Geothermal loops under building

[Rapidrl]

I had an architect pose a question for which I could not give an immediate answer. A 30,000 sf slab on grade building is being constructed. Aside from the structural concerns about disturbing the soil which may degrade the bearing capacity, could most or all of the gshp horizontal loops be buried under the building? The idea would be to trench down 6 to 8', etc. I've done vertical systems and horizontal systems outside of the building, but never under it. The floor of the building will be insulated with 6" eps insulation.

 

[ChrisConley]

In my neck of the woods I'd be afraid of freezing the ground under the building. We do a fair bit of work to prevent frost from getting under slabs, this may not be a concern where you are.

 

[Rapidrl]

Good point about the freezing. Even though the slab will be 8" thick & reinforced, the structural engineer may have some concerns.

 

[tys90]

I've never heard of it being under a slab of any buildings. I've seen them under parking lot slabs. I've also seen them incorporated into footings for larger buildings. There is a local installation in footings but it turned out the heat transfer was less than designed and they had to add supplemental systems when all was said and done.

 

[willard3]

If the loop in under the building, it also will not have the benefit of radiation cooling that the surface of the earth has at night. The building is an insulating barrier.

 

[Rapidrl]

I appreciate the input. These are all valid reasons for not putting it under the building, which is probably why it's not done as standard practice. Thank you!

 

[walkes]

Just as an added point, you may be able to do a horizontal bore under the building with a directional drilling rig.

I would not put a horizontal loop under a parking lot though, too much solar radiation in the summer could hinder heat rejection.

 

[MintJulep]

And when it leaks you have to cut through the slab to find the leak and fix it.

 

[mauricestoker]

You might consider using interlocking block or pavers for parking areas as part of your storm water management, or pervious paving, and use that area for horizontal loop.

 

[sspeare]

You might want to take a closer look at the magnitude and duration of your cooling vs. heating load. Suppose your building load is "cooling-dominated". To put it another way, you have constant high internal heat gains, and a very efficient bldg envelope that results in minimal winter heat loss. You will need heat for only a few hours per year, perhaps none. Even if your borehole field ("geo" well field) is under the parking lot, the borehole field of a cooling-dominated bldg will become "saturated" - at some point the borehole field will be unable to accept additional rejected heat. A ground source heat pump system has to remove heat from the borehole field during the winter, and then reject heat to the field in the summer. If you do expect a reasonable balance of annual cooling load vs. annual heating load, then it won't matter where the borehole field is, as long as you have: a) sized the borehole field properly, and b) selected the heat pumps properly.