Lo-Tech Underground Cooling

[Hip_Shot_Hanna]

Has anyone installed underground "air conditioning"? We are planning to settle in central Texas in about 16 months. What we are looking at doing is a grid of 2000 feet of four inch pvc pipe buried in trenches six feet down and using a fan to force the air through the pipes so that the earth cools the return air.

Here is the url to the page which has a diagram.

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

What we were wondering was whether anyone had done something like this, and if so, do you have any pictures of the junction to the house from the pipes?

We know it works, my husband keeps telling me (as I type this :haha: ) that there is a school in New Mexico that has fitted this type of system, and that it works effectively and it is very inexpensive. THEY use a 1.5 horsepower fan to circulate the air. WE won't need one that big. I'm reluctant to attempt to live in Texas without air conditioning (I've lived there before for 23 years) but I would prefer not to pay the electric company for the privilege.

Cheers
Hip_Shot_Hanna

I've read about this type of energy but am not sure where it was I found it. But I do remember that not only can you cool your house in the summer time with this system but you can heat your house in the winter aslo since the ground is warmer then the outside air. Another way to save you a lot of money on heating and cooling.

[VALENT]

Go for it. I wondered about its possibilities for me but never really looked further into it. The idea works in theory but I dont know about the specifics in practice.

[mtman]

i see they talk about thin wall sewer pipe thats ach. 20 sewer pipe is sch. 40 its pipe not tubing

[moopups]

Yes, this system works and can be improved a bit by adding a catch 'T' at some point in the system so that humid air can be dried by going through filters, thus allowing the moisture to drop into the T area which is pumped out via an automatic float switch. Recommended for outside air supply system.

The same system can also recycle the air that was originally in the home, a single 90 degree manual fitting will give you both options. A bug/critter screen must be used at the intake end, plus the intake needs to be rain protected.

[Hip_Shot_Hanna]

We plan to use pipe, so will be going with the heavier stuff I think.

We had also made plans to drain the condensing water, but thanks for the ideas, as we hadn't seen that addressed, and any input is helpful.

Also, my husband has figured that he will be able to put in truck (not pickup) air filters which are near enough to HEPA standards to filter what comes back into the house AND keep the air going into the pipes cleaner as well. I think we'll have to figure a way to filter the air before the filter though, to keep out larger particles of stuff. (IE dog hair, fibers, etc)

It will be trial and error on the filter thing, I am sure. We are convinced it's the way to manage the climate in the house for us in the location we will be living. As someone said, it will also preheat air in the winter. Since we also hope to have a combination of wind and solar power, it only makes sense from that perspective to have a heat/cool system that won't be a big drain on our power supply. We also plan to be off the grid.

In addition we plan to have a !0,000 gallon rain water storage system to supplement our well. We will be paying cash for everything, and basically have to then find some way of paying taxes on the land etc. We won't have a lot of funds left, and we will have to try and find some sort of health coverage I guess, plus some groceries, though we also plan to grow our own as much as possible.

Lots of PLANS.. lol! But we are willing to work hard to make it happen.

[Jenn]

I expect air is cheaper but why not do a heat pump (heat exchanger) that uses a liquid to circulate through the ground? Might be mroe efficient? My brother's housemates had one put in (he paid for it as his rent when they built the house) and are very pleased with it. They just put theirs shallowly under the lawn guess it's not good ground for digging deep there. A heat pump can use outside air or (but not with a fan for the outside the house part) underground.

Okay what I read about was circulating water through the ground to collect the heat and cool of the earth. Not air.

[WisJim]

The systems that I have seen that worked well used bigger tubing or pipe than 4", I'm thinking it was at least 8", and it was 8 feet down. You have to remember that as the ground cools the air, the air is cooling the ground, and you need more earth in contact with the piping than you might realize. I suspect that you will need at least a 1 or 1 1/2 HP fan or maybe even bigger.

Jim

[agmantoo]

Will you not need to recirculate the air from the inside of the house back through the tubing to avoid the excessive heat grain from only bringing the hotter outside air in through the piping? Seems this excessive heat load would soon overload the system otherwise.

[Cosmic]

You are better off using something bigger in size if you can. Four inch is small and will require quite a bit of power to pump air through compared to say 10 inch, especially over several thousand feet.

I just moved so all my engineering books are still packed away but if you look around on the Web should be able to find the tables that relate resistance pressure drop to pipe size / air flow. Basically you are really interested in surface area in contact with the earth and heat transfer properties.

If you play around with this, you will find the area more than doubles if you change from 4 inch to 6 inch. You want a large internal area to minimize pumping power. Plus if you use larger pipe can use less feet, again it all comes back to the surface area in contact with the earth. Also better to use as thin a walled pipe as possible.

If you use small diameter piping, best to circulate water and put that through a heat exchanger to create cool air. All in all this type system should use far less power for the same cooling compared to most air conditioning systems.

These systems do not have to be built in piping, anything that creates a free void area underground will work. Including something made like a tunnel using bricks, cement blocks or what have you. Those type designs will tend to have very low pumping power requirements because their internal areas are large and have very low system resistance curves.

[Hip_Shot_Hanna]

This system will have a bundle of nine four inch pipes coming into and out of the house, which will then be spaced in concentric half squares, one inside the other, and not one long tube as it probably sounded.

The reason for using this system is because no compressor is involved, and no exchange except that of circulating the hot air out of the house and the cooler air back in. We realize that the air will gradually warm up to some degree and are considering placing soaker hoses along side the pipes to add some moisture to the soil for a more efficient dissipation of the heat. We do not plan to try and cool 100 degree air from outside continuously, but to continue to cool the air that is in the house.

The fan will be the ONLY electric using part of the whole system. Unlike a heat pump. We had one of those and our electric bill was still topping $200 a month years ago.

This system isn't passive as it requires a fan to move the air, but otherwise it's just using the earth's thermal mass to heat and cool. No heat exchangers other than the system itself.

I appreciate the input - especially the engineering stuff! We've read about people who are using the system I've described in Arizona where it works very well, but the info about the fan may be especially pertinent.

In a couple of years, we'll let you know how it works!

[farminghandyman]

one may need a radial fan/ pressure blower to move the air that distance, effectively, vs a standard fan,

but I read in a paper one time a guy heating his green house this way in the winter time in northern nebraska, (it was not hot but was above freezing),

[bgak47]

It sounds like a lot of work to me! 2000 ft of trenches 6/8ft in the ground? Uderground pipes? Fans. How much cool air will you get when it is 105 outside? I'll stick to regular airconditioning.

[Janon]

You may be able to drain the excess moisture, but you'll still be left with mold and mildew forming in the pipes. I'd be very skeptical about how efficient or even how practical an underground air system would be in somewhere like Texas.

I'd look to conventional A/C, better insulation in the structure, shade tree placement, window treatments, being able to isolate only the living area of the house, etc. before a system such as underground air piping. They also have "zone air-conditioning" which requires no ducting. Similar to central air where the a/c unit is outside, but with the fan units mounted on the walls or ceiling - this system allows a more direct use of the cool air without it travelling through lots of ductwork to un-needed or uninsulated areas.

Around here, most older homes had many interior walls and interior doors - not because that was "trendy", but because the house was much easier to heat when you didn't have to heat all of it.

cheers,

[Hip_Shot_Hanna]

bgak.. how much air conditioning - our sources, and we've looked long and hard at it, come up with around 8 tons of air conditioning by the end of the summer. We aren't planning to dig by hand - no way!

The mold and mildew have been one of the things we think about too - hence the truck air filters as I mentioned before. AND we do intend to have good insulation, shade trees, etc.

We don't plan to have a huge house - 1300 - 1500 square feet.

Good points all! Thanks

Deb

[Cyngbaeld]

I haven't done this YET. I have been studying on it and really want to try it myself. And I do think it is workable even in Texas, where I am heading end of this month (Lord help me!). The ground temp where I expect to be is prob around 69*. The limiting factor will be how much heat can be dissapated into the ground. I noticed that the website author will do a site workup for a fee. Don't have any idea if what you get is worth the fee?

I'm prob not going to recirculate the house air because I need a great deal of fresh air thru the house to keep from being sick. I was thinking of using much larger pipes and running it under the veggie garden because that is the one place I am sure the ground will be moist. I thought I might put a sump in the low spot and run the water to the veggies. Expect I'll need a bigger system too. I don't do well with refrigerated air because of build up of pollutants in the house when it is shut up. And it is too humid there for the swamp cooler like we used in western colorado.

Can you run all the pipes into a wooden box with holes cut in the back for the pipes? Then use caulk around the pipes. You can set your blower fan into the box with your filter on the side that faces the house.

[Cosmic]

If you ignore the engineering approach to these types of projects, you do so at your peril. Example, the type system you describe basically has little "Volume".

2000 feet of four inch pipe only has something like 1300 gallons of air volume. You can pump that down in a relatively short time, the ability of the ground to recharge via some method of heat transfer comes into play. PVC probably has a relevatively poor heat transfer coefficient.

The best systems design for these type applications will feature large internal stored volumes, both for low pumping power requirements and to avoid getting into very dependent heat transfer problems.

I may have to consider one of these type systems myself. Bought a Fixer-Upper. The basement back wall is fully exposed and probably will be heated by the sun in summer. Next to it is a two car garage built of wood on piers. Pier system is toast and rotten, will have to tear down the garage. Thinking of building a cement block wall system to "Fill in the Hill" and allowing pouring a conventional cement slab. Lends itself to building in one of those "Natural A/C system. I would tend to go with a "Tunnel Design" that would have a large volume, maybe be 120 feet long in total. Easy to do as the construction space is totally exposed and will have to be totally back filled.

In those designs, you factor out much of the heat transfer considerations and the system has "Mass" and ability to store energy as part of its own walls.

I had some experience with this with my last house. Used the basement and sucked cool air up into the house via an attic venting scheme. I just don't believe a low volume, small diameter piping system in what one would go with if you have the option of building new. Way to many ways to get it wrong, basically you are trying to design something to work without Mother Nature. Always make Mother Nature and "Natural Methods" work for you as the primary method. Large volume, inherent stored energy mass systems are way to be thinking for new systems.

Again always comes back to what materials can you get cheap, cost to construct, what site factors can you take advantage of.

[Darren]

I think Cosmic is correct in suggesting the use of the largest size piping and the longest length you can install to provide as much surface area for heat transfer while minimizing friction. The downfall of these systems is mold growth. Mold spores can be as small as 1 micron. If someone in your household has allergies, you'll need an effective filtration system. Air filtration systems aren't normally rated by micron size. See the link.

http://www.filtercouncil.org/techdata/tsbs/04-3.pdf

The other factor about truck air cleaners is that the system is usually designed to take advantage of inertial airflow to remove particles from the air stream. That's going to be dependent of the speed of the air flow. When you compare the higher air speed of the flow into an engine which can be considered a high speed air pump to your air flow, you're not going to have enough air speed to work with an inertial dust collection system. A truck system operates to remove a lot of the dust before the air gets to the filter. Otherwise filter life is poor.

In the eighties I considered building a system in Mississippi but I didn't have enough land to install the length of piping needed. I was looking at 8" sch 40 PVC. That was also before the mold problem was known. I don't see a problem with building a system that will work. Just make sure you cover all the bases. Otherwise you'll waste your money and end up with something that's a problem.

[Cosmic]

To one of my former points. PVC is probably not the material for a good system. Did a little digging around to find its heat transfer properties. See this source.

http://www.roymech.co.uk/Related/Thermo.html

In general most plastics are more insulators than good conductors of heat. PVC compares to something like plywood, the better materials are metals, especially ones like copper. This is a pretty good table because it is relating to heat transfer via conduction on both sides of the material for something that is buried or in immersion exactly what this application requires.

If you think about it, never see heat exchangers made out of PVC, just not suited.

One thing I can tell you from the prior experience is these type systems can be overloaded. My former basement was 2 foot stone walls with a concrete floor. Stored a lot of cooling but if you got something like a +90 F day and tried to suck too much cooling out of the basement the system basically "failed". Its recharge capacity was limited for high flow situations. You had to understand exactly how to tune and setup the flows to get it to cool the house. Of course that was a "Once thru type system" where air from under a porch, entered the basement, was cooled, passed thru the upper stories and out the attic window.

Plus not all soils are equal. Some don't have good heat transfer characteristics themselves. Somewhere there is probably a web site with a lot of design info. Something like a nomograph to do a typical ballpark design. If I was going to plug serious money into such a system would drill a well and use a water based system, got to be far more predictable in the design.