All of my greenhouses are hand built with 4x4s, 2x6s, etc. and are much larger than this one. They range from 28x48x10 to 40x60x12.
This is my "experimental blue house" . My original idea is that blue cycads look green here in my part of central Florida because the rain washes off the wax that makes these plants look blue. All I did was bury 4 fence posts 12 feet apart and tack some plastic on the corners.


Soon after I put this together we were about to have the coldest winter I have seen since 1983. I had to turn this into a greenhouse, which was easy enough, but then I had to figure how I was going to heat it. We were about to see 9 nights that would go into the teens F and my lowest low would go down to 15F, which is not expected here in Florida. I have small propane heaters for the larger greenhouses, but those cost about $24 a night, per greenhouse (4) to run. So I have set it up that small water emitters drop well water in the greenhouses, and that heats all of them at the same time. Since the water itself is free, it costs 50 cents each night to heat all the greenhouses. However, this one is not close to the irrigated area. I thought about running a hose under the wall of the greenhouse, but the hose froze up when I tried that.
I thought about using a black barrel filled with water to solar heat the greenhouse, but even that can go wrong. Many times around here, it will be cloudy and rain for up to 3 days before the freeze events blow in. That makes any solar heating devise useless. So, how could I heat this greenhouse in a way that never needed power, a machine, or anything that could go wrong in the middle of the night and ruin a bunch of multi $1000 plants.
Around 1984 I thought I wanted to be a contractor so I bought all the books I needed to study and get my license. In one of the books it showed how to build a Trombe wall on a house, which is a window on the south side of your house and more or less, you can heat your entire home with the electricity of one 4 inch fan. This south wall is filled with materials like brick, rock, or in this case, water bags, which are heated by the sun hitting the window, but the key was table that showed a btu rating for each material. I cubic foot of water, in the right situation would produce a constant 62.4 BTUs.
So, the $10 geothermal greenhouse heating devise that can never fail.



I buried a 55 gallon drum mostly in the ground and covered it with a half drum that I had sitting around that normally would be a planting container. There are two small holes on each side in the lower barrel right above ground level. I also cut a 3 inch hole in the top. I filled the bottom barrel with about 40 gallons of water. The cold air in the greenhouse sinks to the bottom and goes into the small holes, the air is heated by the water, which is constantly heated by the ground. The hot air rises and goes out the top hole, and that creates a vacuum, which pulls in more cold air into the bottom holes. During a freeze of 15F, and for the last four years, for that matter, no plants even had tip damage. I went into the greenhouse when it was 20 outside and took some readings, the water in the barrel was 63 degrees and the air in the barrel coming out was 58 degrees, which apparently, is enough to heat this 12x12x6 mini greenhouse. This was just a prototype, but there are all kinds of ways to use this same kind of thing and heat huge greenhouses as well. Tom

Interesting!!

...and useful.

TOM! Loving your posts about your greenhouses!

Thanks for this information.

Very interesting! I may have to just try this!

Is the bottom barrel open at the top or closed with just the small holes above ground level?

abhege said:Is the bottom barrel open at the top or closed with just the small holes above ground level?

There is free air flow where the two barrels are connected, so the top of the bottom barrel had no top and I just placed the half barrel that I had used at one time as a tree container, on top of it.

I'll let you know if we get to do this. I really would like to try it out.

Since my GH is not in the ground but rather sits on top of the deck, I am wondering if I could use giant thermal container that usually holds drinks, the kind with a spout...to hold hot water for the air exchange? I could rig a top on it?? What do you think @cycadjungle??

The only problem I can think of would be how long the water would stay warm. You would probably have to check the temps during the night the first time just to see if you needed to add more warm water or not.
Also, how about that deck? Is it wood and leak coldness into the bottom of the greenhouse? Tom

Burying the barrel is clearly the trick here, as it helps conserve heat and also draws on the much higher temperature of the soil to keep the water from cooling too fast. I'm not clear, though, on why the need for the half barrel cover unless it is just to add a little extra insulation to the setup.

I wonder how much better this would work if you had an aquarium water heater in the barrel all night long - I'd think it wouldn't cost much and would yield more heat.

Yes, the cold snap caught me by surprise and I only got 1/2 of the deck floor covered before getting the plants in on time. Next year, the floor will be taken care of well before the plants go in, plastic underlay of some sort and I/O carpet. I am going to try the thermos barrel for water. I have an aquarium heater that will keep the water warm. Worth a try to keep heat and humidity inside on cold days.

LariAnn said:Burying the barrel is clearly the trick here, as it helps conserve heat and also draws on the much higher temperature of the soil to keep the water from cooling too fast. I'm not clear, though, on why the need for the half barrel cover unless it is just to add a little extra insulation to the setup.

I wonder how much better this would work if you had an aquarium water heater in the barrel all night long - I'd think it wouldn't cost much and would yield more heat.

I used the half barrel to increase the amount of air within the barrel system. With this, there is the equivalent of 45 gallons of heated air plus the 40 gallons of "warm" water. The heater would add to the higher temperature of the water, but I wanted to make something that works totally without any power or machines of any kind. For that matter, I could use a solar panel to remotely produce electricity that could be used to heat the greenhouse.

I got a barrel today so tomorrow I start digging!!! I'm excited to see if this can help heat my 16x12 greenhouse.

ABBEGE I will be interested to see how it works for you.

abhege said: I got a barrel today so tomorrow I start digging!!! I'm excited to see if this can help heat my 16x12 greenhouse.

So, how air tight is your greenhouse, and what material is it made of? The better it is on these two factors, the better the barrel will work. Having the extra air holding volume in the barrel system makes a huge difference. If you only have one barrel to use holding air and water in the same drum will reduce the warm air volume which may lower the over all temperature. If i were to do this over, I might stack two barrels together and seal up where they join and bury those, so I could put more water in the barrel and have more below ground air space which would probably double it's effectiveness. Tom

Yeah, I haven't started digging yet...holidays interfered with that. Polycarb panels and we only just caulked all the seams/edges. I didn't have a clue about this until I read the forums. Wish I had known when we were putting ours up, but still, better late than never. We have straw bales on the outside perimeter and we use the water cooler bottles painted black and filled with water to help heat it as well. Every little bit helps.

Over a year later and I am finally getting around to doing this!

I can relate to that, Arlene -- I've been working on a plan for what I can cut back on next spring so that I can get to a couple of projects that have been put off for 2 years!