Earth Tubes - Geothermal Air Heat Exchange

MacEntyre

MacEntyre

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This has been done before in NC, and is well documented. I wanted to try it, to see if it is worth doing in the Hoop Houses at New Garden Farm. So, I gathered our leftover PVC piping and designed a small system for heating and cooling my shop.

Earth Tubes are sometimes called a Climate Battery, but I don't like that name because it doesn't work the way in which that name evolved. All it is, is a heat exchanger for air, buried in the ground. I first saw it in the video describing a Kansas farmer growing citrus all year round, in a greenhouse heated by pipes buried 8' deep. You blow air down the pipe, and it comes up 50*F, or thereabouts, all year long. In the winter, it provides heated air. In the summer it provides cooled air.

Some folks make them using perforated flexible pipe. The idea is that any water that condenses will exit through the perforations. I chose to install pump outs instead.

The big question is, how deep must you dig to get a relatively constant temperature? I chose three feet. If this works, we will be able to use this system to heat and cool our Hoop Houses and our big Greenhouse, allowing us to grow year round. We just need to figure out how deep to dig, how much length of pipe is required, and what air flow works best.

Here is how it works. A squirrel cage fan blows air into a 6" flexible, single wall, non-perforated pipe that goes out of my shop and down to the underground heat exchanger. It first flows through an 8" PVC pipe 18' long to an 8" manifold. The manifold has three 4" pipes connected to it, each 20' long. The 4" pipes connect to another 8" manifold, to which is connected the 6" flex that goes back up into the shop.

Here is the Air Heat Exchanger where I assembled it.

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We then laid it in the hole.

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I connected a pump out pipe to each manifold using 3/4" pipe. The manifolds are slightly low on that end. I also hammered a 4" pipe into the ground, so I can monitor the ground temperature using a remote digital thermometer. Finally, I connected the flex pipe that will lead up into the shop.

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Filled in all...

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...and covered it with leaf mulch that's a few years old. The whole area is going to become a vegetable and flower garden.

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Now I need to rake out the leaf mulch and mix it with the clay. I will run the flex into the shop. Where the flex is above ground, I will cover it and insulate it. In about a week, I will connect the fan and see what happens!

You can see the shadow of the solar panels that will power the squirrel cage fan. Once installed, this is free HVAC!
 
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When we were designing our house, geothermal hvac was just beginning. I was seriously interested but couldn’t find anybody in the area prepared to install such a system. I’m anxious to know how it works for you.
 
Me. said:
That’s pretty impressive.

It makes me wonder....if you made something like this and used mulch for the fill, I wonder how long you could get heat from the decomposing mulch?
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That would work fine, but it needs to be done above ground, or it will fill with water... besides it will only last one season. A Mulch Heater generates as much as 150F, so it's better for heating water, and transferring it using a shell and tube heat exchanger.

I'm going to do a Mulch Heater one day soon... based on Jean Pain design improved by Mother Earth News. It will have to be loaded in the Fall and unloaded in the Spring. I'm trying to figure out how to make it durable, and easy to load and unload.

Jeppo said:
When we were designing our house, geothermal hvac was just beginning. I was seriously interested but couldn’t find anybody in the area prepared to install such a system. I’m anxious to know how it works for you.
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Traditional Geothermal uses two wells... one for getting water out, and one for reinjecting water. You have to go deeper than 30' to get water of constant temperature, and then you can make a heat pump using the temperature difference.

What I'm doing is a newer development, inexpensive but effective. There is one problem... think about what happens if you supplement this heater in winter, say with a wood stove. The Geothermal becomes a cooler! All of your wood stove heated air gets cooled by the 50*F air from Geeothermal! That's why it's great for Hoop Houses and Greenhouses, with no supplemental heat.
 
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Very interesting, I'm thinking if this were used on a living space you would have to treat it or have a way to remove mold and mildew.
 
MacEntyre said:
The big question is, how deep must you dig to get a relatively constant temperature? I chose three feet. If this works, we will be able to use this system to heat and cool our Hoop Houses and our big Greenhouse, allowing us to grow year round. We just need to figure out how deep to dig, how much length of pipe is required, and what air flow works best.
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Looking forward to see your results through the year.
Typically you want to dig 6+ feet to get the stable 50 to 55 deg temperature. The bonus is the earth's mass has a 6 month delay so it's 50deg in the summer and 55 in the winter.
I've a close friend that builds houses that heat and cool themselves on this principle, I started working for him @ 1984. He's got a concept patent on the process, is in NC State textbooks and is located in Wake Forest.
Enertia Building Systems
http://www.enertia.com/howitworks.html
 
Hopefully, there will be no mold and mildew... but that's why it has pump outs installed. It is possible that hot air will condense when it is cooled, and water will collect in the PVC manifolds. As I mentioned, some folks use perf pipe to let the water out (...or in!). So, it's not appropriate for residential air.

Instead, you could make a PEX heat exchanger, and run water underground to bring it to 50*F, and exchange it with air above ground using a shell and tube heat exchanger. That's also been done before, in the 50s and 60s.
 
Pink_Vapor said:
Enertia Building Systems
http://www.enertia.com/howitworks.html
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Cool! I know deeper is better, but I wanted to try this at 3' because I am limited in what I can dig. Don't have a backhoe...

My house is similar to that design. It has a daylight basement that is always 10 or 15 degrees cooler than the ground floor in summer. The HVAC system we installed uses that by mixing all air in a single return. I like the chimney effect in the Enertia design.
 
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MacEntyre said:
Hopefully, there will be no mold and mildew... but that's why it has pump outs installed. It is possible that hot air will condense when it is cooled, and water will collect in the PVC manifolds. As I mentioned, some folks use perf pipe to let the water out (...or in!). So, it's not appropriate for residential air.

Instead, you could make a PEX heat exchanger, and run water underground to bring it to 50*F, and exchange it with air above ground using a shell and tube heat exchanger. That's also been done before, in the 50s and 60s.
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Yes but moving air is soo much easier. I like this and may have try one sometime.
 
Yup... lots easier to handle air only. I will install humidity and temperature sensors in the pump out tubes, so I will know if it gets wet in the manifolds.
 
One problem with these type of systems is that they don’t dehumidify (unlike a standard AC unit). Yes, needs to be 6’ deep or so.

Are you recirculating air from the house? If so a home dehumidifier can address the humidity issue.
 
Scsmith42 said:
Yes, needs to be 6’ deep or so.
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Jury's out on that... perhaps that is how deep it needs to be to eliminate all fluctuations, but I'm not certain of that.

Scsmith42 said:
One problem with these type of systems is that they don’t dehumidify (unlike a standard AC unit).
...
Are you recirculating air from the house? If so a home dehumidifier can address the humidity issue.
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It's a shop, an uninsulated, highly breathable garage. Dehumidifying would add a tremendous amount to the energy costs, which otherwise are very low.

This is not a Residential solution. It's a for a Greenhouse!
 
So humidity is going to be a problem in the summer, but if we’re talking greenhouse cooling why not draw air from outside? Outside air is probably dryer and cooler than the greenhouse, run it through this piping to cool it some and then into the greenhouse. Vent the greenhouse at the top as usual.

In the winter you’d probably recirculate.

While this works, the capacity is tiny compared to a liquid system, the advantage is that it’s cheap and easy.
 
JimB said:
So humidity is going to be a problem in the summer, but if we’re talking greenhouse cooling why not draw air from outside? Outside air is probably dryer and cooler than the greenhouse, run it through this piping to cool it some and then into the greenhouse. Vent the greenhouse at the top as usual.
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Ya'll are thinkin' Residential... why is humidity a problem when we water the plants daily?

For three seasons, we must ventilate (raise the skirts of the hoop house cover) pretty early in the day. That takes care of humidity in a greenhouse and a hoop house. This system will delay that requirement to ventilate a little bit each morning. If the plants get above 90F they show it. As soon as it gets to 40F outside on a sunny day, the upper layer of our hoop houses are 90F and climbing fast until we ventilate.

JimB said:
In the winter you’d probably recirculate.
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Definitely... and that's where this system will shine. Fewer days below freezing extends the growing season, and makes seedlings grow much, much faster.

JimB said:
While this works, the capacity is tiny compared to a liquid system, the advantage is that it’s cheap and easy.
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That's true. It all depends on how many volume changes you can get per hour. Ideally, you want to turn over the entyre volume every 10 or 15 minutes. If you can do that, and still get 50F air out of it, it is well worth it in an uninsulated structure.
 
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So, JimB, you can see that heating is more important than cooling for a greenhouse/hoophouse.
For my shop, cooling will have a big effect. The big garage door will eliminate the humidity. ;)
Extending the growing season, and eliminating frosty mornings are the goals in a greenhouse.
 
Yeah I was thinking about cooling rather than heating.

If this doesn’t work, a simple solar setup that stores heat in 55gal drums of water would easily eliminate all nights below freezing.
 
JimB said:
Yeah I was thinking about cooling rather than heating.

If this doesn’t work, a simple solar setup that stores heat in 55gal drums of water would easily eliminate all nights below freezing.
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Would that work for an uninsulated plastic covered Hoop House 14' x 100' with a 9' ceiling (just under 14,000 cubic feet)?

What maximum temperature would you expect a drum to reach in the middle of winter?
 
MacEntyre said:
Would that work for an uninsulated plastic covered Hoop House 14' x 100' with a 9' ceiling (just under 14,000 cubic feet)?

What maximum temperature would you expect a drum to reach in the middle of winter?
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Put a row of black 55 gal drums on the south side and they will heat up during the day and give off heat at night.
I would think that if the 100' side was est-west then that would be a lot of mass/heat.
 
ronn47 said:
Put a row of black 55 gal drums on the south side and they will heat up during the day and give off heat at night.
I would think that if the 100' side was est-west then that would be a lot of mass/heat.
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It is oriented East-West... therefore, your drums will shade one third of the plants. They will die warm. That ain't a good trade-off!

But you didn't say what temp you think the black barrels would achieve on a sunny winter day in NC.
 
MacEntyre said:
Would that work for an uninsulated plastic covered Hoop House 14' x 100' with a 9' ceiling (just under 14,000 cubic feet)?

What maximum temperature would you expect a drum to reach in the middle of winter?
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Had 4 panels with 350gal of storage installed in northern Indiana, biggest problem was boiling the water and vapor locking the system, even in the middle of winter with snow on the panels.

You’re gonna need something pretty big to heat such a long structure. Maybe make a radiator from long pieces of conduit running under the plants? You could just lay the barrels on their sides under the plant racks, but then you don’t have a good way to turn the heat off other than draining the system into something like a pond or swimming pool. Maybe better to bury a bunch 1/2 way or more in the ground then plumb them together.

You’ll also need to think about the power to run the pump. I think your biggest challenge may be getting the heat out of storage and into the air.

Not as cheap as barrels, but someone on here was selling old square poly containers, I think they were 100+ gal previously used for molasses.
 
JimB said:
Had 4 panels with 350gal of storage installed in northern Indiana, biggest problem was boiling the water and vapor locking the system, even in the middle of winter with snow on the panels.

You’re gonna need something pretty big to heat such a long structure. Maybe make a radiator from long pieces of conduit running under the plants? You could just lay the barrels on their sides under the plant racks, but then you don’t have a good way to turn the heat off other than draining the system into something like a pond or swimming pool. Maybe better to bury a bunch 1/2 way or more in the ground then plumb them together.

You’ll also need to think about the power to run the pump. I think your biggest challenge may be getting the heat out of storage and into the air.

Not as cheap as barrels, but someone on here was selling old square poly containers, I think they were 100+ gal previously used for molasses.
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Plants are growing in the ground, not on racks... anyway, that's the only way I'd try that, using panels to heat the water.

Sounds like a lot of trouble to get above 40 degrees F, which is all the plants need. It's a lot easier to protect plants, than to heat a house. We're gonna try these Earth Tubes (tha's the hippie name) and see how well it works.
 
Got a bunch of projects piled up on me, and haven't connected 'em yet. I only have to cut a hole in a wall and bring the flex through. One piece of flex can stop just inside. T'other will go up to the ceiling and along the wall for a bit.
 
MacEntyre said:
What maximum temperature would you expect a drum to reach in the middle of winter?
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It’s a little too early for me to run numbers, but back of a napkin: temperature difference in deg F x gallons per minutes / 24 = 24,000 BTU should give you a rough idea of how much heat you can store in the water.

Over heating and over pressurizing would be a potential issue in summer with no way to dump the heat.
 
thank you for this thread. I'll be darned if almost everything I think of with a "I need to check out someone doing this locally" does not wind up posting something similar on this forum! I as well saw that guy growing citrus in Nebraska! I know a guy who did a ground loop (with antifreeze) heat exchanger for hvac near here in Wake Forest, but I definitely need to check your stuff out. thank you again
 
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The Korner’s Folly house in Kernersville has a similar system for climate control in the house before the advent of central air/heating.
 
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Was it a direct air heat exchanger?
I wonder how they handled condensation.
And, I wonder how deep it was.
I'll look it up and see if'n anyone knows.
 
From one of their newsletters:

http://www.icontact-archive.com/XPq9AxC01rv_Oqf9XrRxA9gOuKJyyfqX?w=3

"When Jule built Körner's Folly, air conditioning wasn't an option. People had to get creative, and Jule Körner got really creative. He dug tunnels underneath the house and placed venting grates in the floors so cool air could rise through the living spaces, creating an early geo-thermal air conditioning system."
​
The way they explained it in the tour was more like your system.
 
Cool air doesn't rise, but if there were vents in the eaves/roof such that the hot air left the building and if the rest of the building was tight enough not to leak much air, it would be possible to pull the cooled air inside.

The other thing I'd wonder about is PVC. It's a pretty good thermal insulator, so I'd question the amount of thermal transfer between the air and the soil. While I can't think if a durable solution to fix that, I think it's going to be a key limitation. Aluminum tubing would be great, but it's expensive . Making the exchanged bigger / longer would increase the thermal transfer but also increase the pumping loss, so there's no free lunch there.

I've got about 100' of PVC pipe buried a couple feet underground to run my ham radio cables across the backyard. I was advised to tee the low end of the run (one going to the surface with the cables the other open on the bottom side ) and dig a basin filled with gravel (french drain) so that when (not if) it condenses in the pipe, the condensation has a place to go.
 
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JohnFreeman said:
Cool air doesn't rise, but if there were vents in the eaves/roof such that the hot air left the building and if the rest of the building was tight enough not to leak much air, it would be possible to pull the cooled air inside.

I've got about 100' of PVC pipe buried a couple feet underground to run my ham radio cables across the backyard. I was advised to tee the low end of the run (one going to the surface with the cables the other open on the bottom side ) and dig a basin filled with gravel (french drain) so that when (not if) it condenses in the pipe, the condensation has a place to go.
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Yep. The part about cool air rising was a big red flag. My recollection was that the system was more like MacEntyre's. The guide at the house explained it in some detail.

Korner's Folly is well worth a visit. The man who designed and lived in it, Jule Korner, was behind all the Bull Durham murals and signs back in the day. Made him a small fortune. His house shows what a genius he was, not just a great marketing artist.
 
JimB said:
Yeah I was thinking about cooling rather than heating.

If this doesn’t work, a simple solar setup that stores heat in 55gal drums of water would easily eliminate all nights below freezing.
Click to expand...

Water has amazing heat storage capabilities (specific heat = 1) . I think this is the way to go.
 
MacEntyre said:
For a Hoop House? ???
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For any structure... yup!

Nothing stores as much thermal energy as water does. It's odd that something so common has such a unique property .
 
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