Cooling Tubes

Does anyone here have experience with earthship-type cooling tubes?

I have several questions, including:

1. How to maintain a healthy environment in them? Is mold/mildew a concern?

2. How deep must the gravel layer be? What size or sizes of gravel should be used?

3. HOW EFFECTIVE ARE THEY? Have their efficiencies been documented?

Where do you live? There is no universal answer.

Oh, right!

I'm in upstate New York (near Monticello) where it's about 50 degrees Fahrenheit (10 degrees Celsius) two feet (60 centimeters) down in the earth all year.

I don't have any practical knowledge about cooling tubes, but I have worked through the exercise of looking for an alternative to standard air conditioning. My take on cooling tubes is this:

1. Plastic pipe (of any type) is non-conductive, ergo acts as an insulator.
2. Concrete or Terra-Cotta pipe will grow mold or mildew if the air has a relative humidity of 20% and the ground temperature is below the dew point.
3. The Aldo Leopold center aside, there aren't a lot of practical examples of this type of construction that make a great deal of difference in the United States. What I have found hints that this was a common tactic in the middle east, but I haven't found much on practical examples. Earthships are well documented, but details seem to be fleeting, including efficiencies.
4. In a humid climate, (ie anything with more than a 50% relative humidity) the air coming in will drop so much condensation inside the tube that it would serve practically as a passive water source. This condensation may or may not also coat the inside of your dwelling depending on temperature and humidity of the dwelling itself.
5. Direct flow of air through this type of system will likely at some point bring mold spores into your air, effectively polluting your air with potential neurotoxins.

I do know that a friend of mine made up one about 100 feet long out of 4" schedule 30 plastic pipe. It was buried approximately 6 feet under ground in the middle and only the last few feet on either end had around 2 or three feet of cover. Im told it had about an 8 to 1 slope, so about i foot of rise per every eight feet of run. The temperatures in Southern Iowa, where he was, was somewhere in the realm of 90 degrees Fahrenheit. He put a digital thermometer in each end. The air going in was 90, and the air coming out was 88.6. This was done as an experiment to see if it was feasible to do in our area for himself, myself, and another two families that are trying to get off-grid.


That said, I don't want to discourage you.

Temperature exchanging is a relatively low tech innovation used in every furnace and air conditioner, and even your car. It should be feasible to create a system to pull the air through your structure and get that temperature into the air through use of something like a plate exchanger, all without polluting your air with excess humidity and mold spores.

If you don't have terribly hot summers, you might get away with the humidity issues and just putting some kind of filter over the vent where it comes in may be enough to deal with the mold/mildew (I would be more concerned with smell if all else is eliminated).

And while inefficient, thin-wall plastic pipe is still very economical, so even though the temperature exchange isn't much, it may be enough if your climate isn't terribly hot in the summer.

So basically, knock yourself out.

Oh, and as for the gravel, I don't know the purpose of it in the Earthship design, perhaps drainage, but I really can't speak to that with any certainty.


Please fill us in if you learn more.

The simple answer: If you live in Upstate New York, you don't need cooling tubes. It only gets above 70 degrees during the Summer months and rarely above 80. For 9 months out of the year you are only concerned with heat gain, and heat loss is EASY.

Cooling tubes are really only for hot, arid areas like Taos, NM in the summer. Using them in Monticello is going to cause humidity issues, and condensation makes then LESS efficient (water gives up energy as it condenses) Using the condensation as a method of getting water is also pointless: You get 46" precip per year. That's more than I get in Seattle.

The HVAC design for your Earthship should therefore be primarily tailored for solar heat gain and solar hot water, which is no easy feat in an area with less than 50 cloudless days per year. I would also recommend an air exchanger system (something blatently overlooked in Earthship design) so that you can maintain indoor air quality in the 9 months that your gravity skylights are closed. This system WILL put an additional strain on your solar electrical system, so more panels.

I would also recommend using vertical greenhouse windows, even though the "ideal" winter angle is more like 60 degrees, because sloped glass is more trouble than it's worth in a damp climate.

Also, once you have built your structure, don't expect it to be really comfortable until the thermal mass has had time to charge. I would recommend ditching the cob fireplace recommended by Reynolds in Comfort in any Climate and instead build a Rocket Mass Heater into the structure. You may find that you need to burn wood a lot in the first year but less in subsequent years after things have stabilized. (The biggest heat suck in new earth plaster buildings is actually all the moisture evaporating, taking with it the latent heat of vaporization)

In summary: Cooling tubes. More trouble than they're worth unless you live in a climate with more "cooling degree days" than "heating degree days".

Just a couple of thoughts.. One really does need to get down about 8 ft below grade to get the maximum amount of heat exchange..

Plastic is ok but using galvanized metal is much more efficient at heat exchange. If you look at ground source heat pumps 2 inch pipe with about 500 feet per ton of heat exchange.. and then they need a condenser unit to extract the heat from the inside of the house or extract the heat from the ground.. This really does become a matter of volume of surface area to exchange the heat.. Since this is passive.. expect to need a heck of a lot more surface area than you might imagine.

Consider this.. the latest earthships are seeing installations of 80 foot of 1 foot steel culvert for a 1200 foot space

as to condensation.. not really a factor if you are moving enough air at a slow speed because of the diameter of the pipe.. the smaller diameter the air speed must by nature move a lot faster to move the same volume of air

I am just beginning to plan out our new green/sustainable home here in Yucatan Mexico. Our biggest challenge is cooling. It is hot and sunny all the time other than when it is raining. I was hoping a cooling tube system would work. But I also was concerned with condensation and mold. I am leaning that it might just not work for this area. any suggestions?

Honestly I think that for earth tubes to work one needs a lot of actual surface area to operate correctly plastic tubing is not very good at changing the delta eg cool of the earth to the air inside of the tubes. So while some are using bigger piping made of metal culvert.. actually think that 4-6 inch plastic piping is better for a few reasons.. one can push adequate air thru the individual pipes.. You can seal the pipes at the joints easily. Very difficult to do with metal piping (expensive)

Here is a quick thumbnail on the amount of 4 inch piping length that one should aim for for each 600 square foot of conditioned space.. to make this easy .. 1:1 ratio.. or 600 feet of piping runs. the other thing to make note is that you dont want to actually have a single 600 foot run but break it up to 6 runs.. Why.. back pressure.. it is easier to flow air thru shorter runs because it just takes less effort with a short tube over a long tube.. think of it this way.. blow thru a staw... then go outside and try blowing thru the end of a 100 ft water hose.. even though the hose is bigger.. takes a lot more effort (friction in the tube plus trying to move that much mass of the air) is way harder.

Since you are talking the Yucatan I would think you likely need to add about 10% more runs.. and try to get them as close to 8 foot below grade as you can.. If you have a way to make the pipes have a slope in grade.. where they can drain _3/4 inch pipe drain is really all you need at the lowest point.. you will not need to be concerned about water related problems of mold.

Hope this gives you a bit of perspective

steve pailet wrote: . . . you will not need to be concerned about water related problems of mold.

Steve, can you explain why mold/mildew wouldn't be a problem? Thanks.

(And thanks, everyone, for all your really good information.)

We have earth air tubes but we use them for heating rather than cooling since we live in the north country - mountains of northern Vermont.

They are very effective. They bring in sub zero outdoor winter air and raise it into the positive thirties or forties depending on how cold it started. That's free heat.

Since we live on ledge we could not put ours very deep. They're about 12" down. We put pink foam closed cell insulation on top of them which create s an umbrulla that diverts the winter cold and frost line away from the pipes. Our pipes are four 70' long lines of 4" diameter ABS drain pipe. It is all sloped to drain out the air inflow.

We have had no problem with mold or mildew. We did not use a lot of gravel although I might in the future.

I like the system so much that I'm going to build a much bigger one for our on-farm butcher shop.

See:

http://sugarmtnfarm.com/2008/09/05/earth-air-tubes/

as Walter and I said.. you have to have a way to drain the piping .. at its lowest point. the idea being if there is no moisture there is no mold problem condensation is a reality due to reaching dew point.. more on the cooling than the heating side..

so bottom line.. if you put in 1/4 in per foot fall down the the lowest point and drain it there.. I pick this amount of fall for two reasons.. it is identical to what you would use on your drain waste vent plumbing.. so there is no need to think .. now how much did he say.. it is all the same.. and the second part is it likely will give you the maximum drainage that will remove any sediment that could other wise collect over time ( dust that might get into the system)

As I said.. consider the easy rules of thumb.. just makes these things a lot simpler.. and when you have to deal with outside helpers .. dont confuse them.. they already suffer with being gorillas doing fine construction.. LOL

Hey Folks,

Some general rules of thumb about cooling tubes (I've worked with engineers from Buro Happold on designing houses that will implement them in the Saudi Desert): If you are in a humid environment, they don't work. Historically they were widely adopted in the mediterranean and middle east--anyone wanting some good informaiton on them can look into Nader Khalili's book on "ceramic" architecture, where he talks about old persian ways for both capturing water and banking the cold.

If you are in a hot/dry area, there are usually 3 components that go into cooling tubes, only one part of which is the actual tube. Usually at the junction between the room you're trying to cool, and the tube, there is some kind of evaporative cooling: damp rags, an unfired clay pot filled with water, moistened charcoal, or the like. The tubes bring in pre-cooled air but the added humidifier can drop the incoming air temperature by another 7-10 degrees F. The third component has traditionally been a solar chimney--a tower connected to the house with a blackened top (we'll use cast iron) to heat up and draw hot air out of the house. This creates lower air pressure inside the home, which in turn stimulates a draw through the cooling tubes (assuming your house is well sealed!). Those 3 elements together, combined with good earth berming and good house location, is what is allowing me to build in Saudi Arabia without needing an air conditioner. I don't have data because we're starting the build for our first one in about 6 weeks, but the engineers have looked at it and said it should work...my fingers are crossed.

For high humidity areas it will be beneficial to construct a dehumidifier inline with the cooling tube. You can use the cool water from the cistern, cool it further using a Peltier element, or passive/air cooled heat-sink which will condense the water vapor as it travels along the length of the pipe. The Peltier element, though uses power fairly inefficiently will give you a much cooler airflow than a fully passive system. Also, because of the way the thermal exchange works you can effectively use one side of the Peltier element to cool one water pipe while you use the other side to warm another. This could allow you to save some energy on heating water for bathing.

a couple of things to keep in mind. open vs closed loop..

Open loop in a humid environment is definitely going to be bringing in humid air in large quantities. So the need for some sort of mechanical dehumidifying likely will be needed..

Closed loop will not have this problem for two reasons.. first and most importantly you will be circulating make up air.. Air entering the loop from inside of the house.

The second reason is that the temperature difference between the house and the ground will not be as great so reaching the dew point is going to be a bit more difficult hence less humidity ..

All that said.. I am of the opinion that one needs to install one open loop to bring fresh air into the home.. using the exhaust points that are natural in a home. the bathroom exhaust fans and the kitchen vent fan to move the stale and humid air back out of the house.

Generally speaking the open loop because it is open to much higher or lower Outside temperatures will loose its ability to charge the ground or discharge the ground of heat. So after 15 -- 30 minutes it will stop dehumidifying or adding heat or coolith to the home. One can only move so much heat in a short period via ground transfer. once this equilibrium has been reached it takes time for the ground to return to its natural temperature, at which time you can start using it effectively to remove or add heat back into the ground to be used in the house. It will again be discharged and useless in 15 - 30 minutes

Because each set of tubes has its own purpose.. on is for constant fresh air.. and the other is for actual setting of temperature within the envelope of the house... I think both are needed

peltier modules interesting concept of this application.. wondering how many dozen of them one would have to put around the house to have them be effective in dehumidifying.. I had one on my sail boat.. it indeed works.. does not draw a lot of current.. but wow it took days to bring a well insulated box down to refrigeration temperatures. Had three inches of expanded foam.

Nicholas Sanders wrote:I am just beginning to plan out our new green/sustainable home here in Yucatan Mexico. Our biggest challenge is cooling. It is hot and sunny all the time other than when it is raining. I was hoping a cooling tube system would work. But I also was concerned with condensation and mold. I am leaning that it might just not work for this area. any suggestions?

Have you considered an evaporating tower? If you have access to water, then you can build a tower where water is slowly released at the top. As is wicks down the outside, it evaporates, cooling the surface of the material.

Here is a video of an example of the principle in action:
http://www.youtube.com/watch?v=nROyfgGxMDw

steve pailet wrote:peltier modules interesting concept of this application.. wondering how many dozen of them one would have to put around the house to have them be effective in dehumidifying.. I had one on my sail boat.. it indeed works.. does not draw a lot of current.. but wow it took days to bring a well insulated box down to refrigeration temperatures. Had three inches of expanded foam.

The thermal transfer rate is directly related to the energy applied. It does, however, become more and more costly for smaller and smaller thermal transfer gains as you apply more energy. Mostly due to the heat produced by the electronic components themselves. Also in relation to the temperature difference between the two ends of the element. The ideal thing would be to have two water lines running parallel to each other. one for cold running in one direction, and one for hot running the opposite direction. Each element would transfer heat from the cold pipe to the hot pipe, and you could run as many as needed in parallel. You could set up a system to adjust the current to the elements based on the temp of the cold pipe to keep it at a constant temperature no matter what the temp outside.

Also of interest...they can be used to generate small amounts of electricity, so the system could be configured to feed back to itself to save some energy.

I'd explore other options if your working with a standard house. It could be opening windows over night would do the job or shading out some patio area that heats the ground near the house. Find where the heat is coming from. If you have some white curtains to shut during the day it will reflect the light out. Dark curtains or any light coming in can heat a house a lot the moment it is warm enough outside for the glass to be ineffective at draining the heat.  You could paint the roof white but that will make it colder in winter. Insulating the roof and possibly walls would probably be better year round. For cooling tubes maybe using water and connecting it to existing radiators would be better.  You would just need a small pump or if there is some higher ground on the northside of the house you might not even need that. The problem is that the cool air  sinks so it could just pull warm air down the chimney into the house and flow the wrong way. The solar chimney might be good enough to cool it down without any additional cooling tubes if the problem is just lack of suction. I am more experienced with the heating tubes where the condensation issue is perhaps less. However it can cause condensation inside if the walls are cold. It is effective tho and you can put it thru a solar panel after to push it into the house and increase ventilation. My idea is to have quite a big pipe so lots of preheated ventilation can be provided. This keeps the house pressurised and cold air won't leak in so much elsewhere. If you run a fire then instead of cold air coming in everywhere to feed it the big vent pipe can provide it and you get a better gain and hopefully keeps it dry. My concern with it is more plastic smell from the pipes. If there is enough ventilation then it should reduce condensation problem. If you wanted cooled air then probably best to look if there is a prevailing wind direction in these hot periods that could be used to push it in thru the pipe. Otherwise it might flow the wrong way and bring air down the chimney (which could already be happening and causing the issue).