Can I run a geothermal loop in footing trenches instead of across the landscape?
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 geothermal 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?
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I think it is worthy of a try, but have not done such. Water is a great conductor and if the water is the right temperature and flow like you say it should work very well.
Eitan,
Have you considered investing your money into the enclosure instead of the equipment?
Eitan,
I'd advise caution, and I'd consult an engineer familiar with GSHP design. During the winter, you'll be lowering your soil temperature, and it's possible that you'll freeze your soil.
You'll also get much better heat transfer in the waterlogged trench than in the drained and dry footing trench.
Our footing trench will not dry in the winter, unless there is a serious drought.
Our frost depth is only 12 inches. If the fluid flowing to the loop never drops below 35 degrees, the footing is not going to freeze. I doubt that anyone in Western Washington ever ends up with fluid below 35 degrees. I'll talk this over with our designer. I know of one nearby 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.
Regarding the enclosure: We are renovating our 1933 home, and sealing and insulating it like crazy. I understand that sealing and insulating is the cheapest, greenest way to reduce energy consumption. The reason we are considering geothermal is that we need some kind of efficient heat source (we currently heat with a gas fake-woodstove), our solar exposure is less than optimal, those tax credits make geo comparable in cost to an efficient gas boiler, and it seems marginally greener than the gas boiler. I don't think we can get our older house tight enough to where we would not need any heat source. But maybe we will be able to reduce the size of the system.
If there is a perforated drainpipe next to the footing, then water can never rise above the bottom of that pipe. That's the purpose of a footing drain - to keep the footing trench dry.
The purpose of a drywell (more accurately, a percolation chamber) is to create a reservoir for percolation of water into the surrounding soil. A properly designed drywell should not remain flooded.
I am misnaming the drywell. We are required to have a drainage system that will catch our groundwater and stormwater and then release it into native vegetation and on down the slope to our wetland. Our "drywell" will be wet all winter long. Also, our topsoil is too wet in the winter to allow for much percolation into it. That's why our "drywell" will be releasing its water to surface.
Our current footing trench is never dry in the winter; it is washed gravel that conducts water into the perforated drainpipe, which flows and flows. The new footing we are building could be backfilled with a material that would better conduct heat into a pipe. But I am quickly concluding that running geo pipe in a spiral in the not dry not-drywell likely makes more sense than running it in the footing trench. We should just use the footing trench to collect the 50 degree water and use the perforated pipe to bring it to the not dry not-drywell.
Are you sure putting your pipe runs in the dry well is wise? We are putting a foundation and basement in under our house in Seattle which is on Glacial TIll/hardpan, so I have seen how much water can flow at the soil hardpan interface. Interesting idea to capture heat from that flow. But the deep cold snaps here occur when there is no rainfall, so wondering if there is a chance that you could get an extended cold snap so that your heating demand would be highest precisely when your water flow over the pipes in the well is the lowest. Will the volume of flowing water in your dry well be sufficient to heat your house over an extended cold snap without there being a danger of the well freezing solid?
So your "footing trench" is a shallow French drain (not a perimeter drain at the level of the footing), and your "drywell" is a retention pond?
David: You're right that when it's coldest it's drier, but no way is our retention pond/drywell ever going to freeze, since it is underground, and fed by the groundwater. It really doesn't stop flowing all winter. And it never gets that cold here for that long. Our house is at the bottom of about eight wooded acres which seem to drain into our foundation ditch, which acts like a french drain.
Robert: The footing trench is a like a french drain, but not that shallow; it is about 3-4 feet deep, slightly down into the hardpan layer, and it is at the level of the footing. The drywell (which has not been built yet) will be like an underground retention pond which drains to the surface. If it was an actual retention pond, we'd have to worry about it freezing solid in an extreme cold snap. Underground, it's not going to freeze, even if we are extracting heat from it.
Eitan, I applaud your experimenting enthusiasm. Don't let naysayers dissuade you in your quest.
Much can be learned from trial and error.
Post your progress.
Such as how expensive an error can be.
AJ believes he's not subject to the laws of physics.
I do? AJ will talk for AJ. Some good posts on this thread and then you have to take it personal. Time for Christmas activities here, Robert I'll enjoy your spam you like sending me now.
Peace out
Merry Christmas GBAers
I'm not building the system. My installer has installed several hundred systems; he's not going to build me something that he thinks is not going to work. I am not an engineer, but I do understand that if this is not properly engineered, it could fail. But if it works, it reduces the length of trench we need to dig through the woods.
Once again, I hope to find some local test data which I can factor into my analysis. We have a mild temperate climate here, so the laws of physics treat us more kindly than they do in upstate New York, for example.
Around the DC area which is only slightly cooler than where you are, it is very normal for loops to be below freezing by the end of winter, and every (closed loop) system uses antifreeze heat transfer fluid.
It has more to do with the ratio of loop length to annual heating load than what climate you're in.
Your situation is interesting. It certainly sounds like you'll have good transfer to the water in the ground, and you may not go below freezing. However, if you do gamble with shorter loop length and any of a number of wacky situations come up, you may find yourself more at risk for freezing the ground/ground water around the loops (if the loops are shorter they'll be colder).
Locking up your "not-drywell" as a giant ice cube could have significant impacts both for your foundation water management and for heat transfer ability of your loops.
As long as your loops don't ever bring the water/ground to below freezing, all that water will help you. But if it ever does get over that tipping point, it could be a long recovery.
One other funny thought/question: do you suppose the ground water gets pretty cold during cold spells in winter? I know the near-surface water around here gets pretty cold in winter. Do you have any information about that? A lot of people have this idea that the ground stays at a constant temperature, but that is only way down below the surface, and before you start cooling it off with a geothermal system--then it gets below freezing as mentioned.
Good questions. We only have a five degree advantage over you in the winters, so that helps a little. I am not proposing shortening the loop lengths; just putting one of them in the not-drywell. We'll still have the recommended 600 feet per ton.
A giant not-drywell icecube? We don't want that. But assuming the slightly cooled water drains away and is replaced by new water, it's not going to freeze.
Our unheated daylight basement has maintained 50 degrees in the winter; that's what makes me think we have 50 degree ground temperature down three feet. But you're probably right that it probably gets at least a little colder than that in mid to late winter.
In the past, I have never checked the temperature of our foundation drain where it comes to the surface. This week it was 51 degrees. And we had an unusually cold November, though December has been mild. I don't know about your design frost depth in D.C., but here it is just a foot. So the water temperature 3-4 feet down may not stay as high as 50, but it definitely does not drop anywhere near 32. But it does sound like it would be pretty stupid to run the pipe next to the footing; I'm not even going to ask my installer about that, he'd probably laugh (nicely).
Boy, I would really benefit from some more local data.
There is no such thing as a stupid question, right? So don't be afraid to ask your geothermal contractor. And, if he's done hundreds of installs in your climate, then he's the person to ask - not cyberspacemen.
(But I agree it would be foolish to use your footing trench.)