and disruption of digging trenches.
Cheaper than fossil fuels?
This will depend on your energy prices, for example if you pay 4.0p/kWh for mains gas and 15.5p/kWh for electricity, comparing a typical 92% seasonal efficiency gas boiler, this would require you to maintain a SCOP of ~3.6 to be comparable. Depending on your system set-up this may not be possible so worth checking out before deciding.
Consideration with other fuels that require delivery such as LPG and Oil and the GSHP may be more competitive, plus other benefits such as removing and storage tank and deliveries from your life. At the time of writing, oil prices were around 55p/Litre, or 5.5p/kWh delivered, meaning the ’break-even’ COP is 2.7.
A useful resource for oil price trends is https://www.boilerjuice.com/heating-oil-prices/
For LPG, prices are currently around 6.5 p/kWh delivered, giving a break-even COP of 2.3 (price figures from ‘Which?’)
A Useful resource for fuel conversion factors is https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/47732/7309-cca-draft-technical-guidance-app-b.xls
One selling point suppliers always reach for is the recent increase in the Renewable Heat Incentive (RHI), when you receive quotations for ASHP systems paybacks look very satisfactory. However, most don’t include installation costs and importantly ground works. Groundworks are a significant cost, and, on many projects, we have worked on, once factored in, the RHI payment really only covers a significant portion, not all of the capital cost uplift, when you consider the whole installation.
So, you have a high-performance building fabric, great energy efficiency measures, running the GSHP system at optimum settings but one crucial element is the ground and having an accurate appraisal of ground conditions. These will vary immensely from destination to destination either sandy or rocky, dry or saturated, these conditions will not only affect your cost in ground works and potential array size but also your best anticipated efficiency, so this is crucial and often over looked by an eager client.
Is it all worth it…?
This will be a balance of benefits offered to your particular situation, over and above solely cost or return on investment; one factor might be building regulations compliance for example. Before considering “what type of system do I want?” Do what you can to ensure that the heating temperatures are as low as possible, through great building insulation, and hot water usage is as low as practical. By doing this you can see significant efficiency improvements in whatever system you choose to use.
If you have any further questions, please ask us.
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