My ground conditions are clay, which is wet in places. I have dug a load of trial holes around the field at 1500mm deep where the collectors will go. All these had some water in them after 12 hours. I had a trench open at about 600mm deep for a week which remained dry.
Is it worth going down any deeper than 1500 with the pipework?
I'll have a 20t swing on site, so depth isnt a problem...
My instinct says to go as deep as practicable whilst I have the digger on site. The pipes will be laid in a 5ft wide trench with a pipe at each side, covered with topsoil first before backfilling.
Any benefits of going to 2m or so?
Cheers

Yes Nick, the pipework is 40mm so a 1500mm deep trench will have 1460 above the pipework. In fact the depth will be a bit deeper once all the topsoil is replaced, about another 200mm or so.

Posted By: Ed Davies - ground source is essentially a solar collector (actual geothermal heat from below being negligible in most cases) so heat must flow down from above to replace that extracted. The usual advice is about 1 metre down with the idea of getting reasonable stability without too much insulation. Maybe somebody who knows properly will be along to advise otherwise for your case but getting just into the water seems to me to be a good idea from the point of view of getting good heat flow and probably roughly consistent with one metre depth.

That all ties in with what we picked up while considering (& eventually dropping the idea of) a GSHP.
Our ground has a water table that is much higher than that & were told that they'd need to run a pump to keep the trench dry(ish) during construction.

Edit to Add: "during constructon".

... see the attached

Posted By: skyewrightOur ground has a water table that is much higher than that & were told that they'd need to run a pump to keep the trench dry(ish).

Why would you need to keep the trench dry(ish)? Is it just that dry below-zero soil has a better thermal conductivity than ice? Other than that, I thought being in the water was better.

Posted By: DarylPAre you sure, the ground is a solar collector, but the heat flow in the heating season is not downwards.

Bit difficult to parse that sentence but yes, during the heating season the heat flow in the soil will be upwards so not being too close to the surface is a good idea but over the whole year the net flow must be downwards; the heat taken out by the GSHP pipes needs to be replaced. If the pipes are too deep there will be too much insulation for that to happen so the ground wouldn't warm up enough in the summer before the next winter comes along.

How deep that it is is another question; from what I've read about 1 metre seems to commonly be about right but that's rather second-hand information.

Boreholes are bit different as they rely on heat transfer by water movement (which slinkies can use too, of course) as well as having ground available in more directions.

It must depend on the area that the pipework covers and how much heat is being taken out of the ground by the heat pump. (If you go too deep, it may take more than year for the ground to recover.)

Posted By: Ed Daviesom what I've read about 1 metre seems to commonly be about right but that's rather second-hand information.

I recently read that pipes coming to/ from the manifold should be insulated at 1M deep to protect the ground from freezing/ heaving..

So much conflicting info!

2m minimum, and the area of the heat field is very important too small wrt amount of heat being drawn out has led to big expensive problems.

Boreholes work well

I have never understood how boreholes recover unless they are below the water table. As the cold earth is a long way from the surface. However I never did like 3d maths!

Posted By: Ed DaviesWhy would you need to keep the trench dry(ish)? Is it just that dry below-zero soil has a better thermal conductivity than ice? Other than that, I thought being in the water was better.

I should (& now have) added "...during construction".

They said they needed a clear view to be sure the pipe was properly in the bottom of the trench, and then properly buried during backfill.

At the time we "walked the ground" (& for most of this year come to that) the water table was just slightly above ground level and flowing (very slowly) across the site, so quite some pump would have been required!

If we'd gone ahead the ground conditions, with an almost constant slow flow of water through it, would have been ideal (very good thermal contact & heat loss replenished by the passing water).

Got to be remembered that Woodgnome is in Cornwall, so most of the energy is transferred via water, which is warm here. We rarely get a frost (think I had 2 last year).

I think, and from what I understand from my mate that used to work at Kensa, that soil type is important. Down here, and especially on the North Coast, there is little soil (in the scheme of things). Bit different on the South Coast where there is more clay (what St Austell is built on).
Without doing a lot of research into the local soil type and water table, I suspect that there would not be much difference between 1 or 2 metres depth.

There is very little energy moving upwards from the Earth Core, about 65 mW/m^2
https://en.wikipedia.org/wiki/Geothermal_gradient
But there are places where it is much, much greater (volcanoes for one).

We don't need much heating in November, but generally February is a cold month and we do. This has a lot more to do with ocean temperature and weather patterns than stored energy in the top meter or 5 of the ground.

Posted By: BeauOurs are between 1m and 1.2 m depth for single pipe not a slinky as per our suppliers recommendation.

What is the pipe length and the output of your GSHP. This is really the important bit I think. The pipe length 'sets' the land area needed, coil the pipe up and you probably need more pipe overall as the energy harvesting area is reduced.

Could read though this, funny units thought.
http://geokiss.com/res-design/GSHPDesignRec2.pdf

I had a similar conversation with a heat pump installer that i trusted more than some about depths. I too was asking if it was worth going lower than 1.2m (after that you need to be shoring up the sides according to the regs).
He said that it wasn't worth the effort and that it can even be beneficial to be nearer the surface because there can be more movement in water which is good for obvious reasons. I too am in a clay area with a high water table in winter months.

You only need to shore up the sides, if someone is going into the trench. (Unless the ground is so unstable that the sides fall in.)

Posted By: woodgnomeIs the pond any good for the collectors?

Yes, the overflow rate alone is more than enough. For a 10kW heatpump, you need around 20l per minute (easy to calculate - take the specific heat capacity of water, a reasonable temperature drop of around 5C and convert power units into energy by factoring the time, and that will give you your required flow rate). This is just less than your lowest rate of 30kl/day.

Paul in Montreal.