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Earth sheltered home earth tube question  RSS feed

 
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We are building into a hillside at the top front end will be near bottom 60 wide by 22 deep and am looking for air tube advice...i.e. How long especially. We will use positive pressure will a fan at inlets (and a critter filter) with lengthwise perforations for moisture drainage and gravel bed also lengthwise. Length needed main question besides how long the air needs to be in the tube . THANKS in advance.
 
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Hi Mike,

I just saw this question and it got me thinking that I'd seen these calculations before.

Unfortunately it's complicated and I can't give you a number that will work for everyone, but the attached white paper goes through the math, and has an example for a home in Southern Germany.

Everything's in metric unfortunately.

The concept behind the math is that essentially, what matters is the temperature of the air you're trying to heat/cool, the temperature of the earth where to tube is passing through, the thermal conductivity of the tube itself, the size of the pipe, and how long it stays in the tube before coming into your house.

I've never done the math, but you can probably come up with a good approximation by comparing his +/- 138' of 5" diameter pipe, buried 2-5 feet below ground, to pre-condition the air for Southern Germany. If you are in a milder climate you can probably use less, if you're in a more extreme climate you will probably have to use more.
Filename: Calculation-Tools-for-Earth-Heat-Exchangers.pdf
File size: 77 Kbytes
 
mike lamere
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Nicola Marchi wrote:Hi Mike,

I just saw this question and it got me thinking that I'd seen these calculations before.

Unfortunately it's complicated and I can't give you a number that will work for everyone, but the attached white paper goes through the math, and has an example for a home in Southern Germany.

Everything's in metric unfortunately.

The concept behind the math is that essentially, what matters is the temperature of the air you're trying to heat/cool, the temperature of the earth where to tube is passing through, the thermal conductivity of the tube itself, the size of the pipe, and how long it stays in the tube before coming into your house.

I've never done the math, but you can probably come up with a good approximation by comparing his +/- 138' of 5" diameter pipe, buried 2-5 feet below ground, to pre-condition the air for Southern Germany. If you are in a milder climate you can probably use less, if you're in a more extreme climate you will probably have to use more.



THAT is some serious math...the contractor is getting with a designer for this aspect of the build, those equations are a bit arcane for my simple mind but thanks anyway. I have DL'd it in case he asks.
 
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You might want to do some more research.  Specifically look for studies on the efficiency of Earth Tubes as well as the numerous problems people have with them.

When properly engineered and installed, using passive ventilation techniques and ideal locations, they can often be useful.  
But, from what I've read, the majority of the earth tube installations are not very effective and/or problematic.  Radon gas and mold are common problems.
 
And if you are going to use active ventilation (fans) then the energy costs vs the cooling/heating is marginal at best.  In many(most?) cases you'd be better off using a Mini-split heat pump.

Some helpful articles:
http://www.greenbuildingadvisor.com/articles/dept/musings/all-about-earth-tubes


This study shows that the COP for an Earth Tube can rival a Mini-Split heat pump, however, this was at the beginning of the cool and heating seasons, when earth tubes are most effective. As the season goes on earth tubes become less effective as the surrounding Earth cools/warms.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.504.5677&rep=rep1&type=pdf

Note: the tubes in this study were buried 3 meters (~10 feet) down, one of the most common problems with ineffective earth tubes was from not burying them deep enough.


I'm not saying you shouldn't try earth tubes, but I really encourage you to do a LOT of research before you commit to them.
 
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