Using a compost pile to heat water

Asked December 8, 2016, 11:28 PM EST

Our house has 4 300' x 1/2" dia pex radiant loops in the basement floor which have never been used. I'm attempting to build a compost water heater with sawdust, wood chips, chicken litter, and horse manure to heat our house and water while the pile is rotting. After the compost is done the pile will be used on our garden, and the pile will be re-built to continue heating our house and water. The corral panels with poultry netting will keep the south side of the pile our of the road, and may allow for better air circulation.

The questions are.... To keep from having to turn the compost pile could I add drip tape through the pile for moisture, and tubes for air, or could the drip tape do both air and water at different times? Or will water tubes need to be higher in the pile than the air tubes? I've always thought that heat rises, and water migrates down. However lately I've been corrected saying heat migrates evenly down as much as up, it's only hot air that rises???

Oregon County Missouri

1 Response

I have been working on this since you sent me the email yesterday.

I think the plan has a good possibility of working. The compost pile temperature may not be as high as you would like. There have been several researchers that have gotten fairly goo results by pumping air and water into the pile as needed. It never quite matches a good mixing of the pile, but it does work. You are right heat in a solid material moves from an area of high temperature to areas of lower temperature. A good compost pile has air moving in it, so there is some warming of that air and it will rise through the pile. That is how you loose moisture out of the pile. So, your Idea of putting water in as needed near the top and adding air near the bottom should be good.

Carbon to Nitrogen ratios of 25 to 1 or 30 to 1 seem to give us the best reaction and heat. Pile temperature may get to the neighborhood of 160 degrees. That does not mean your water temperature will be 160 degrees, nor the floor. The pile will always be warmer than the water in side the piping, and the water inside the piping will always be warmer than the floor. There will also be some heat loss in the piping from the pile to the home floor. The transport piping should be insulated to minimize that loss.

See our guides on composting.
Making and Using Compost
On-Farm Composting - NRAES-54
Field Guide to On-Farm Composting - NRAES-114

The last two will need to be ordered, the University of Missouri Extension - Oregon County office can do that for you.