For those comparing to the guy with the pit greenhouse in Alliance, NE (panhandle of western NE)… he is roughly in the red area with 5.5.-6.0 kWh/m^2/Day with fairly high solar potential. Very little rainfall and winter clouds sometimes blown away by winds off the front range (you can see the effect of that along the front range).
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Find some land near a nuclear power plant…ask nicely for their waste heat. A nuke plant is about 33% efficient so dumps the rest as heat. SO a 1000MW plant has to dump about 2000MW (7 billion btu/hr) of heat. That should keep a football sized greenhouse (or 10) toasty.
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A lot more than 10
But that is any power plant. Not just nukes.
PM’d you…
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I’ve been researching recessed/walipini greenhouses, in combination with geothermal heating, for some time now (and by researching I mean watching a lot of youtube videos mostly) in hopes to do this in a few years when my very small kids are older and more independent.
I don’t know how much youtube watching you’ve done, but in addition to the ‘citrus in the snow’ example, you might want to look at LDSprepper’s videos on youtube. He claims to make his system work in southeast Idaho, which is probably bordline 5b/6a. He does it with an above ground high tunnel, and buried tubing. I think a key part of his system is that he puts up styrofoam insulation on the north half of the building in the winter. VergePermaculture also has some videos, and he is apparently an actual engineer(hvac?) and has run some numbers. Zone 4 in Canada, I think. They’re both trying to sell you their expertise (especially Verge) so keep that in mind.
In general you have to be careful with youtube videos, because a lot of people seem to go into it with no real knowledge, just copying things they see. But tons of people have indeed attempted these things. Sometimes you have to do a bit of searching, and a lot of them have videos of the building of their greenhouse, but not really any follow-ups. Which makes one wonder.
@BG1977 it sounds to me like you shouldn’t have to worry about water from the water table. Grading will indeed be important, most likely, to keep surface runoff from infiltrating. Depends on your site.
There are many greenhouse calculators out there (top google result ACF Greenhouse Surface Area Calculator) however I think they all deal with ‘conventional’ greenhouses - meaning totally above ground. Still better than nothing. With a greenhouse, it’s all about the surface area that’s exposed. So basically you can calculate the heat requirements for a much shorter than normal greenhouse - just use very short sidwalls in the calculator. At a 4’ below ground depth you may be reducing your exposed area by as much as 40%, depending on the exact design. That’s huge. Beyond that, you’ll kind of have to just guess at the heat gain you get from the exposed ground and earth walls.
The question of sun angle is directly related to use. If you plan on growing greens, then ya 4’ might be a bit deep. If you raise them up on tables that will negate a lot of the loss, but at 4’ depth the area right by the south wall probably will still shade the table top. If you are just over-wintering citrus, they’ll be raised above the floor somewhat anway just by virtue of pot and trunk. You could put them on boxes to get the canopy into the light, or maybe they can just idle for the winter? The point is, it’s not all about how much of the floor is in shade, unless you’d be growing greens in the ground. It’s also about the height of the leaves. Which can be modified by tables and platforms, if you’re not in-ground.
Is it worth it? From pure economics I’d guess no. But a lot of what hobbyist fruit growers do is not, in a purely dollars and cents perspective. A lot depends on how much it costs you to build the greenhouse, and dig the hole.
@PaulinKansas6b Be careful of your tubing design. I watch a lot of youtube videos where people try to feed multiple laterals into too-small trunks. The cross-sections need to add up. Also, my guess is that you’ll get no meaningful cooling from those tubes. A wet wall would seem to be a much better bet, if you have the circulation path. That’s one disadvantage of extremely sunken greenhouses potentially - no good wall area for a wet wall.
My own personal plan is to do a water-based design. As disc4tw alludes to, liquid circulation systems should be much more efficient. Unfortunately I’ve found only 1 guy on youtube that attempted such, and it seemed a half-hearted attempt, with some serious flaws. But water can hold probably 4-5 THOUSAND times as much heat per unit of volume, vs air, depending on the humidity of the air. Water holds more heat than any other safe and commonly available fluid, in fact, and by a large margin. According to the Verge Permaculture guy, most of the heat transfer when using “air” systems actually occurs due to the water in the air, and the condensation/evaporation phase changes.
I suspect that one doesn’t need fancy heat pumps to make use of a liquid system in a greenhouse setting, and it may even be cheaper than an earth tube system - almost certainly so if one has to pay for the digging. Sadly though there seems to be little ‘research’ on a liquid system.
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Or maybe a hot spring, active volcano. When you think of it the whole heating/electrical system in Iceland is based on that. They grow bananas there (or they have).
I do know a local guy who uses one loop geothermal system (because our water table is very shallow so he just exchanges with that) and heats/cools his house. Have no idea what his bills run but he’s had it for a long time. Our well water sticks around the low 50Fs pretty much year round.
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I’d sign up for that forum! I’d be happy to test any semi-apocryphal greenhouse advice in my slightly leaky 300ish square ft greenhouse. Since I’m logging temperatures (including delta vs outside) by the minute, it should be possible to see whether any particular practices impact relative temperatures. I probably should add a sensor to measure how much sunlight there is (very little this time of year), but haven’t gotten around to that yet.
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Another idea: since we can occasionally get walloped with snow. Is there a formula for calculating what inside temp would result in the outside surface of the glazing being warm enough to melt snow on the roof?
I am interested!!! 
Hey if you run across good ideas on making this work in a simple way, please share!
Maybe perhaps a simple water pump to circulate the water and run it through a few car radiators with fans? Car radiators on ebay are only $50.
Hmm…
Something I’ve always wondered about with a greenhouse.
Generally, your heater runs mostly at night, and during the day you often have to ventilate even if it’s cold outside.
But what about cloudy days? If it’s cloudy and 35, will there be any appreciable solar gain? If you sent your heater at 45°, would it be beneficial to have a two-stage thermostat in this case, so that you can push it a little warmer during the day if solar gain is insufficient to heat it up? Maybe have the daytime temp set at 60?
Using my citrus example, if you have a stretch of cloudy weather in the greenhouse, and it’s basically stuck at 45° inside, day and night for a week or two, it may not kill citrus, but it’s probably not good for it, either, especially if there’s ripening fruit.
It depends on how thick the cloud cover is and of course other factors like sun angle. We only have a few days a yr here where solar gain in my greenhouse is near zero. Even with a moderate overcast it will usually heat up 20-30F.
I grew potted and in ground citrus in mine for several yrs. They all did wonderfully even with nights at 37F and days as cool as possible, usually less than 60F, for 45-60 days in order to chill stone fruit. The citrus didn’t drop leaves and still matured fruit. They bloomed soon after it started warming up in Jan/Feb. So even with potted citrus and media temps in the 40F range citrus did great unlike when bought inside a house for winter.
For citrus in your area 45 at night should be fine maybe even 37 like I ran. Heating too much by day might be worse than 45-60. If the roots are not very active due to cold media, then warm and dry by day might be the reason indoor citrus often drops it’s leaves.
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100%. Cloud thickness + low sun angle (elevation of only 21° at solar noon today) mean that here in Seattle this time of year the greenhouse may be only 2-3 degrees over ambient at solar noon on many days, and sometimes the heaters run day and night.
Yesterday it was mostly cloudy, but thinner than usual clouds, and there were two brief sunny periods. Even for the one that happened near solar noon the greenhouse peaked at under 9°F over ambient:
On a fully sunny day around the winter solstice here (sun angle of 19° at solar noon) it probably would struggle to top +20°F. Sun angle matters a lot!
My greenhouse hits 90F about 90% of days even in winter. I’m at 30 North so I think the sun is still 40-45 above horizon in winter. In summer that would be near 80 degrees. It seems nearly straight overhead. It’s wonderful. My sunroom provides most of the heat for my home and the greenhouse is nearly always warm and inviting. Can’t believe I might move north.
I had to close off the sunroom just after solar noon today. The house hit 90F just from sunroom heat by solar noon. I don’t know why more people here don’t use solar heat. It saves me hundreds in heating every winter.
If I do move to Denver I might try getting a place with a walkout basement that faces south. Then build a lean to greenhouse on the south side. Excess heat from Oct thru May could help heat the house. And I’d have a warm place to tend plants most days during that period. Or at least I think I would. THey may not get as much sunshine hours in winter as we do but it’s by no means a PNW situation.
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A friend years ago built a wood framed greenhouse with polycarb panels. Wasn’t huge, about the size of a large shed…12x16 maybe. It was heated with nat gas (he works in hvac). This is near Milwaukee/near L Michigan…so warmer then here in winter but still cold. One thing he said was that he saw little to no growth starting late fall until late Jan…by early Feb things started pushing…he had citrus and some tropicals…it was mainly to start cannas from seed in winter to get them sized up for spring (to sell i think)> I haven’t talked to him in a long time but i’m pretty sure he gave up because i know it was costing him a lot to heat. Moral of story…really need a sunnier climate with longer days unless you have money to burn.
I’m intermediate between all your locales: cloudier and colder than Texas, warmer and a tad sunnier than Seattle or Milwaukee. Sun angle on the solstice at noon is 28°.
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Here in Howard County Maryland I’m on a hilltop with exceptionally good sun exposure. I see a lot of solar gain pretty much every single day. It’s a whole lot less when it’s cloudy of course but its extremely rare for my greenhouse to not be in the 80’s at least during the early afternoon even in January and February. The only time my heaters run during the day is when it’s crazy windy and no sun at all plus temps below freezing all day long- which is pretty rare. This has been maybe the most surprising thing of all to me about having my own greenhouse. Well, that and how fast it cools down at night!
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Thank you. That’s interesting about how yours warms up as well. And amen on the quick cool off at night. So many people say that their greenhouse stays 10-20F warmer at night than outside. That with little heat storage or anything else to keep it warm. Even after being 90F for 8 hrs mine quickly cools off to outdoor temperature after the sun goes down. It will seem to hold +3-6F sometimes if there is a very heavy overcast at night. Those periods are very rare here.
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Can’t Believe you would leave that setup. It sounds awesome!
So basically, in a snowstorm is about the only time it doesn’t gain much, or maybe those windy polar vortex type days?
What do you grow in yours?