We live in Western Washington, where the winters are mild (mean temps in Dec through Feb of 39), and wet. During the winter, our land becomes saturated with groundwater about 2 to 3 feet underground, just above a layer of impermeable hardpan. The land is mildly sloped, and the water flows underground down the slope until much of it surfaces in a wetland.

We are considering installing the typical 300 feet of trench five feet deep for a horizontal ground loop, one layer of 3 pipes at the bottom of the trench and a second layer up two feet from the trench bottom. But since the underground is so wet two to three feet down, and never falls much (if any) below 50 degrees at that shallow depth, do we really need to dig such a deep trench?

Which leads to my big question: We have just dug a foundation footing trench about 150 feet long which will tie into a drainage pipe leading to an underground dry well (ironic how a dry well is full of water!). What if we were to run the PE loop pipe in the footing, alongside the drain pipe, and into the drywell? It seems to me that in our climate, the wet ground and groundwater will provide a steady stream of 50 degree extractable heat. I am assuming that wet ground or groundwater is at least as good a heat conductor as backfilled dirt. In my current concept, I think it might be ideal to backfill over the PE with recycled glass pellets (locally available, cheap, pack well, and would conduct heat well for this purpose as well as keeping the water flowing).

In the summer, the ground is dry, but our loads are small, and the ground likely warms up both from the top down and the bottom up.

We are insulating on the outside of the foundation wall, so if this lowered the footing level soil temperature from 50 to 45, we won't notice it.

Has anyone done a loop like this? Any comments on whether you think it would work, and if so, would we still need 1800 feet of pipe?

Thanks!

Unless there is a heckuva underground water flow around that proposed loop, I don't see it providing for the necessary heat transfer. Worst case it freezes the ground right below the footer and heaves it...that could be very bad!

First time a drought occurred, you'd be SOL if your loop wasn't designed for dry soil

It could be all about ROI. If you design for drought conditions (worst case), your first cost can escalate. If you know what you are risking by factoring in all the tolerances in a system, you can make some pretty good working systems, that only fail in a extreme worst case situations. It really depends on what the customer wants and can tolerate. Would you be happy saving $20K up front but if there was a drought, you might have to drill more holes later? You get my point. Or do you want a guarantee that the system always works no matter what the weather conditions? If you factor in the comfort requirements, duct design, heat pump requirements, building envelope, soil conditions over time, then you can begin to make trade offs on loop design... only then. It doesn't hurt to experiment if you have the stomach for learning from failure.

Posted By gonegeo on 23 Dec 2010 10:44 AM
... It doesn't hurt to experiment if you have the stomach for learning from failure.

This is a great line

Ah, experimentation. We have a subterranean creek that flows non-stop from October through April; the flow varies, but it never stops. (I know, until we get hit with a drought!) I understand the theoretical potential risk of chilling the ground by the footing to the freeze point, causing frost heave. But if the fluid flowing to the loop never drops below 35 degrees, the footing is not going to freeze. (Also, our frost depth for buildings is 12 inches!) I realize that these systems work even with fluid below freezing thanks to the Methanol solution in the loop. But I doubt that anyone in Western Washington ever ends up with fluid below 35 degrees. I'll talk with our designer. I know of one installation that is being closely monitored, so I will try to get information about what kind of fluid temperature range they have experienced.

If I learn that the fluid temperature could drop below 32 degrees in extreme weather, another possible alternative is to run part of the loop through the flooded drywell, which will catch all of the groundwater flow. This would eliminate any risk of freezing the footing, while still taking advantage of a shallow flow of 50 degree water. Perhaps we will end up doing a traditional trench for three loops and have the fourth loop use the wet drywell. Then we could measure its performance. Ah, experimentation.

Since you seem bound and determined to do this...

You can probably design a system so as not to require EWT below freezing, skip the methanol or other antifreeze, and set the system's freeze protection for "water only" "well" or however it is labeled. It'll lock out of it sees 30*F or lower. I'm in Florida and we do not add antifreeze to closed loops, so we set freeze protection as I described.

Better a lockout and use of more expensive source of heat than frost heaving a foundation footer.

Typical closed loop design plans for EWT no cooler than 20-25 below deep ground temp for the area, and no higher than 20-25 deep ground temp for the area, with 20 being better than 25. What are your deep ground temps?

One of the great advantages of these sites is the ability to learn from OTHER PEOPLES' failures...cheap tuition!

Have you considered open loop?
Joe