# Geothermal Cooling



## tamilee (Apr 13, 2005)

Hi All;
I want to make a geothermal air air cooling system for the house. I was going to do this a couple of years ago but got side tracked by a hurricane and family emergencies.
Now, Lord willing, I plan to start in amonth or 2 when the finances are in a little better shape.
I have a few questions:
1.)Is three feet deep for the pvc line in the cooling field? 
2.) How do I determine the linear feet needed?
3.) Theoretically all ground should be the same temperature at a certain depth but my soil is mostly sand, how does this effect the temperature below the "topsoil".
4.) I plan to put the cooling filed on the side of the house where the duct work begins, this is in the shade in the morning and late afternoon but in the heat of the day it gets direct sun. How much will this cause the air tempertaure to rise? I do not want to draw up cool air, only to loose most of the reduced temperture at the junction of pipes and duct work. 

I plan to build one with pvc pipe and have a grid of pipe connected with elbows. Would it be better to have several layers of smaller pipe or one fairly large?
Would a single solar fan (dc) be enough to circulate the air? I was hoping to be able to use the system offered by Real Goods that can power up 3 dc lights. The investment would run between $1,2000.00 and $1,500.00 depending on the amount of pipe. 
I would love to hear from anyone who has a system like this and would also like to know if it can cool a house in coastal NC.
Any input would be apprecitaed.
tamilee


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## moonwolf (Sep 20, 2004)

can you explain the contents of your 'cooling field' ? I might be missing something from reading your post. I get it that you want to bury your lines and all, but sorta lost beyond what you want to accomplish with what you are doing without the use of a heat pump. 

Having said that, I've thought about cirulating water from my deep cold water source pond (about 35 degrees) in a closed system through a similar heat exchanger that is used for heating from the wood furnace in winter.


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## fordson major (Jul 12, 2003)

moonwolf said:


> Having said that, I've thought about cirulating water from my deep cold water source pond (about 35 degrees) in a closed system through a similar heat exchanger that is used for heating from the wood furnace in winter.


we are looking into a system similar too this too cool my bros. house, parents cottage and kennel building. tapping into a swimming pool would also heat the pool! may now have the ammo to re dredge shepherds spring pond! wow, could even stock some fish! :dance:


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## 12vman (Feb 17, 2004)

tamilee..

Is this something you are talking about..?

http://mb-soft.com/solar/saving.html


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## WisJim (Jan 14, 2004)

Friends of mine have a system that I imagine is similar to what you are talking about. It is buried 8 feet deep, 3 feet isn't enough. It is 12 inch pvc, and is a couple of hundred feet long. It does make a difference. Sand will not work as well as heavier soil, but this is the kind of thing that you need to try to find out if it will work.


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## tamilee (Apr 13, 2005)

12vman said:


> tamilee..
> 
> Is this something you are talking about..?
> 
> http://mb-soft.com/solar/saving.html


 HI 12v;
Good to hear from you again. Yes, that is the system I was talking about. I lost the link when my hard drive was fried last year.
Thanks.
tamilee


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## tamilee (Apr 13, 2005)

moonwolf said:


> can you explain the contents of your 'cooling field' ? I might be missing something from reading your post. I get it that you want to bury your lines and all, but sorta lost beyond what you want to accomplish with what you are doing without the use of a heat pump.
> 
> Having said that, I've thought about cirulating water from my deep cold water source pond (about 35 degrees) in a closed system through a similar heat exchanger that is used for heating from the wood furnace in winter.


HI moonwolf;
I'd like to make a gris of pvc pipe and have all of the tie into the house's duct work. I plan to place a solar powered fan to draw the air from the ground into the house.
12v man's link is the link which explains how to do this. I had lost the link last year and was trying to recall from memoery how it needs to be done .
tamilee


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## fordson major (Jul 12, 2003)

first heard of such a system in college, they were looking at it for heating/cooling fresh air for pig barns. they were using field drain tile nonperf, they did have some water intrusion and radon gas was another concern. have not seen any of these systems in practical use round us for this reason. check on earth gases in your area before proceeding, lot of money and time.


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## WisJim (Jan 14, 2004)

The piping that is buried should be sealed, except for a drain on the lowest end (for condensed water vapor to drain out), so radon shouldn't be a major concern.


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## ET1 SS (Oct 22, 2005)

WisJim said:


> The piping that is buried should be sealed, except for a drain on the lowest ed (for condensed water vapor to drain out), so radon shouldn't be a major concern.


I don't think he was talking about making the pipes out of concrete. Rather PVC, there is no radon in PVC.


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## fishhead (Jul 19, 2006)

You could jet down a small 1/2 cpvc pipe using a garden hose and then check the temp with a thermometer at various depths to see how deep you have to bury the piping. I don't see why soil type would have anything to do with the system temp.

I've thought about using a similar system if I ever build.

The water you collect will be distilled so you could set up a collection container and use it for washing clothes.

As far as linear feet needed you'll have to calculate the heat transfer rate of the pipe and the air flow speed. 

The nice thing about a system like this is it will cool and dry the air in your home. I've never understood why earth sheltered home builders don't do this.


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## blue gecko (Jun 14, 2006)

We put a peripheral air system around the basement when we built the house. It's a closed loop system circulating ~90 feet (6-8+ feet deep) through a 12 inch pvc pipe. The pipe is a male/female end set up so there is no place for water to collect and mold to grow. At the lowest point the air is exposed to a gravel bed that is tied into the french drain. A fan circulates air through it and through the basement. To move it to the rest of the house we use a solar flue/natural draft/attic fan or the fan on the central HVAC. When it's really hot we still use the peripheral system and the air intake for the HVAC is in the basement so we get a little help for the AC.

I know this is not the system you are trying to do but I though you might find it interesting.

Have you thought of setting up a swamp cooler type of system: Use a cistern filled with Xlinked hose like you use for radiant heating and sand. Keep the sand damp and let evaporative cooling become your chiller. Circulate fluid through the hose and blow air across it to transfer the excess heat from your house to the fluid. The air coming past it would be cooled and could be circulated within your house. You'd want your cistern to be porous so the water could evaporate readily. Maybe a two layer system would work best with a cistern to catch rainwater then trickle feed into a wiremesh box with a gravel "wall" and sand in the middle with your tubing. 

As you can tell I'm just thinking out loud but maybe it will give you ideas and perhaps spark a discussion.


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## fishhead (Jul 19, 2006)

Have you ever checked the temp of the air entering the system and the temp exiting?

Also, the flow rate?

Those would be interesting.



blue gecko said:


> We put a peripheral air system around the basement when we built the house. It's a closed loop system circulating ~90 feet (6-8+ feet deep) through a 12 inch pvc pipe. The pipe is a male/female end set up so there is no place for water to collect and mold to grow. At the lowest point the air is exposed to a gravel bed that is tied into the french drain. A fan circulates air through it and through the basement. To move it to the rest of the house we use a solar flue/natural draft/attic fan or the fan on the central HVAC. When it's really hot we still use the peripheral system and the air intake for the HVAC is in the basement so we get a little help for the AC.
> 
> I know this is not the system you are trying to do but I though you might find it interesting.
> 
> ...


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## whistler (Apr 20, 2005)

tamilee said:


> 3.) Theoretically all ground should be the same temperature at a certain depth but my soil is mostly sand, how does this effect the temperature below the "topsoil".


The bigger problem with sand is not how it affects the temperature of the topsoil, but how well it absorbs/releases heat. Unless the sand is very wet, the heat transfering abilities are nil. It's not really the soil that does the bulk of the heat transference, it's the soil moisture. 

I know a guy whose contractor put in nearly a quarter mile of pvc buried 8' deep. Because the sandy soil didn't give up heat well (he was using a heat pump, but the same principles apply to cooling) the air entering the underground network emerged the same temperature as it went in. As the house got cooler through heat loss to the winter weather, the air coursing through the network became cooler and cooler until the heat pump fried itself trying to extract heat from air that was below freezing.

I'm not saying that you shouldn't go for it, because I don't know the specifics of your situation. I would just talk with more people who are experienced with soils similar in profile to yours.

Whistler

Edited to add: We are looking at moving to a farm located on an expansive sand plain and so I have done some reseach on using this type of system in sandy soil.


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## fishhead (Jul 19, 2006)

Wouldn't that put the frost line at 8'?

Must have been one of our recent drought winters.


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## whistler (Apr 20, 2005)

fishhead said:


> Wouldn't that put the frost line at 8'?
> 
> Must have been one of our recent drought winters.


No. The ground was still 54 degrees. 

The premise behind this system is to take cold air and force it through the tubes. The warmth of the ground should heat the air inside the tubes to 54 degrees. A heat pump then extracts heat from this 54 degree warmed air sending it into the house, usually via a heat exhanger. The cooled air is then send back through the tubes to be warmed by the heat of the earth. And the cycle begins again.

The key step in this process is that the 54 degree earth gives up some of its warmth to the air in the tube. It's like putting an ice cube in a glass of warm water. The cube doesn't actually cool the water, it absorbs the heat of the warm water. In the case of a home heating system, the air is the ice cube and the earth is the glass of water.

In sandy soil, the earth has a hard time giving up the warmth to the air within the tube. The reason for this has to do with soil moisture (water). 

Water is a much better conductor of heat than air. This is why 60 degree water feels colder than 60 degree air, it is more effective at stealing your body heat. For this reason home insulation products are designed to trap air and not moisture. Because sandy soils drain much more readily than clay or loam, there tends to be less water and more airspace between soil particles. This has an insulating effect between the tube and the earth, preventing the heat within the ground from moving into the tube.

In the case that I mentioned, air from the tubes came out the same temperature as it went in. The earth was unable to pass its heat on due to the insulating effects of the airspace in sandy soil. This caused the heat pump to work harder and harder to extract non-existant heat for the house. Eventually this fried the heat pump.

Sorry if this sound condescending. I'm just trying to explain, that's all.

Whistler


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## SolarGary (Sep 8, 2005)

Hi,
John Hait in his good book on PAHS (Passive Annual Heat Storage) says that the ground temperature at 20 ft underground is constant, and is equal to the year round average air temperature at that location. According to him, depths above 20 ft will vary though out the year -- the amount of variation decreases as you approach the 20 ft down level. And, the temperature at 20 ft below ground level is going to be warmer in Texas than in Montana.

His book is quite good, and has a quite a bit of information on ground cooling/heating tubes. He has built a number of homes that actually use these, and buried a lot of temperature sensors to keep track of the actual temperatures. 

Here is a little review of his book:
http://www.builditsolar.com/Projects/SolarHomes/haitbookreview.htm
His website:
http://www.earthshelters.com/

I managed to get a copy through my libraries interlibrary loan program.

Gary


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## fishhead (Jul 19, 2006)

Whistler,

Not at all.

I'm surprised to learn that soil moisture has such a significant impact on heat transfer to the pipe. I never would have thought about it. I did speak with Paul Hunt from Pine River MN and he said the dry sand under their solar heated building provides all the insulation they need.

One solution might be to put down a waterproof liner to hold the soil contacting the pipe in a permanent waterlogged condition.


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## blue gecko (Jun 14, 2006)

fishhead said:


> Have you ever checked the temp of the air entering the system and the temp exiting?
> 
> Also, the flow rate?
> 
> Those would be interesting.


I haven't checked it specifically as in a thermometer inside the tube at each end. We've incorporated a greenhouse into the system that has drums of water to help levelize the temps. The only real info I have at this time is winter things. The min temp the greenhouse got down to was 40F the minimum temp the basement got down to was 58. Most of the time the basement temp hung around 63F-65F. Right now its 45F outside, the basement is 66F and the greenhouse is about the same, the main floor of the house is 67F. 

In the summer the warmest its gotten in the basement is 78F but that's been without using the intake tube. Now that it's a closed loop and I can separate the greenhouse from it during the summer I should be able to increase the efficiency of the system. It's a work in progress. I DO need to put thermometers at each end and I'm looking for a better fan that I can put right on the pipe. I think that should improve the system significantly. The temp of the air should be a direct correlation to how fast it moves through the tube and the amount of turbulence in the flow. The tube has 4 right angles in it that create turbulence and the idea is to also help drop moisture out of the system and move more air across the surface of the tube. 

The house itself has a two story solar flue to help circulate air through the house. In that area (we call it the solarium) there is a trickle fountain we use to increase the humidity in the winter time. In the summer we only run it enough to keep the fish happy.


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