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randyb

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             I am fairly new to alternative energies. But I do have a couple of harbor freight solar panels for charging my rv batteries [smile] I didn't stay at a holiday in express lately so need some input.
             Question is about earth tubes for cooling. I am entertaining the idea of building a 1200 sq ft single story extremely well insulated tight home on concrete slab on 2 acres that has natural drainage. I have not shot it with a transit but it appears I should be able to go at least 6 feet down and still have gravity drain for them.  So the worry of moisture and mold is minimal. I have been searching the net looking for information on the length and depth needed for sandy soil in Arkansas.  From the looks of the lay of land I should be able to get at least 4 runs of 200 feet. probably longer if necessary. This would be a closed system circulating the air from inside the house. Also my plan for summer cooling is an angled ceiling with exhaust vents at the approximately 10 ft eves with computer fans on thermostats to vent to hotter upper air outside. My idea is that heat rises and a 7 1/2 foot to 10 foot ceiling that pekes against a wall would trap the hot air close to the vent. My plan is to also use slow moving air via the 12v computer fans to circulate the air through the 4 earth tubes rising from floor located close to each corner of the house. I have yet to find any definitive information on depth or length needed for a 4" schedule 40 pvc system versus square footage or climate etc.
              Anyone here with experience for such a system ? Is my plan a good one for my area ? Thanks in advance.

SolarInterested

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Reply with quote  #2 
Welcome to our forums. Can't give you any advice but here's a link to other links

http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Other

 

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Garage_Hermit

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Reply with quote  #3 
Hi, welcome on board !

For a start, I would not use PVC at all for breathing air (it contains volatile organic compounds - otherwise known as VOC's).  Schedule 40 PVC is not meant for your application.  OK, I happen to use it for experimenting with, but that is not quite the same thing as putting it into a new-build home that will stand for perhaps 300 years...

As an aside, PVC is an insulant, so it would not be as good as metal, at transferring heat/coolth.
So earth tubes are generally made of special stainless steel, and they do not come cheap.

Generally, earth tubes are an OPEN circuit - in the Summer, they cool the (otherwise hot) air intake feeding the house, and in Winter, they warm the (otherwise cold...) air intake feeding the house...

So earth tubes warm or cool *VENTILATION* air - they are not really for producing heating or cooling effect for SPACE heating, which your application seems to be...

http://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger

If you are talking about 4 air-changes per hour, and your house is 1200 sq.ft, presumably that is around 12000 cubic foot, and you are looking at 48,000 Cu.ft/hr...

The duct size for shifting 12,000 CFM can be calculated at HERE...
http://www.engineeringtoolbox.com/equivalent-diameter-d_443.html

if you go down the table, you will see that the duct size required to shift 12,000 CFM is 20 x 24 inches, which is "some tube".

I am not trying to wreck your project, I just wanted to supply some basic ideas to respond to your interrogations...  But please don't take my word for any of this, I am no expert...

So this makes it plain that an earth tube is not the right tool for cooling one's house: why not just dump the warm air outside, like you say,  and use mass cool air (from your north side, for example - preferably over a water feature - to draw cooler air into the home.

There are references on here to Solar, Chimneys, I believe in Texas...

Will try & dig you out some links.   http://www.simplysolar.supporttopics.com/post/solar-exhaust-chimney-6351079?pid=1278087477&highlight=solar+chimney#post1278087477


Best regards,

Garage_Hermit


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randyb

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Reply with quote  #4 
Thanks for the response garage hermit. I have looked at the wiki before. But have not looked at the duct sizing chart, thanks.  I think my cubic foot will be more like 10,000. so I'm looking at a needed 40,000 cu ft per hour needed ? The chart does not show a 4" pipe but does show a 4.9" round duct. The 4.9" shows an airflow of 200 cu ft per minute. Multiplying that times 60 wound indicate one 4.9 would circulate 12000 cu ft per hour. Hillbilly math tells me to deduct 20% for the downsize = 9600 cu ft per hour for a 4". If I install 4 tubes I should get 38,400 cu ft per hour. If these calculations are correct the air should change 4 times per hour for the whole house. That is not taking into consideration the 4 4" vent tubes mounted at the highest point in the ceiling. Which would help dispel the rising heat functioning sort of like the chimney method in mini fashion.
           From these calculations if I'm applying them correctly would seem to show that the sizing of the pipes should do the trick. Please correct me if I'm wrong. The ducting chart will help me decide the type of fan to use to get the maximum amount of air possible through the ducts but does not give me optimum airflow to take advantage of air changing thermally. The air has to have time to cool in the underground.
          I may be better off to use an open system which would create a certain amount of chimney effect forcing the rising hot air out. And not sure if this would be a benefit either since the exhaust tubes will be forced air. Open system looks to be a detriment since it could be pulling in outside air possibly around 100 degrees. But maybe not I can place the intakes in a shaded area and if the required length we're known for optimum.
           If I install this system I will need decide between open or closed. I had leaned towards closed because it makes more sense to recycle cooler air from inside the house. But i can see a huge advantage of pulling in fresh air. But may be able to test both with 2 risers installed where the pipe makes it's loop back close to the 90's which could be plugged. 
           I am also blessed with having my own backhoe so I can take my time and not have to worry about pushing it too fast to minimize cost and the possibility of making changes. Of course would rather get it right the first time. [smile]
           What I am missing here is information on desired length, depth and optimum airflow for this area and soil type. 
          
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