Anybody have any experience with placing horizontal loops under a basement slab? We have done lots of vertical loops in bore holes, but I have a client who's now interested in over-excavating the basement slightly, and putting the loops horizontally under the slab.

It can be done of course but you do give up solar gain. You also want to ensure you don't impact foundation with changing ground temps or heat up basement floor in the summer while you are cooling house.
Good Luck,
Joe

I would think that would trigger addition of sub slab insulation if that is not already planned

That would be one way to help ensure you didn't impact the slab :)

Thanks for the super fast responses! Impressive. I should elaborate on this situation a little bit:

I'm a PE and a General Contractor. We design and build houses and light commercial structures, and additionally we perform vertical bore hole geothermal installations using a Techno Drill rig and down the hole hammer. We do our houses through the NAHB National Green Building Standard.

In this case, my "Client" is a government backed research group that is building a model energy home. The idea to put the loops under the slab is as a test for both constructibility concerns and loop performance testing. There will be a separate vertical bore hole field to alternatively run the system if desired by selected valving at a manifold.

The client has done extensive thermal modeling and there will definitely be under slab insulation. My motive in posting here was to see if anyone had any experience with this method of placing loops horizontally under the slab. The models are nice, but nothing beats real world experience!

Thanks again for those fast responses.

I'm not sure if this is what you mean but...

We have several homes where the horizontal bore goes under the home. Both home sites were steep, forested hills. The bores started in the front yard, ran under the home and down the hill. The supply and return headers did the same and were manifolded in the front yard.

Bergy

Yea, that's pretty much it. Except this site is urban infill in the city on a flat lot. So the idea is to keep the loops within the footprint of the house. The notion is to dig the basement an extra few feet deep, lay the loops out, then fill on top of them before pacing the slab.

Without knowing the details, it seems like a lot of pipe for a small footprint.

Bergy

Yes, that is one of my concerns. The system is small at only a design load of 1.5 Tons, but the problem remains how to fit 400 feet of geo pipe (or more) into a 30x50 footprint.

Vertical or DX would be my bet. How did you arrive at the 400' number? Best soil to shorten pipe is soil that is not great to support foundations.
A DX horizontal bed uses about 500sf/ton that would fit neatly.
j

A PE I am working with here in Texas pointed me to the forum note. Have asked him to help with a new home for my brother. Loops below polystyrene insulation below the slab. Did you do this project? Did things work out? Please Email me [email protected]

My major concern would be that you meet the specs for the total amount of pipe in the ground and the spec for total volume of soil. Ie, feet of pipe, feet of trench and cubic feet of soil (aka trench/loop spacing) all have to be correct.

Also, would soil just under a house be much dryer than adjacent soil - quite possibly.

I just today finished up 13 tons of load of VERTICAL under a soon to be house on a tough to build lot that is bay front and has a 5' building set back . Why not vertical underneath instead of horizontal? We plan to manifold above grade, should prove to keep me interesting.
Eric

new builds we always try to drill under the foundation. there is no difference between underneath or beside the house in terms of performance.

Would be nice to know results...

Seeing heaved foundations and basement walls qracked with both dx and gle---

in 52 deg "blue-line" is leaving at 27 deg or colder to the field...

at 34-35 deg to 4 tons with one pump, about 29 is coldest leaving in sizing loop to load (may be 5-pipes on 4 ton, not ever slinkies then,

I wonder if the 400 ft was enough and hope it was ~ 6 ft away from basement with the 2" board, used when we get in to connection 2 pipes near entry at jumping slabs in retrofits, etc... but eith 35-deg loops is safer still and 2 pumps.

gt....where are you and what is your background on geothermal?