Not sure if this is the correct section to post...

I am installing my 6kW Heat Pump next month. Over the winter i renovated the entire house, Insulated, installed underfloor heating on both floors in all rooms with 116mm pipe spacing.

I have purchased the heat pump and just need to dig the loops in the field.

Location:
At the bottom of the field, 50m from the house, its a valley with a steam running through it. During the summer it dries up typically.
I will run straight pipes parallel with the stream, digging 1.5m deep i can get down to below stream level.
Its the end of August now, we have had a damp august, stream is dry, but when i dig to 1.5m its wet clay going into sand at the bottom of the trench. See photos. (IMG_3334 is after 48 hours) plenty of water in the ground all year i suspect.

I purchased a report from the British geological survey. They confirmed we are on sandstone with a thermal conductivity of 2.59W/m.K.
I spoke to them, as this is the bedrock figure and we wont hit that, they suggested 2.5W/m.K. as i have just hit unconsolidated sand.

Looking at the MCS lookup tables, wet sand is 2.4W/m.K.. So i have gone with that figure.
Average ground temp at 1m is 11.6c from their report.
I measured the ground at 1.5m immediately the excavator dug it and it is currently 16.3C in August.

Sizing
I plan to install enough to satisfy the house demand when we hopefully extend in a couple of years! This would mean upgrading to a 8-9kW heat pump. I have run calcs for both initial and proposed sizes. I intend to install enough pipe now to satisfy the larger pump.

I have a old 3Ton Kubota i plan to spend a few days sitting on to do this work!

Pipe will be PE100 32mm.
I dont plan to blind with sand. There is 0% rock in the ground and at 1.5m i am into sand.

The MCS tables only reference 25mm pipe. Using 32mm i expect a slight increase in thermal absorption ,but i wont attempt to factor that into the design

My Question Is:

The tables talk about 750mm pipe spacing.

I plan to dig a 400mm wide trench (its the bucket i have). At 400mm it seems reasonable to lay 2 pipes in the trench at 400mm centres.

MCS calculations and table suggest 182m of pipe for my 6kW.
Then 335m when i upgrade to 9kW.

Would it seem unreasonable to lay 400m of pipe in 2x 200m trench, pipes spaced at 400mm in the trenches, with 1.5m between trenches. With the plan to be well oversized initially, and suitable for when i put the bigger pump in.

I read somewhere that 300mm is good practice to avoid thermal interference between pipes.
But as MCS tables reference 750mm, does my extra 65m (20%) cover the fact im 400mm centres.....?

Initial Dig Conditions. Clay into Sand(Grey).
https://postimg.org/image/epei37y3n/

Ground Temp measurement during dig.
https://postimg.org/image/93s2z5xer/

After 48 Hours.
https://postimg.org/image/f5zppnlur/

Sizing calcs for current 6kW HP.
https://postimg.org/image/6p07eqh5v/

Future 8kW HP Calcs.
https://postimg.org/image/f9y4jngpv/

Location.
https://postimg.org/image/ebhrhd3dv/

Posted By: geoheatedThe tables talk about 750mm pipe spacing.

I plan to dig a 400mm wide trench (its the bucket i have). At 400mm it seems reasonable to lay 2 pipes in the trench at 400mm centres.

MCS calculations and table suggest 182m of pipe for my 6kW.
Then 335m when i upgrade to 9kW.

Would it seem unreasonable to lay 400m of pipe in 2x 200m trench, pipes spaced at 400mm in the trenches, with 1.5m between trenches. With the plan to be well oversized initially, and suitable for when i put the bigger pump in.

If you work out the total area that the MCS figures imply and make sure you cover that area, I can't see how that would be worse. So they want 750 x 335000 = 251250000 mm². For every metre of your dual pipe trench you cover 500 x (750+400) = 575000 mm² so I think you need to lay two pipes in 251250000/575000 = 437 m of pipe in 219 m of trench.

+1 and may as well do a bigger hear pump now or insulate better which will be cheaper in the long run.

How deep? And how will you turn the corner at the end of the trench?

2m deep every time for me and never use slinkies - shallow = inefficient too smaller heat field = inefficient and frozen ground

Rob, maybe talking about the same thing, but straight water pipes can be laid by a "mole plough" implement behind a tractor, it cuts a slit that closes itself behind the pipe without needing backfill.
This one is apparently 700mm deep:
https://youtu.be/C1FXgpZknJc

(Edit: the gshp manufacturers don't mention using these, presumably for some good reason...)


New Ashps are about as efficient as gshps nowadays. Given you don't use much heat, would any small advantage in efficiency outweigh the extra installation costs?

I think youve already mentioned it but you can get 1200mm deep pedestrian controlled trenchers that cut a slot 100-150 wide. See Ditchwitch.co.uk

I dont think you need a MCS installer unless youre going for one of the subsidy schemes. As I understand it you only need an F gas fitter if your install involoves breaking into the refrigerant system and recharging.

I appreciate ASHPs efficiency may have improved but I cant see them being as efficient as an equivalent age GSHP when the temp drops and you need heating the most.

MCS020 requires that both the ASHP and the installer have been certified by MCS.

It also includes a procedure for calculating noise levels to check acceptability under permitted development.

This implies that DIY ASHP installs are not permitted development under this provision. I suppose you could put in a planning application to show that it wouldn't cause a nuisance, or you could chance it.

Of course there's legal issues about DIY installs of gas/oil/electric heating too, and electrical hookup to a HP

Phil, you can check on the certified SCOP values for heatpumps at

https://mcscertified.com/product-directory/

For example:

Kensa 3kW GSHP SCOP= 3.52
Mitsubishi ecodan 5kW ASHP SCOP = 4.57

Both at 35degC CH water flow temperature

(SCOP = coefficient of performance, averaged over a standardised set of weather conditions to represent a heating season)

I have heard that the 1.2m depth is the level below which you should have a trench box, so only digging the trench this deep can be done without one. I think deeper is better but maybe diminishing returns comes in to play? (At some points I went deeper than this, and at one point slightly shallower due to a mahoosive rock that wouldn't budge).

We used slinkies (Kensa system) and I dug the trench about 1500mm wide using a 900mm riddle bucket (overlapping each pass), because it proved easier to clear out the bucket on our heavy clay. There was also a 1500mm flat bucket that I occasionally used to scrape out the trench base where necessary.

I believe that a straight pipe can be put in each side of a single trench but they should be 1m apart, so you would have to use a 1m bucket. We have a section approx 45m long from the manifold (at the edge of the field) to the barn where there is one larger pipe laid down each side of a single 900mm trench (the width of the bucket that I had).

I looked into hiring a trencher for a narrow trench and using straight pipe (as opposed to slinkies) but in hindsight it would have covered a larger area of the field so would have cut through more land drains, and there are a few random larger rocks which I think it would have struggled with. Plus you need to dig out to turn around and change direction so you need a digger anyway as well. I think this might have resulted in a neater job and a field that recovered quicker.

The design spec was for 2 x 40m slinkies but I dug the trenches to 50m as there was enough space, and we cable-tied a few more slinky loops in to use up the spare header length on the pipe. So it should end up slightly oversized, without spending any more money.

It took me 2 days to dig the trenches and 4 days to backfill (carefully) with a 10 tonne JCB360 machine.
Trenches are on 5m centres.

Rolling out and cable-tying the slinkies was an "experience", in a rush, in the p***ing rain, trying not to fall out with SWMBO. I can see why companies prefer straight pipe.

On a quick read of https://www.planningportal.co.uk/info/200130/common_projects/27/heat_pumps/2 I noticed another perhaps interesting condition for an ASHP to be permitted development:

"Used solely for heating purposes"

That's annoying. Though how they would tell or enforce it I don't know.

Posted By: Peter_in_HungaryWith regard to laying GSHP collector pipes - no one has yet mentioned moleing the pipes in. That is 2 trenches as deep as you want as far apart as needed and then a moleing machine to put the pipes between the two. Is this ever done

Ive no idea if its ever done but there would be limited contact between the ground and piping so I guess there would be limited heat transfer??

Some great insights there people!
I had not seen that mcs document, and I think it indicates 104m (East Anglia, wet clay, 3MWh/year extracted + 1MWh elec). I notice that the tables in the doc are split into various runtimes, 1200hours, 1800hours, etc, which makes sense. If I used a slinky, that mcs doc suggests a 32m long trench, with 128m of pipe in it.

Kensa suggest more simplistically 100m of pipe for a 2kW heatpump, either as a slinky in a 1.2x1.2x20m hole, or straight. As they don't offer a 2kW unit, they would suggest a 3kW hp, and 150m of pipe. The figures do end up similar for us, but the mcs doc indicates it's more about the total yearly heat demand than the heatpump power, as Will pointed out.

Very good point about turning a trencher, I hadn't considered it! I guess I'd have to do a series of parallel straights with top and tail straights, so several 200mm radius bends. I'm expecting bending the pipe to fit those bends would be "annoying". I guess it could be filled with hot water to help, and worst case the pipe would end up riding up on the bends, and not living at the bottom of the trench. Can you get things that help bending it to a 200mm radius?

I'm guessing the PP ashp for cooling issue is because people go outdoors more in warmer weather, so noise is more likely to cause offence than during the winter. Gshp avoids this issue, and gives much cheaper limited cooling.

I did hire a trencher last year, dig a load of holes in our garden, and put in ~160m of 25mm dia pipes comprised of 4x40m loops, and connect them all up to a heatpump. It's a relatively small system as we have lots of EWI insulation now, which made it plausible to completely do diy. The digging happened back last November, the garden was like a farmers field for a month or so after, but grass seed grows even in winter here it seems. The trencher itself was easy to use, but there was a lot of manual digging at the corners - trenching 90degrees to an existing trench does work, however it backfills a lot of the original which has to be manually removed. Getting the pipe in and laying at the bottom was pleasantly simple - we unrolled it into the narrow trench and it naturally sat at the bottom, then we used cold forming benders and a heatgun to form any tight corners, spraying on cold water to set the bend before removing the bender.

The system has been running since early Dec this year, we've been gas free since then. We do still have the boiler; it's plumbed in series, and there's a 3-way changeover switch so we could swap back if needed - the wires being the original old school call4heat stuff. SWMBO would not be happy if the place ended up cold.
I hoped and tried to use tap water instead of a glycol mix in the groundloop. That worked really well for a month, then the heat exchanger started to ice up, so I swapped out all of the water from one of the 4 loops for £200 of glycol and then turned the whole thing back on to let it all mix together over time. It's stopped the freezing up for sure, but I have noticed it's not as good - both the heat transfer is worse, and also the flow is lower.
There's a calibrated elec meter inline with the heatpump - it now reads 620kWh after 77days operation, and does all our heating, and almost all the DHW - we have an immersion that comes on once a week overnight. It's got an elec meter too, and uses ~1kWh/day average - some of that is the immersion manual "boost" button being pressed.

I did a complete writeup for the system as a pdf, and lodged it on ecorenovator as they don't mind huge files, it just seemed the simplest way to get the story all in one place. If you want loads of diy gshp details, please follow the link, download the over-long pdf, and enjoy!

https://ecorenovator.org/forum/attachment.php?attachmentid=9418&d=1641225224

You're too kind!
I wanted to be involved, as the heatpump involves very interesting engineering. I then proceeded to tie myself in mental knots - diy fit monobloc ashp? - the planning issue put me off. Buying a pre-made gshp and then diy fit? - all the ones I found have f-gasses so I couldn't fully service it (unlikely to be needed admittedly), there aren't many units and the ones I found wouldn't fit where I wanted. Also diy heatpump install generally voids the guarantee of a commercial unit.
In the end I persuaded myself it was simplest, cheapest - hobby time is free after all, and certainly the most interesting to start from scratch.