heat transfer question about using water stove to heat cold frame

[Ericcc]

I have an outdoor wood heated water stove with a pump to circulate the hot water to my house regulated by a thermostat in the house. I'm wondering about using hot water from my water stove to also heat a cold frame (which is basically a box with glass on top, big enough to hold some flats of pepper plants). I'd like to have a cold frame about 3'x12' (very roughly). If I want to keep a box that's insulated on the bottom and sides but with old storm windows simply laid across the top and an insulated blanket over the glass top every night at around 80-85 degrees through the night and on rainy days starting in March when nighttime lows can drop as low as 20-25 degrees with daytime highs as low as 40 but averaging more like 40 at night and 60 during the day and then continue using this cold frame until mid-May, can anyone recommend a relatively cheap and easy way or give me any advice on what kind of tubing to use to radiate heat below my pepper flats, how much of it to use, and ways to regulate the temperature in the cold frame? I have 3/4" pex running to my house that I would presumably T off of, and the water temperature can vary between 120 and pretty close to boiling.

 

[salecker]

Find a CI Rad put a TRV valve on it and set for the temp you want.No power required,simple set and forget.
I have them throughout our house,temp never varies more than a few degrees.

 

[Ericcc]

I just read up on TRV valves and they seem like they might be a perfect solution. Thank you! I assume I could use a TRV with simply some kind of tubing run back and forth under my plants (for bottom heat, which I think would be better for the plants) instead of using a radiator? Could I get fittings to make those transitions simply enough? (I'm really ignorant when it comes to plumbing things. It took me over a half hour to figure out what a CI Rad is.) Would copper tubing be the way to go? Or would some kind of plastic work just as well and be a lot cheaper (something that could take the heat and also make pretty tight bends back and forth every so many inches)?

 

[maple1]

I'm not fully grasping what you're thinking, but check out Pex. It is used for under floor heat, between joist spaces.

 

[Ericcc]

Yes, 3/4" pex is what I used when I ran the lines from my outdoor water stove to my house. My concerns with pex, at least the 3/4" pex that I'm familiar with, is that it's stiff enough that I don't think I could bend it enough to run it back and forth under where the plants will go on a 3'x12' table. I assume in order to heat the table well and evenly I'm going to need to have lines every 12" max, probably more like 4-6" apart, and I doubt if I could get 3/4" pex to bend tight enough to run lines even 2' apart. I also really don't know how well pex would transfer heat. It seems very possible that pex would transfer heat well enough, especially if the lines running the length of the table every 4-6", but I just don't know.

Another question: do you all think think if I built my cold frame 3' higher in elevation than the water level in my water stove and not too far away that there would be enough natural circulation from the hot water wanting to rise that I wouldn't need to use or depend on a pump?

 

[maple1]

I don't know what your 'outdoor water stove' is. But 3/4" is pretty small to move a whole lot of heat around.

Google infloor or underfloor heating & pex. I think it usually uses 1/2", it bends enough to get at least a couple of runs in a 16" joist space. There is also 3/8" although less common so might be harder to find it & fittings local.

Unsure on your other question. Much horizontalness will usually take the zing out of convection.

 

[E Yoder]

I doubt you could rely on convection flow.

I did a cold frame with heated media once.

I did 1/2” pex buried under the growing media.
A mixing valve to turn the water temp down, I'm not sure if full temp water would get the growing media too hot? I turned it down to like 120 degree water if I remember correctly.
I used a Ranco temp control to turn the pump on and off. It has a probe you can hang in the air or insert in the soil. A TRV would be simpler tho. I'm not sure if it would heat evenly tho as it throttles down in warmer weather. Just the beginning of the loop would get full temp water.

 

[Bad LP]

Years ago we built a grow room for flowers starting from seed. It was indoors and before PEX was available around here around 24 years ago. We used 3/8 copper tubing, a Taco 007 and a 5 gallon electric water heater.

 

[velvetfoot]

This might be a specialty item for growers. Maybe you could find a place that specialized in that stuff.
More than a few years ago my wife was looking at plants to buy in a greenhouse, and I noticed that the grower's wife had an outside boiler and there were pipes in the dirt. So, it's probably a "thing".
That's all I got.