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Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 5:13 pm
by nowty
(nowtyproject for seaching)
As my original project story in 2018 died with other content on another forum, I thought I would re-document some of it, as it’s a quite interesting low cost project using an off the shelf heatpump to provide spaceheating and hotwater. I'll do it in several bits as and when I have the time to re-write it.

Nearly a decade ago I fixed a 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 I could tap into for,

(1) Pumping out to harvest the water for garden irrigation and for flushing toilets.
(2) To incorporate a heat exchanger for a heatpump as the water temp ranges from a high of 14 degrees in summer to a min of 7 degrees in winter.

The water harvesting has so for yielded nearly 400,000 litres of water but the daily amount averages about 150 litres so significantly below the 20,000 litre daily limit which would require an abstraction licence from the Environment Agency.


The underground river
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Looking down my manhole (pipes in the picture are for the water harvesting)
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Back then I could not find much technical data for a water sourced heat pump so its was several tears later in 2018 that I found some good technical info on the Kensa heatpump website. The critical value I found for a water sourced heat pump in moving water was there is a 3 litres per minute per kW output rule.

So I did a simple test to measure the flow rate after it had not rained for 2 months. I measured about 15 litres a min so theoretically I could extract 5kW or so. And given that in winter the flow is generally better, maybe 7 or 8 kW was possible. The Kensa website also said one submersible mat with 250m of 32mm HDPE pipe can provide 6kW. But the only space I had in the pool of water was 600mm wide by about less than a metre wide.

Obviously, that amount of pipe had no chance of fitting, but what if I used copper pipe as the thermal conductivity of copper is 800 times more than HPDE pipe. But it was not as simple as that as the limiting factor is the thermal conductivity of water itself.

Then I found on the Engineering Toolbox website that the overall heat transfer for copper as a heat exchanger in still water is circa 400w / k m2. Therefore, for an assumed 5 degree temp difference and an 8kW extraction I would need a copper surface area of 8000 / (400 x 5) = 4 m2.

Coils of 25m of 10mm copper microbore pipe were available, were easy to lower down the manhole and each one has a surface area of 0.79 m2. So I needed at least 5 of them to reach the 4m2 target.

Ground source heatpumps were pricey bits if kit, but I found an 8kW ex display one which had been in the showroom of Cool Energy on ebay being sold for around £1k. So I took a punt and bought it along with 5 coils of copper pipe.


My purchased heatpump
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Inside the Heatpump
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Ground Source Heat Exchanger
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Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 6:10 pm
by Stinsy
This is one of the most compelling “When life gives you lemons…” stories I’ve ever encountered!

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 7:54 pm
by Sunrisemike
wow, not much in the box, just a couple of fridge compressors!!

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 7:59 pm
by nowty
Sunrisemike wrote: Sat Nov 27, 2021 7:54 pm wow, not much in the box, just a couple of fridge compressors!!
Actually only one fridge compressor and two water heat exchangers, that's one reason why I cannot comprehend everyone says they cost £10k and you need a £5k grant to smooth the cost, It cost me £1k, ok it was ex display but it was as new.

Plus there's a few electrics in it,
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Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 9:25 pm
by nowty
In the end I installed a 22mm copper pipe from the cold circuit of the heatpump through the driveway and down the manhole to a manifold connecting 6 x 25m coils of 10mm microbore in parallel.

I air pressure tested the ground loop by using a pressure sprayer up to 2 bar, unfortunately it leaked badly. Using leak detection spray (soapy liquid) I quickly found 4 compression joints which were pretty bad. Copious amounts of sealant compound (Fernox LS-X Leak Sealer) and PTFE tape sorted it.


22mm copper pipe through the driveway and down the manhole.
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6 x parallel copper coil manifold.
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Copper coils sunk below the waterline and cable tied together for rigidity.
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Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 9:43 pm
by spread-tee
Thanks for taking the time to re-do this, it was a good thread, such a shame they all disappeared. I don't know if I can be rrsed to resurrect CRAC!!

Forgive me for being pedantic but it looks to me as if you have wound PTFE tape around the threads of those compression joints, It doesn't do any good there, a turn or two around the olive only on a comp joint.

Desp

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 10:24 pm
by nowty
spread-tee wrote: Sat Nov 27, 2021 9:43 pm Thanks for taking the time to re-do this, it was a good thread, such a shame they all disappeared. I don't know if I can be rrsed to resurrect CRAC!!

Forgive me for being pedantic but it looks to me as if you have wound PTFE tape around the threads of those compression joints, It doesn't do any good there, a turn or two around the olive only on a comp joint.

Desp
My own experience with compression joints, especially if they don't quite line up perfectly is a layer of Fernox LS-X Leak Sealer (its similar to thick silicone sealant), then a few wraps of gas grade PTFE tape (not the standard thin PTFE tape for water). I have found there are different qualities of compression joints, the best ones seem to be the "Yorkshire" brand which are expensive and bullet proof BUT the pipes have to be lined up perfectly. So I prefer the cheap ones where you can get away with an unperfect line up but you then need the PTFE tape and sealant to make it good.

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sat Nov 27, 2021 10:42 pm
by Oldgreybeard
I discovered the Screwfix "Liquid PTFE" a few years ago. Seems to be brilliant on compression fittings, although a plumber friend reckoned it was a bodge. A thin smear on the pipe, and another smear over the olive, and it seems to just make compression joints foolproof. The only slight problem is in taking them apart afterwards, but warming the joint up seems to make the stuff soften and break free,

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sun Nov 28, 2021 9:03 am
by spread-tee
Fittings are pretty variable in quality that's for sure, personally I solder all copper work,I only use compression if I cant use poly plumb or the pipework wont drain out enough for a solder joint.

Desp

Re: Ground Source Heat Pump at Nowty Towers

Posted: Sun Nov 28, 2021 12:29 pm
by Joeboy
spread-tee wrote: Sun Nov 28, 2021 9:03 am Fittings are pretty variable in quality that's for sure, personally I solder all copper work,I only use compression if I cant use poly plumb or the pipework wont drain out enough for a solder joint.

Desp
Same here. Love a Yorkshire fitting and the smell of flux in the morning!