Radiant heat in greenhouse soil

Hello all! Wondering if anyone has done this or feels they can speak on this approach we are considering. Our plan is to house a wood boiler that can heat water that is pumped through Pex tubing and back through a closed system. We are likely to lay the tubing down 12 inches and space it 18 inches apart. Tubing will likely be 5/8" to 3/4" diameter.

My question isn't so much about our ability to run the system, but rather about its effectiveness as it is running. Will the heat that is produced reach the root systems of things like tomatoes? We do plan on laying some sort of insulation beneath the tubing to direct the heat where we want it. I would much rather lay it deep like this so that we don't have worry about hitting the lines as we work the soil.
Also, we will incorporate a 300 gallon tank for storing the heat produced in the water as we circulate.

We have already installed about fifty 55-gallon steel barrels on the inside perimeter of our 30'x72' high tunnel and filled them with water for insulation from outside soil temperatures, as well as to moderate temps inside the greenhouse.

Thanks for your thoughts!! -Jesse

Let me add a couple things. We are housing the boiler just "outside" the greenhouse. And the barrels we installed were dig 2 ft into the soil, and have black plastic housing them as well as a black roofing membrane on top of the to collect heat and we'll use that surface for our starts. Thanks again and enjoy your days!

Will the pipes be buried in the earth floor and are the plants also at floor level? I don't see why it wouldn't work; I am not an expert but have been reading up in radiant floor heating a little for the house I am building. But radiant floor heating works in houses so why not a greenhouse? But I wonder if it would help to add a layer of gravel or sand for the pipes to lay in so the heat will transfer to the more conductive material and then spread more evenly. You would effectively have a big radiating slab under the earth floor.

What made you decide on the bigger pipe size rather than 1/2"?

Also I'm curious about what boiler you are going to use.

PEX used in radiant floors can be used under a concrete slab, so I see no reason why this would not work. The density of the soil would be less than concrete (I hope). I think the moisture in the soil would also promote conduction as well as radiant heating.

Assuming you have good insulation (>R50) between the "earth" and the greenhouse floor.
Then it a very effective way to maintain the temp of the greenhouse.
Now plastic is a good insulator so I am not too sure how good it will at releasing the heat in the water, in a timely manner.
You are going to have to somehow make sure that the temp of the water in the piping never get over 200F.
If it does the water will turn to gas/steam and BOOM you have an explosion, that you will have to replace/repair.


So how do you get that good R50 insulation and keep the water below 200F

Insulation.
It will have to waterproof, hold alot of weight 1ft of soil + human walking + tables/etc, not compress and lose its insulative value.
You could build a concrete platform on pillars and insulate the "crawl space".
Then add soil on top of the platform. Then plant your tomatoes.

How do you keep the water under 200F
Make sure what ever heats your water never get to 200F.
You could use a solar water heater, solar concentration, set to a temp below 200F
You could also use a heat exchanger to separate the hot fuel from the 200F radiant heating pipe/water.
You could somehow limit the burn rate of your fuel.
Maybe a valve that reduces Fuel and or Oxygen the closer the water get to 150F and also auto shutoff once the water get to 150F.

I use pex under my beds but use solar panels to heat the water. This spring will be season 2 for this particular greenhouse. You can see the pex before the dirt was put on top app 18 inches deep.

Jesse Coker wrote:Hello all! Wondering if anyone has done this or feels they can speak on this approach we are considering. Our plan is to house a wood boiler that can heat water that is pumped through Pex tubing and back through a closed system. We are likely to lay the tubing down 12 inches and space it 18 inches apart. Tubing will likely be 5/8" to 3/4" diameter.

My question isn't so much about our ability to run the system, but rather about its effectiveness as it is running. Will the heat that is produced reach the root systems of things like tomatoes? We do plan on laying some sort of insulation beneath the tubing to direct the heat where we want it. I would much rather lay it deep like this so that we don't have worry about hitting the lines as we work the soil.
Also, we will incorporate a 300 gallon tank for storing the heat produced in the water as we circulate.

We have already installed about fifty 55-gallon steel barrels on the inside perimeter of our 30'x72' high tunnel and filled them with water for insulation from outside soil temperatures, as well as to moderate temps inside the greenhouse.

Thanks for your thoughts!! -Jesse

Heat always rises........... Heat will penetrate downwards some too. This includes geo-thermal heat. With your buried barrels. it does not sound like you have this option.
Dig down at least a foot around the perimeter of the GH. 9 feet away. lay down 2" styro SM and bury that too. Have your tubing run outside of the GH also. Geothermal will help warm the ground around the outside as well and you won't have cool GH perimeter. The frost will not penetrate the ground around the GH. This extra might not be huge in RI, but in the Grt White North it makes a big Diff....

While it is true that heat by rise by convection in water/air.
Heat is also transmitted by radiation (sunlight, electric heater, halogen lamp) and by conduction thru solid object (metal spoon, earth, etc).
When heat is transferred by convection or radiation it does not really follow the up/down rule.

Coog Kelly wrote:
Heat always rises........... Heat will penetrate downwards some too. This includes geo-thermal heat. With your buried barrels. it does not sound like you have this option.
Dig down at least a foot around the perimeter of the GH. 9 feet away. lay down 2" styro SM and bury that too. Have your tubing run outside of the GH also. Geothermal will help warm the ground around the outside as well and you won't have cool GH perimeter. The frost will not penetrate the ground around the GH. This extra might not be huge in RI, but in the Grt White North it makes a big Diff....

Did the pex heating system work in the long run? Would you recommend insulation under the pex?

In my case the pex experiment was a success. That particular green house collapsed under 3.5 ft of snow and we now have greenhouse 3.0. In this green house I am using a 4 inch duct fan that draws heat from the center of the greenhouse peak and t's off down the center of the greenhouse under again under about 18 inches of soil. I did have this similar set up of heated air in the greenhouse I had the pex in. I did not run pex in this new greenhouse. Greenhouse 3.0 was completed last fall and this will be the first season trial. I am running a trial with a Chinese diesel heaters in this new greenhouse ducting the outlets into the winter double covered beds.

Hello, I was exploring the same option for the same size greenhouse. I was thinking about putting a small plastic pool, use a water pump to push the water in the infloor pipes. Also use an electric water heater or just a hot lamp on a metallic portion of the infloor pipe to heat the water. It is a closed circle, and the gradual walking of the pool is a bonus to help increase the temperature. I would love to hear what you think!