I am no expert on ground source heating but I found going through the process of having it installed that there were real practical elements that I was not getting or understanding from researching the web or talking to ground source heating contractors. Also there are a lot of comments such as ‘its more suited to new builds’ & ‘the building should be well insulated’. Neither are applicable in our case so I wanted to share our findings of having a ground source heating system installed into an older stone property.
What is Ground Source Heating?
A meter below the ground the temperature remains fairly constant throughout the year. Fluctuating between 8 to 12 degrees winter to summer. This heat energy acts as a large reservoir and as such can be used with a heat engine to heat your home. This energy is from the sun and is sustainable and renewable. Your sourcing the heat energy for this from the ground and involves running pipes in land close to your property.
How much land does it require?
There are several factors that affect this. In modern houses with modern materials and techniques insulation can be very good and the heat loss will be lower (than say a 150 yr old stone cottage in the middle of nowhere!) meaning you need a lot less heat to warm the property. So to give some frame of reference for Chester House we are installing 6 loops that are 50m out and 50m back again. This was calculated by our Ground Source Heating contractor. This is going in a pasture field behind the house.
Is it suitable for a stone cottage?
Well I hope so or I will be gutted! Its not for the faint hearted and the advice is to improve the insulation as much as possible and because the heat is ‘low grade heat’ it tends to be more suitable for use with underfloor heating. So an older cottage which probably has an un-insulated concrete floor has to come up. It might be possible to go over the top of existing floors but in our case the ceiling height was already very low. We found out one of the reasons for this when we excavated the kitchen floor. Breaking the concrete with a jack hammer showed a first layer of 100mm thick. Here there was a damp proof membrane over a previous 100mm thick concrete layer – probably the original floor which had no damp protection.
How far down do I need to excavate the concrete floors?
This is where we had some conflicting information. So wherever you excavate to, you will need to add hardcore to 100mm and then poor a new sub-floor of concrete at least 100mm thick. On top of this you need 100mm of insulation – Celotex or Kingspan are brands in the UK. On top of this goes the underfloor pipes on clips and screed is poored onto the pipes. Pipes and screeds will be 65 mm thick. Finally don’t forget the final layer of flooring – in our case some slate tiles ~14mm thick but we did consider reclaimed flags of 40mm. So your looking at 400 mm. Consider what you will do with all the material that is excavated. Rather than going to landfill we are reusing the excavated material to fill in large pot holes and dips on the track to the house. Consider also about pouring the new concrete, we have a logistical nightmare with getting the concrete lorry to the property. Don’t forget to put in a damp proof membrane as well.
How much does it cost?
You will obviously need to get a quote for the heat pump, pipework and installation but don’t forget to add in the costs for excavating the concrete, adding hardcore and pooring a new sub-floor, digging the trenches for the pipework in the field and consider that to do it properly partitioning walls may need to come out and insulating materials will need to be installed, walls reinstated, skirting boards, doorways, walls re-plastered and repainted. If your going to do some of this anywhere it might not be too painful. It also depends how much you can do yourself. We had a team of tradesmen.
Where does the Heat Pump go?
The associated equipment needs somewhere to go and can make some noise with the pump running for long periods. Something like a basement or utility room will be required. There is a heat pump that needs access around it, expansion vessel and a hot water tank. We placed it in an outbuilding that was very close to the house.
Underfloor Heating Detail
Although we had some illustrations from the installers about how the pipes are installed and where the insulation goes in different situations it was not that clear and had obviously been through the marketing department – so no use nor ornament to anybody. We also wanted to leave some of the beams exposed. The following drawing I created after quizzing various people. It shows a cross sectional view of how the underfloor heating pipes sit in an aluminium tray and the insulation sits between the joists and in our case because we expose the beams sits on slates attached to the inside of the beams. Anyhow I’ve tried to make it explicitly clear in the image – PDF.
Similarly on the concrete floors on the ground floor you may find that the ground source heating installers will talk about and specify the pipe work and installation but will be rather vague about the sub-floor. Our guys in the beginning were talking about ‘needing to go down 165mm’. However this is just for there bit and does not account for needing a decent subfloor below this and the truth is you need to excavate 400 mm minimum.
Chester House Farm 