Hairmetal4ever, some of the information that has been the most useful to me in designing the SHCS for my relatively small planned greenhouse has come from people in the UK where the climate is definitely different from where I live in southern Arizona.

My husband and I are just finishing digging the subterranean section and hope to finish that and start on the greenhouse structure this spring. Therefore, my responses to your questions are not based on personal knowledge. Much of what I understand has come from the following sites:

http://www.permaculture.co.uk/articles/how-build-solar-greenhouse

http://www.sunnyjohn.com/indexpages/shcs_faq.htm

http://forums.gardenweb.com/discussions/1607983/subterranean-heating-cooling-system

http://forums.gardenweb.com/discussions/1613384/subterranean-heating-cooling-system-continued?n=29

1. Hex2006 explained to me on one of the threads on this site how running the system at night using ouside air in the summer could cool down the underground storage space to increase the cooling capacity during the summer. This is essentially adding a means to take cool night air in underground and also exhausting it outside to drop some of the accumulated stored heat. During the day the air would come from inside the greenhouse and also exhaust into the greenhouse.

2. Definitely backup systems are a good idea.

3. One of the responses I read related to this was where the SHCS system was run in conjunction to the supplemental heat. There was a comment to the effect that it was cheaper to heat from a few degrees below freezing than from several degrees below freezing. Also, using the system to cool during sunny winter days helps recharge the system for the cold winter nights.

4. My understanding is that the temperature differential is affected by the number of greenhouse air changes per hour. This is controlled by fan speed and tube capacity.

5. The size of the system is related to the volume of the greenhouse. I will have to look up the ratio. Usually the system is under the greenhouse but my system will include some space outside the greenhouse.

Thanks for the reply.

My comments/questions to your replies:

1. That’s a good idea, but our summer nights aren’t always that “cool”. Some nights we stay in the low to mid 70s with muggy humidity. That said if I do “close up” the GH in late May, most nights will be cool enough for this idea to work.
2. Yes, of course.
3. I would do this. I wonder HOW warm you can keep it…50 degrees? 45 degrees?
4. This makes sense, but I think the air needs to “sit” in the tubes long enough for temperature change to occur, so blowing through TOO fast might not have the same effect.
5. My idea was, to add cooling capacity, to have a second set of tubes outside, and a relay switch. As weather warms in spring and the tubes under the GH get too warm for effective cooling, you can switch to the outside tubes, which should have stabilized at the “default” subsoil temperature of 50\-something degrees. By this time, heating probably is less of a need anyway.

I also wonder if there’s a way to monitor the temperature deep inside the tubes, as a sort of way to know just how warm or cool potentially you can get.

3. I have no clue now but hope to know better this time next year.

4. Hex2006 (or maybe Steve333) posted results with his variable speed fan. At the slower speeds the temperature difference between intake and exhaust stayed the same until the entire tube length was used for the water phase change or condensation while the overall greenhouse temperature kept dropping. In the first post someobe was quoting an earlier post. I found another post where Hex2006 was using 20 air changes per hour and maxed the fan at 45 air changes without reaching the point where the greenhouse temperature stopped dropping. For those of us with smaller greenhouses, this is good news. (Edited after rereading earlier posts.)

5. I am putting in both an inside and outside system. During the winter I will have a hoophouse around my greenhouse. I plan on trying various setti gs to see what works best for me.

There is a way to monitor the temps deep in the tubes but I don't remember who did it or how. I think it was on the SunnyJohn site.

Hi

I`d recommend designing for a minimum of 20 airchanges an hour with small/medium greenhouses. If the air idrops to near soil temperature in the first 2ft of tube and your tube is 20ft long, the air will simply coast through the remaining 18ft without any further cooling. Increasing the airchanges per hour brings more tube length into play. Putting all the greenhouse air underground for a short period of time is vastly more efficient than putting some of the greenhouse air undergound for a long period of time.

One suggestion. As I'm sure you know, the temps in the northern part of Maryland are nothing like the temps in the most Southern parts of Maryland, and of course trees, etc to be considered. I would suggest when looking for information, that you make sure that those you talk to, know just what area in Maryland you live in. I know it's a simple statement, but it does make a huge difference. My best friend lives in the most north western part of MD and I live in the most South Eastern part, and have really different temps to deal with, and I find we often have to make sure the information is correct for what your zone is. Her being in a high altitude, and wooded area, is very different than my low beach area, and that makes even ground temps. very different too. (Just in case others need to know this too.) Good luck, Sheila

Just to clarify point 4
"At the slower speeds the temperature difference between intake and
exhaust stayed the same until the entire tube length was used for the
water phase change or condensation while the overall greenhouse
temperature kept dropping"

Upto a certain number of airchanges per hour, the air from the tubes will be at the same temperature as the soilmass.
If you are pulling heat from the soilmass to temper the greenhouse air, (heat mode) this is the ideal thing.to see as it shows you are taking the maximum amount possible. .
If you are trying to cool the greenhouse air, then its not a good thing to see. To get maximum cooling efficiency, you should increase the flow rate until the air leaving the tubes is a few degrees lower than the air entering the tubes (typically air taken from the apex of the greenhouse) In practice its very difficult to achieve even when you have a lot of fan power available. Its virtually impossible early in the season when the soilmass is still relatively cold after the winter,