Installer pouring sand on top of ground loop pipes before backfiling

I have been watching a Geo Thermal install in a nearby town. After the pipe was layed in the horizontal loops the installer dumped truckloads of sand on top of each pipe loop. I saw two huge piles of sand and he was back filling sand over the pipes with an excavator.

I was always told that sand does not conduct heat, hence why you put sand around insulated pipes to and from an outside boiler.

Does this sound strange to anyone else. The ground seemed to have clay and shale.

 

Could be to protect the pipes from damage during backfill. This is pretty common.

 

Sand

Yes, it is generally a poor material for heat transfer but necessary if you have sharp/large native materials. Though utility guys get obsessed about it, and spec it in to the detrimant of the ground loop.

 

yes i believed it was a poor material for heat transfer, that is why I was asking.

 

Sand or stone dust/screenings is used when it is difficult to get good compaction around the loops and to protect the loops from any sharp rocks.

Moist sand is actually a little better conductor than moist silt/clay at 0.9 vs 0.75 BTU-Hr-Ft. If it stays completely dry, then silt/clay would be slightly better at 0.5 vs. 0.35, but this is rare.


-Adam

 

If you happened upon a DX installation then you would have seen "crusher dust" not sand. Others have used a dry grout as well.

Are you sure you saw "sand" or was it something that appeared to be sand?
J

 

It was sand, not beach sand, it was sand like they spread with salt trucks.

 

sand not so bad

Since you have clay, in dry whether, clay will shrink, pulling away from the pipes, thus no (well, actually minimal) heat exchange. Sand will move to fill in the voids and help reduce the drying effect since it will retain moisture better than clay & will thus aid in the heat transfer during dry weather. If shale (dust) is available, it would perform better than sand, but sand is far better than clay.

The ideal transfer is for the ground loop pipes to be in water (or as close as possible - liquid to liquid heat transfer is more efficient than liquid to air heat transfer). My preference for both consistency & efficiency is a 250 ft vertical well per ton (especially if the water table is within 50 ft of the surface) - note that the actual well depth required per ton will vary by the soil conditions and area of the country.

Remember, the installer needs recommendations, so his goal is to create as efficient a loop as the local soil conditions will allow, within his budget.

I hope that helps.

 

Hi, are you talking about a 250 well with a closed loop, open loop or standing column?

 

In the water table is always good, but your "preference" is 250' wells? Are you a well driller?
My preference is the least expensive loop that performs within design and fits the property. That is most often horizontal in my AO.
joe