Wood heat for a greenhouse

I am running through schemes to heat a 24x60' hoop house with wood. Currently it is heated with an LPG unit heater and consumes around 2000 gallons annually. The current useage is only March through the end of the heating season, and our farm manager would like to expand that to year round use. The easiest scheme I can see is to use the existing LPG tank (1000 gallons) as a heat source by moving it inside the greenhouse and heating the water with a wood boiler. How can I calculate the heat output of a bare tank at various temperatures? We could partially bury the tank in the ground if the heat output is too great. Obviously such a scheme would require maintenance fires to prevent the system from freezing up since that amount of glycol is not possible. Alternatively, we could buy a hydronic unit heater and run a glycol system. I see some advantages to the propane tank: first, it is free and a unit heater is not. Second, we could build frames over the tank and rotate seedlings through that hot zone.

May I assume you want the tank in there for storage?

If yes -- it will be too inconsistent of a heat source for good plant life and production

You will want to circulate a more consistent temperature liquid for a constant stabilize plant/soil heat source - thus keeping the soil warm and the air around the plants warm enough

A tank of say --160 degree stored energy will give off too much heat causing the plants to stretch and grow too quickly - and when it cools -- with the plants in a hi growth rate -- the crop will suddenly be stunted and susceptible to necrosis.

There is a thread floating around here somewhere that talks about putting radiant pex right in the benches for the plants. This will minimize the heat requirement for the greenhouse by keeping the heat right where you need it.

I am not a botanist, but this method seems to make sense.

cheers

Tuo,

I would use the Runtal site to attempt to get BTU ouput at temperature "X" info. They manufacture steel radiant panels and I bet you can easily extrapolate their data based on size and temperature, although their opanels also have some steel fins too. I have a number of Runtal panels and they work well. You will have temp stratification in the tank that you'll need to take into consideration.

Mike

OK, At this point I will be heating the greenhouse (80 kbtu/hr max heat loss) with a combination of a hydronic unit heater for the air and pex in the soil in some locations. The heat source will be wood, and potentially solar as well. I do not believe that pressurized storage is viable due to freeze potential for the system - glycol is not affordable in bulk! Is there any way around this? We could keep a fire going all winter if there was a good way to keep loops circulating (not 24/7) as needed to prevent freezing. That is more of a control issue. A 1000 gallon tank is available.

Open vessel: We have many older sap tanks available, or could do buried epdm etc. This would allow the boiler loop and solar loop to be glycol. Heat transfer is more expensive than the large expansion tank required in the pressurized option. Would we need separate coils for the solar and wood? The solar system could be an active pressurized setup. How cold can a glycol system get without damage?

Any comments or examples of real working systems under these conditions would be helpful.

Your crop will do far better if you give it a constant temperature to live in. That's hard to do if you don't circulate 24/7 with variable energy by degree day load.

That said you can reduce the energy necessary to heat the place at night, if you pull a thermal curtain over the crop when the sun goes down - and slowly expose the crop again in the morning - slowly--- to heat the air above and prevent the cold air from shocking or freezing the plants. This system works best if you use a gas air scorcher overhead to warm the air - or build a drain back system to protect the overhead hot water to air exchanger

If the greenhouse is seasonal -- build the wet system so you can drain it for off season protection - but also build some way so you can refill the radiation system with liquid hot enough that is will not freeze before the system fills and the water returns in the loops. I wouldn't use chemical protection either due to cost and efficiency loss, and the possible loss to the ground water. There is another issue - these days you don't just dump amendments on the bench and let them leach into the soil under the structure - you should provide a way to recover and dispose of that mess.

This project is only limited by your creativity and the crops you wish to raise.

You could use one of these in each greenhouse ans use a central boiler.....
http://www.cozyheat.net/unit.html

Mike

Use the 1000 gallon tank and really heavily insulate it and then run a plate heat exchanger for the beds. You would have the cost of 2 small circulators and the heat exchanger but the tank is free. It would take quite a while for a well insulated tank to get close to freezing after you heat it up to 190 degrees and with the lines for the air handler and beds, you could put in the glycol mix. For the expense of a little wood, you could skip the solar install for many, many years and still be money ahead. This is just one option - many to choose from and many choices to be made but sounds like a cool project.

Have you investigated the non-hydronic options? Check out rocket fired mass heaters.