Hot compost and water heated greenhouse.

[Tiny Medford Farmer]

Hello Everyone,
I am constructing a pvc greenhouse. It's a lean to desgin that is 10x10. It will be 7 feet high where it is attached to my house and 6 feet high on the other end. My ultimate goal is to produce warm weather veggies in the winter. My plan is to run poly tubing up and down all of the walls, across the ceiling, and under the sand floor. The tubing will be hooked to an 80gph fountain pump which costs about $12. The pump will be inside of a 5 gallon bucket of water. The tubing will leave the bucket and twist through a drum of hot compost which I believe gets to about 140 degrees. My thought is that a constant recirculation of this water will be enough to keep the green house around 80 degrees during the winter. The greenhouse will have two layers of 6 mil plastic with bubble wrap inbetween. I will also have a barrel of water inside to help hold the heat in. There will be a 10x1 potato bed that will be 2 feet deep. And an L shaped bed that will be a total of 19 feet long. It will be 16 inches deep and 2 feet wide. If I can actually get all of this to work, I would like to add in a temperature controller that turns the fountain pump on and off accordingly. Does anyone have a similar system? Or any thoughts on the subject?

[Alan in Vermont]

My thoughts would be that your pump won't have power enough to push water through all the tubing you are planning and that it will take one awful big "drum" of compost for a heat source. I think you need to do more research, starting with how many BTUs are required to heat 700 cf. of virtually uninsulated space. Then figure out how much you are going to get from the sun during the day and how to store the excess for nights/cloudy days. Compost may reach 140° but that is in the center of a pile. How fast does it produce heat? That would determine the BTU yield that you could use. What size tubing and what velocity for the fluid passing through it are optimum for heat collection?

How many of those things have you figured out already?

[applestar]

Alan has a good point. I think my 60gph fountain pump can only raise water to maximum of 2 feet high when the pre-filter sponge is pristine and not clogged by algae, etc. The specs should be on the package.

The design concept is cool though. I hope you get it to work.

[soil]

i would consider trying to hold heat in some sort of thermal mass to slowly release it over the night.

nut to go along with your question. look up jean pain and his method. its like what you want but better.

[freedhardwoods]

Unless you have at least a 40 or 50 ton pile of compost, I can pretty safely say that even if your pump could push the water (I don't think it will), all you will have is an elaborate compost cooler, not a greenhouse heater. My guess is you might raise the temp in your greenhouse by 5* above the outside temperature on a cloudy day. The sun shining in would make much more heat than your compost pile.

[Tiny Medford Farmer]

Thanks for the input everyone. Looks like i have some more research to do. My only experience with heat exchange is with brewing beer. So that's where I have been drawing my thoughts from.

[tedln]

Pumps are rated by maximum ft. of head which simply stated amounts to maximum discharge pressure. In order to properly select or size a pump, you need to know what the suction pressure will be and then calculate the pressure needed to achieve your purpose in gallons per minute or hour. You will need to know the pipe diameter to determine the restriction size and the total length in feet of the pipe to determine friction losses. You also will need to calculate the difference between the pump suction elevation and the maximum elevation on the pump discharge side. All of the calculations are available in most mechanical engineering handbooks (at the library or probably on the internet now). You should be able to google "fluid flow calculations".

In order to size the coils in your compost bed, you will need to determine the heat transfer rate of the material your coils are constructed of and the most efficient rate of flow of the water through the coil. You need to remember that while you are transferring heat from the compost pile, you are effectively cooling the compost pile which if not properly controlled can effect the composting process.

Just a few of the things that need to be determined!

Good Luck

Ted