I'm thinking about installing earth tubes for a small 10x10 hoop greenhouse as a test before building a permanent structure.
I was told by one company they only use solid pipe and go down at least 6'. They say because of the air flow and using the black corrugated HDPE tubing, there won't be any condensation so no need for a drain and going down into the ground water (4' water table) is ok. They also claim the HDPE will not grow mold. They claim they've personally done 13 installs and haven't had a condensation problem in any of them. Also, none of their numerous partners have ever reported an issue in their installs in 30 years. That sounds impossible but that's what they said.
The other company I talked to says I'll have condensation and mold with solid tubing and to only use perforated tubing and (obviously) don't go into the ground water.
So here are my questions
1. Has anyone had first hand experience or known someone who has had a condensation issue? And what about with mold in HDPE tubing?
2. Is it true if the tubing is in ground water saturated soil, there will be no warming of the soil when I blow excess hot air through the tubes?
3. If #2 is true, I assume I won't cool the ground temp of 55 degree's either by blowing cold air through the tubes for approx. 6 months of the year. Is that correct?
4. Do these numbers sound like they're in the ballpark:
5600 heating degree days, 10x10 hoophouse, 2 layers of 6 mil polyethylene, north wall attached to house, to keep the temp at 50 degrees I'll need approx 1 million btu's of gain/month avg during the heating season; actual heat gain should be about 1.5M btu's/mo. avg. during the heating season.
142 cu. yards of sand * 20 degrees = 1M btu's storage (ie would have to heat a 30x32x4 area 20 degrees (from 50 to 70) to store 1M btu's);
5. Is there a rule of thumb or a reasonably simple formula for calculating the total volume of tubing needed? The company I'm working with tell me its based on cu ft of the greenhouse and, they imply, heat loss.
6. The company I've been talking to does not provide any real scientific numbers, just rules of thumb. Is there a good book on the subject that gets into the science?
7. The installs I've heard of and seen don't insulate below the tubes. I assume that's because the temp differential of the warmed earth and the ground below it is probably 20 degrees at very most, so there won't be enough heat loss to justify the cost.
I know that's a lot of questions but I learned my lesson on my last solarproject that didn't work as advertised so I'm really doing my due diligence before plunking down the cash to do this. And if it doesn't work, its an expensive and time consuming fix. I really appreciate you taking the time to provide any insight.
Unfortunately I have yet to find any good sources for clear information on earth tubes and I have been looking for years. General rules of thumb seems to be about as good as it gets. From my understanding and in my work it is VERY important that you use perforated tubing. Anytime warm humid air comes in contact with a cold surface it will condense moisture, regardless of if it is corrugated or not. The primary function for the corrugation is to slow the air down as it passes through the tube enabling more temperature exchange.
I would be very cautious about going down into the water table, one small hole in the tubing could render the whole system useless as it fills with water. I have not heard before that HDPE will not grow mold but this is what I am always sure to use within my own projects. At first impression I would be VERY wary of this company, moving warm humid air through a cool environment without condensation seems to defy the laws of physics as I understand them. Not saying it's impossible but I would be wary for sure.
So put me in the camp of only using perforated, corrugated, HDPE tubing and don't go down into the water table. I have been installing earth tubes in greenhouses for clients for over 4 years now making sure to use this type of tubing and have yet to run into any issues with mold. I have heard disaster stories of people using solid tubing, and with the corrugations you would not have any way to clean this type of tubing, meaning there wouldn't even be a labor intensive fix if mold is an issue.
1. Yes I have seen mold issues, and it has always been with solid tubing. Solid tubing can be used so long as it pitches to daylight, but this is only possible in very specific situations.
2. If the tubing is in flowing ground water then this will carry all of the heat you are trying to store down hill.
3. I wouldn't expect any noticeable cooling of the ground temperature, although this setup could be used to help cool the air that you are blowing through the tubing.
4. This totally depends on the context of where you are. In many cold climates even if you can keep the greenhouse 50 degrees there isn't enough sun to grow anything. Often times it is better to run a short dormancy to work with the natural cycles for healthier plants and less pest troubles.
5. I base all of this on the cu ft of air volume that I need to move in the summer to cool the greenhouse, then design the system based on this calculation.
6. Rules of thumb is about as good as it gets, at least from what I've come across.
7. I have never insulated below the tubes, but I also build earth sheltered greenhouses that function depends on receiving the ground temperature to help balance out the extremes. 20 degrees sounds like a huge differential, I would expect more in the 5-10 degree range.
But what you said about "not enough sun to grow anything in the winter" seems to be fairly controversial. Do you say that based on experience?
I'm in a very cloudy climate (NW Indiana) and am not interested in using grow lights so that is the big question that needs to be answered now. I know people like Eliot Coleman grow some stuff in the winter, mainly greens, but that isn't that exciting and not worth putting a greenhouse and earth tubes system in for. The prospect of growing tomatoes is though. I've heard people claim that diffuse sun is fine for most plants (esp the diffuse greenhouse cover companies lol).
Also, since you've been installing these systems, do you have any tips on sizing the system....such as the total volume and length of the tubes? 6" or 4"? And also how many cfm of air you need to move?
Paul Ladendorf wrote:what you said about "not enough sun to grow anything in the winter" seems to be fairly controversial. Do you say that based on experience?
This depends on where you are and what you want to grow. Lots of perennial crops you might want to keep a greenhouse warm for require minimum daylight hours during the winter in order to fruit. This means that even if you can keep your greenhouse warm enough these types of crops won't produce fruit without supplemental light. Other crops may grow in the winter but will not be anything close to their full potential under such conditions.
Eliot Coleman has got some great stuff with greenhouses and I use his 4-season gardening techniques often and recommend them to clients. Just to be clear it is a 4-season harvest system, growing cold hearty crops with the last sun of fall and then storing the crops alive in the greenhouse and harvesting throughout the winter. I am eating a salad as we speak grown this way and the greens and more sweet and succulent than at any other time of year. Once you get a taste for this you will realize many types of greens and root crops are the highest quality during the winter.
I've seen tomatoes grown in cloudy northern climates but they have always involved grow lights and they don't taste very good. If you want great tasting tomatoes in the winter the best strategy would be to grown a surplus in the summer and can them. The white tasteless tomatoes you kind find in the store during the winter, this is the reality of winter sunlight ripen tomatoes, there just isn't enough sun for the fruit to develop it's full sweetness and flavor. If your main goal is to grow tomatoes in winter without supplemental light I would recommend putting your resources into a more fruitful project.
I use the 4" pipe so there is maximum temperature transfer, for larger diameter you need longer runs. The cfm and length of tubes really depends on your cooling load during the summer. You will most certainly need additional ventilation as well. Do you plan to use shade cloth? Lots of variables here. Best of luck with your project!!
On growing tomatoes, based on what I've been reading, since they're perennials, they can be overwintered if nothing else so you could have tomatoes super early in the season. I also plan to use the greenhouse to overwinter citrus.
Avg. cooling degree days is about 50 using a base of 80 degrees so cooling really isn't that much of a concern but If I need to shade, I will.
Do you typically backfill with gravel? Since its about $12/ton to have it delivered, since its got about twice the heat capacity as soil, its cheaper than digging a bigger pit and add more tubing to get the extra storage. Neither of the companies I talked to use gravel.
So I assume you use a general formula for sizing that includes heat gain and loss, and cooling load? Can you share anything on that?
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