ASK THE EXPERTS: Earth Tube Questions

Intermediate
Earth tube installation.
Earth tube trenching and installation.

I’ve read, and reread your net-zero home article (“Net-Zero Performance” in HP150) many times. I am going to break ground on an earth tube system near Provo, Utah, modeled upon the one in the article, and I have some questions.

Is antimicrobial trade-name pipe better than stock pipe? What evidence exists showing bacterial overgrowth in well-designed earth tube systems, and mitigation with the more expensive product?

Costs depend greatly on trench depth. In rocky, boulder-strewn ground like mine, I’ll have to strike a balance between trench depth and cost. How about placing polystyrene insulation on top of the pipe at a depth of 3 to 5 feet and then backfilling, instead of digging 10 feet deep?

Chris Anderson • via email

The system I used— the only one I found with an antimicrobial coating in the tubes—is Ecoair by Rehau. Their U.S. division is Amvic (amvicsystem.com).

I read about the issue of mold in a handful of articles by folks who installed earth tubes, but typically theirs were installed in more humid climates than my climate zone 5B in central Colorado. I found as much anecdotal evidence of systems that had no mold issues. While I’m not sure what all the differentiating factors are, I suspect:

  • The use of smooth-walled tubing versus ribbed tubing minimizes the risk and increases airflow
  • Specific microclimate characteristics
  • Slope of the pipe (no low spots) and use of a condensate drain at the low point
  • Number of hours of operation
  • Degree of waterproof seals at tube joints

Still, I did not want to gamble with my family’s health, so I went with the Ecoair. The systems are costly, though. For the roughly 4.5 kBtu per hour heating capacity we gain from the earth tube, we will have a very long payback time to recoup the $6,500 system cost, including trenching, labor, and parts. I knew this when we designed the house, but this was one of about four areas where my wife and I opted for the moral imperative of reducing our ecological footprint—we intentionally did not select the most cost-effective course of action.

As for trench depth, I believe you will be compromising the system’s performance if you don’t bury the pipe deep enough. In the Provo, Utah, area, at a depth of 3 to 5 feet, pipes will barely be below the frost line and ground temperature will fluctuate. At your latitude, the soil temperature will be 50°F to 52°F at approximately 8 feet of depth. By insulating above the pipe, you would reduce part of the surface area available for heat transfer. However, if trench depth is a insurmountable limitation, using insulation would be better than not using insulation. I would use R-10, 2-inch extruded polystyrene foam, and extend the foam 4 feet on either side of the tube. But overall, the deeper the tube, the better. Data from the geothermal heat-pump industry indicates that at depths of 6 meters and greater, the soil temperature equals the mean annual air temperature. At 4 meters below the surface, the soil temperatures vary approximately 3°F; at two meters, about 10°F.

Some installers run a cable the full length of the earth tube during construction, which is used to pull a cleaning rag or plug through the tube once per year. This seems like a good, low-tech way to reduce the risk of mold.

Jim Riggins • EnerSmart Energy Solutions; enersmartenergy.com

Comments (12)

steve pailet's picture

just a couple of comments. Do not use 90 degree bends rather use 30 degree or long sweep 45 degree bends. As mentioned. It slows the flow dramatically.

Second thought. 8 inch tubing is about ten times more expensive than 4 inch tubing. 4" 12.57 volume and the 8" 50.27 so replacing a single 8 inch pipe with 4" pipes is not only better economically but allows you to do a variety of different plumbing within the house along with air flow for specific reasons.

Now what are those reasons? One can run individual vents to places like bathrooms, utility rooms, kitchen, and or into a greenhouse to remove moisture laden air while if running this thru a heat or energy recovery unit.

Generally you always need the same drop in grade as one would do with drain waste venting . 1/4 inch per foot. with a gravel bed to accept the condensation. This is a good idea whether you are doing a closed or open loop system. It will also eliminate the mold problems.

A good rule of thumb is not to go much more than 150 feet with each pipe run.. total length. Why is pretty simple the farther you make your runs the more power needed to flow the air thru the pipe. The higher the velocity of the air flow the less time it has to exchange the heat from air in the pipe into the ground and converse movement.

I am of the opinion that one should run both closed and open loop systems in the same home.. but not connected together. Ten closed loops that are 50 foot in length wide then adding the pipe to take it out into the yard (this is where you end up with 150 foot runs) will provide several tons of cooling or heating depending upon the time of the year.

A couple of 4 foot tubes running out but open into a gravel pit will provide for amazing amount of fresh air into the home but modified to add heating or cooling again dependent upon the time of year

Placing insulation over the piping will indeed force the ground over a broader area to give you better heat exchange. If you can go down atleast 6 feet it is better.. just another comment on spacing.. keep the pipes atleast 6 feet from each other so you end up with 3 feet exchange area per pipe

Fred Golden's picture

Steve,

I don't know how effective 4" diameter pipe will be as a earth tube. When I read about the Colorado Passive house using 12" diameter tube, I thought it might be overkill to put in such large tubing. However each liner foot is about 3.15 square feet of surface area.

Lets say the tubing is 100' long and exactly 12" X 12" square. It would hold 100 cubic feet of air. So air flow of 100 cubic feet per minute the air would spend 1 minute being heated or cooled. Lets compare that to 4"x4" square tubing. It would take 9 liner feet of 4" square tubing to have 1 cubic foot of air. The surface area of 4" diameter is only 1 square foot per liner foot of length. So moving 11 cubic feet per minute would allow the air to be heated for one minute in a 100' long tube, or to move 100 CFM would require 4" diameter tubing nearly 1000 feet long.

I think I will stick with the shorter trench and 12" diameter tubing. At 10' long with 1/4" per foot, that is 2.5" drop per 10'. So 100' would have a drop of about 2 feet. I hate to think of a 1,000' long line, and how deep it might need to be.

I probably will also be using 12 VDC computer fans, and hooking them up to my 12 VDC solar lighting system.

In my local area, they infuse zinc to the roof shingles to prevent mold and mildew. This might be what makes the earth tube mildew resistant. You also have to consider the material weight. 12" diameter tubing will be much more weight per 10' of length than 4" tubing. So cost to manufacture is much higher from a material standpoint too. I think the cost is well justified. I certainly do not want to dig up some moldy tubing and have to replace it in 3-5 years, I want a trouble free house for more than 30 years.

steve pailet's picture

A couple of comments. I would not run just one 4 in pipe.. as you have said it would not be effective. I would be looking at atleast 6 pipes and more likely 12 pipes in and 12 pipes out. A single 12 inch pipe is again fairly useless.

Using a heat recovery ventilator it pushes 400 CFM.. not eleven cfm I have no clue where you figure one would only push 11cfm. Figuring 6 four inch pipes with a run of 100 feet is in my view about right. Beyond 100 feet one starts to get real back pressure. 1000 feet as a single pipe.. That would be equivalent to running the drop from the power pole 1000 ft using a 28 gauge wire and trying to run a stand up range that wants to draw 50 amps. That is not going to work either.

As I have said, the cost of 12 inch pipe is pretty much prohibitive unless you are that guy on the geico commercial made of money. Figure that the cost of 12 inch pipe 20 foot long is about equal to the 6 runs of 4 inch pipe 100 feet long check this link
http://www.neobits.com/gf_piping_sy... 20 foot 12 inch poly pipe $1,472.95 that is just for one length.

http://www.homedepot.com/p/JM-eagle... 4 inch pipe 16.00

I dont know what you are thinking but I am not made of money.. and this pipe wont promote mold or mildew. Yes one does need to drop 1/4 inch per foot. so it needs to drop about 2.5 feet per run of pipe. If one does not the probability of mold growth I think will go up geometrically

Fred Golden's picture

Steve,

If you carefully read the comments that I posted, especially the second paragraph, I figured out that if you used 12" square tubing *no they do not make it square but the math is very easy* 12" square X 100 feet long then 100 cubic feet of air will sit in the tubing at all times. Moving 100 CFM through that length of tubing will cause the air to stay in the tubing for exactly 1 minute, but because real tubing is round, less than 100 cfm, figure the air will enter at one end and leave 55 seconds later. Not much time to warm the air. Moving 400 CFM through a 12" diameter tube, it will only stay in the tubing about 13 seconds?

You asked how I got 11 CFM. That is about how much air will sit in 100 feet of 4" square tubing. Round is smaller, so figure about 10 CFM in a 4" diameter 100' long tube (there are charts that have already calculated how many gallons of water fit into a given pipe, probably really easy to look up the exact figures).. If you try moving 100 CFM through 4" diameter pipes, and expect that air to spend 1 minute in each pipe to warm it, (blow it through fast enough and no heat will transfer) then you might need 10 parallel pipes to achieve enough surface area, and slow down the air speed to a reasonable speed (and pressure drop). 400 CFM = a lot of pipes, and several trenches.

I also pointed out that the surface area of 12" diameter pipe is about 3.2 square feet of surface per liner foot of length. 4" diameter is about 1 square foot per liner foot. So the 10 lengths of 4" pipe will have much more surface area than a length of 12" diameter pipe. But also consider the material costs. The Colorado home owner stated "The extra $6,500 for the earth tube will not save $6,500 in a reasonable length of time, but it was used anyway to keep the house green as possible". So my guess is that I can install a earth tube for about $6,500. No I will not be buying the 12" PVC from Home Depot, their cost seems a bit high, and if it is pipe rated at 200 PSI water pressure, it is probably a lot thicker than I would need for air flow into my home.

If you have 6 runs of 4" pipe, with 10 CFM in each 100' length of tubing, you will have a total of 60 CFM in all of that pipe (600' X 4" diameter). You talked about moving 400 CFM, and that would mean the 400 CFM will spend about 400/60 1/7 of a minute, or about 9 seconds? I don't think it will warm 15F outside air to 45F in that time. But you can try. . . To have the air reach 400 cubic feet of air inside the tubing, then it will take about 4,000 feet of 4" diameter tubing, so it can spend 1 minute in the tubing and warm from the outside temp to withing 10F of the ground temp. This would be about 40 tubes that can be manifolded together at the end near the ERV, and can come from different directions to prevent needing to manifold the air at both ends.

Personally, I do not have time to bury more than about 400 feet of tubing.

Anonymous _2836's picture

I just started researching earth tube heating/cooling, and am primarily interested in cooling. I've been living off grid in the Sonoran Desert near Yucca AZ at an elevation of 2400 ft. Cooling has been achieved with swampers (evaporative coolers). Summer daytime temps are around 100 to 105, nights 80 - to 85. Soil composition is sand, trenching a hundred ft to 10 ft depth is not a problem. I'm considering an 8 or 10 inch pipe. Would using 2 - 45s instead of a 90 improve air flow by making a less abrupt turn? Air flow would be from a 12volt radiator cooling fan powered by a small solar system. The tower bringing the pipe up the side of the house would be 8 ft tall and I would insulate the pipe in the tower with blown in insulation. Is this a feasible approach to cooling in those conditions? Thanks.

steve pailet's picture

living out in Yucca you might also consider a roof top pond with a retractable cover.. because of radiational cooling you would be surprised at just how cool you can go..

Reality.. 4 in tubing / piping is a much better way to go if you have a back hoe that can go 8 foot deep without caving in the sides.

yes two 45s are much better as the real flow will be far superior to even a long turn 90 Remember the bigger the piping the higher the cost and it goes up geometrically. The cost of the fittings even faster.

Fred Golden's picture

Rather than "reinvent the wheel" you might try something that Huel Howser tried a few years ago on his program "California's Gold" while out in the Mohave desert. It was a metal shack (yea right) with water running down the sides into a sump and recirculating. The Railroad companies set these up to give the engineers a comfortable place to sleep at night. And it worked. Water evaporating from the metal sheds cooled the shack within.

The metal conducts heat away from the shack quickly. The water stays cool because it is cooling all the time it is being pumped.

As for the earth tube, I would suggest a larger diameter tube. Figure how many CFM you will need, and then make sure that the tubing will have at least the same cubic feet of air in it that you will withdraw each minute. Your ground temperature is probably about 80F? Lets say it is between 72 and 80F at a depth of 10' underground. This air will come to the surface at say 75 (low end if the air is staying in there more than 60 seconds) and 85F on a 105F day. By then passing this air through a evaporative cooler, you can then cool this 85F air to a comfortable 72F on a dry day.

It might be less expensive to run a evap cooler and hook up a drinking fountain to it. This will cool the water going through it to about 45F with a very small refrigeration compressor. I don't know how cold the air will come out, but my guess is the 45F water will cool it off a LOT, especially if you have a belt drive and slow down the air flow a lot. If the evap cooler comes with a 4" diameter motor pulley, change it to about 2" diameter to slow the air flow. This will allow it to cool the air much better, even though you will not have as much air, it will be much cooler.

Jason Geisler's picture

I have read that a lot of people are factoring in trenching costs for the earth tube, which is understandable - however I live in am area where full depth basements are required and I was wondering if routing the earth tubes around the foundation (which is below the frost line) would be an acceptable solution?

James Riggins's picture

Jason, this is an acceptable solution and one that is used for construction on limited size lots. Just be aware that tube elbows create additional resistance to airflow. A single 90 degree elbow is roughly the equivalent of ten feet of straight pipe. So try to stick with large diameter (8" or more) pipe to minimize the increase in static pressure created by the multiple bends. Make sure the air handler is sized to give you the desired airflow at about 0.5 to 0.7 inches of water column (IWC) static pressure.
Also take care when using this technique to maintain a constant slope that results in only one low point for condensation drainage.
Finally, you don't want the tubes in contact with the foundation wall. You are looking to extract heat from the earth so keep about 2' of soil between foundation and earth tube, and consider insulating the foundation wall, on the tube side, with 1" to 2" extruded polystyrene foam.

zap101's picture

Hi is the tube open to exterior or a closed system venting into an out of the home? I had not considered it to heat the home but to cool it. Do you use it to cool the home? How has it worked out?

James Riggins's picture

Zap101- The earth tube is open to the exterior because it serves as the fresh air source for my energy recovery ventilation (ERV) system. The inlet is on a 6' tower to keep it above snow drifts.
Yes, the earth tube can be used to provide some degree of cooling, but the airflow I use for ventilation is far too low the the flow that would be needed to use the tube as a primary cooling source. We primarily use passive cooling, but on 2 days of record (100+) degree temperatures, and thick wild fire smoke, we did seal the house, and use the tube and ERV to keep inside temps at 76 degrees.
Also realize that if you are in a high humidity area, using an earth tube for cooling will not extract latent heat (humidity) from the house. This is not a problem in my dry mountain climate.
Jim Riggins

zap101's picture

Jim thanks for the additional information.

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