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Tim Malacarne
Posts: 226
Location: South central Illinois, USA
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Question: If you were to supply bottom heat via hot water pipes to beds in a growhole or technically, a coldframe, how deep below the soil surface should the heat pipes be?

Also: How hot do you suppose you'd have to heat the water to also heat the soil in the growing bed? I am figuring that a soil temperature of around 60 degrees F for cool season crops such as spinach, lettuce, and radishes would be about right.

Given: beds will be insulated on the outsides and bottom.

Thank you for your input. I remain

Sincerely, Tim M
 
M Foti
Posts: 171
Location: western n.c.
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This may have been on this site... I saw a guy's video who was building pretty big greenhouses, he used taller than you would really want ceilings and heated the grow beds with air pumped from the top of the greenhouse through a french drain type system under the beds. His first design overheated them, but the second one got it just right. I think he had to bury his heating system a bit deeper. Really worked out well for him as his energy costs were alot lower than having to heat water, although I'd imagine you could use solar hot water heaters (even home made ones) to do the same type of thing.

Either way, he lived in a place with pretty cold winters and was able to keep everything heated without adding much if any additional energy other than his blowers. Really smart way to do it, I was very impressed. The trick they found was to rig the blowers onto a thermostat rather than a solar switch, with the solar only switch the fans would start blowing first thing in the morning and pump a bunch of cold air into the beds, after he switched to a thermostat controlled switch he was only putting heat in when there was heat to be put in...

Might be worth a bit of thought, wouldn't be too hard to build a solar hot water array, or just use high ceilings and hot air... Either one could be supplemented with propane, wood or electric energy for those times when it's needed.
 
M Foti
Posts: 171
Location: western n.c.
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I can't seem to find the original video I was referring to, but the key thing I took away from it was that he was able to do quite well with little or no additional energy (aside from a small blower, or in your case a pump) BUT... he had issues with overheating his beds. I would think in a cold frame like you mentioned this would be even more of an issue since it would possibly have less thermal mass to buffer the heat. For a cold frame (I'm imagining things that aren't huge) water would be the way to go rather than air, but you may want some sort of thermostat to turn the system off if it started to overheat the media. I'd also consider making them deeper than traditional cold frames so you could have more thermal mass... Seems like you're pretty swift so you've probably already thought of this, but it bears mentioning, if you have multiple cold frames and you hook them all up in line, then the first one will be getting warmer than the last one, although that could be mitigated by choosing what to grow in each bed or by figuring out some sort of manifold to distribute the water evenly instead of to one at a time.

 
Tim Malacarne
Posts: 226
Location: South central Illinois, USA
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Wow! That's a really slick idea, circulating hot air that way. I'd of never dreamed! To avoid overheating, I'd imagine he could install some sort of limit switch, don't you think?

We are about 40 miles east of St. Louis, and have had some single digit readings so far this winter, and one night at minus 8. I sowed the spinach and lettuce in October or November I think, it's all alive and doing well... I have a variety of "Winter Lettuce" a lady in Arkansas sent me. Dang good stuff for cold WX. Last spring, we set out a row of them that got buried twice in snow and ice, didn't hardy slow 'em down at all. Every one lived.

I am retired, and have a woodstove in my shop 110' from the coldframe. I intend to install a water tank right up next to the stove, bury some insulated piping, and rig a pump to a re-purposed exercise bike. Also will have a way to use a regular electric pump...

I have calculated that the beds will take a total of 135' of PVC Schedule 40, in 3" that gives around 50 gallons of thermal mass right under the growing beds. If 4" pipe is used, it would be 88 gallons of storage, but I think that'd be too much. I figure to try and make the heating tank 20 gallons. There's a fire in the shop stove every day, through the winter. I figure to hop on the excercise bike 5 or 6 times a day, pump the heated 20 gallons out there in 10 minutes or so.

What do you think?
 
Nick Kitchener
Posts: 477
Location: Thunder Bay, Ontario, Canada
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Good video from a greenhouse designer on the realities of winter growing:
 
Hey, sticks and stones baby. And maybe a wee mention of my stuff:
2017 Rocket Mass Heater Workshop Jamboree - 15 workshops in one event
https://permies.com/wiki/63312/permaculture-projects/Rocket-Mass-Heater-Workshop-Jamboree
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