A few years ago I fixed a major problem under my house which involved building a reinforced concrete box culvert to slightly divert an underground river.

When fixing the problem I purposely created a catchpit to provide a pool of subterranean water,

(a) So I could easily pump out and harvest the water.

(b) To incorporate a heat exchanger for a heatpump as water temp ranges from a high of 14 degrees in summer to a min of 7 degrees in winter.

(a) was done some years ago with over 190,000 litres extracted do far, (averaged 500 litres per day this month for the garden)

but for (b), I could not find much technical data for a water sourced heat pump so its been on the back burner.

Now I have found some good technical info on the Kensa heatpump website. It seems that for a water sourced heat pump in moving water, there is a 3 litres per minute per kW rule. Although it has not rained here for over two months I have measured a flow rate of around 15 litres per min which suggests I could theoretically extract 5 kW or so. Given that in winter the flow is generally better, maybe 7 or 8 kW is possible. The Kensa website also says one submersible mat with 250m of 32mm HDPE pipe can provide 6kW.

Hmmmmmm, I don’t have the space for 250m of 32mm HDPE and the manhole access is only 600mm wide but what if I used copper ?

Its thermal conductivity is 800 times more than HPDE but then 250 / 800 = 0.3m of HPDE obviously would not work as its insanely short. Therefore I think the much lower thermal conductivity of water needs to be taken into account. Then I found on the EngineeringToolbox website that the overall heat transfer for copper as a heat exchanger in still water is circa 400w / k m2.

Therefore assume a 5 degree temp difference and an 8kW extraction I would need a copper surface area of 8000 / (400 x 5) = 4 m2.

I can buy coils of 25m of 10mm copper microbore pipe, each of which are easy to lower down the manhole. Each one has a surface area of 0.79 m2. So I bought one and I found it fitted easily with enough room for 4 or 5 of them. So that’s a total copper surface area of 3 or 4 m2. Mmmmmmm, its now getting interesting.

Ground source heatpumps seem to be pricey bits if kit, but I found an 8kW discontinued and ex display one currently in a showroom on fleebay being sold by a heatpump company for around £1k. So took a punt and it arrives next week.

**Heatpump Spec**Bought the rest of the copper and I have been prototyping the heat exchanger on my lounge floor, much to the dismay of SWMBO.

**Obviously more to follow .....................................................**