I recommend against it. They require too much watching to be safe.

This is a talk I'm having to do. The greenhouse I already work at already has them. I just need some info about them being used to heat greenhouses.

in general, the possibility of gaseous emissions that are bad for your plants make them a poor choice....

Wood stoves would not be a good idea. Too much temperature variation. It would get really hot when first loaded with wood and may get cold by morning esp in zone 6. Isnt TN more like zone 8 ? We are zone 6 here in Canada. You would need to leave a large area around the stove without plants as you may cook them. A pellet stove would be better as you wouldnt have to worry about running out of fuel in the middle of the night, but still not a good idea.

I've used both wood and coal stoves to heat two (15'x40' and 20'x96')of the three structures I have for many years and with fewer notable problems using a wood stove. With the price of heating fuels still soaring wood is a cheap heat source for many of us. I would prefer the safety factor of wood over kerosene heaters and some older, leaky propane burners.

An airtight model would be essential to prevent exhaust smoke from leaking back into the greenhouse. I position a cheap box fan in a corner always blowing air over the stove to keep the plastic film above the stove from getting too hot. A few days supply of wood can be stacked next to the stove(within 1 ft.) than if you were in a living structure because of the damper environment. Running a flue pipe out through a double metal plate with a flue hole cut into it is simple and costs only a few $. The external pipe can be supported by stacked concrete blocks and a section of pipe for vertical support.

Downside is building and tending to the fire nightly, sometimes in the middle of the night. But you can usually get extra BTU out of a woodstove by drafting whereas other fueled furnaces are limited by their designed output range. Heat output is a little harder to control with wood but I never had too hot temperatures overnight. You want to avoid having your exhaust fan kick on when the house is tight and the stove burning or you could have smoke drawn from the stove into the greenhouse. Opening an adjacent door to let heat escape is preferred.

Just as in a heated living unit you should take safety precautions with a smoke detector and carbon monoxide detector if your plants are valuable to you. Although I've burned wetter wood that I would not chance in a house you need to clean your flue if creosote builds in it. that task with the flue setup I described takes about 5 minutes.

One unmentioned alternative is the external wood stove with an internal heat exchanger. The costs quadruple but the safety and heat regulation factors are better.

I can't think of a better heat choice than wood for the small commercial grower who wants to MAKE money in a type of business where heat is a defining season extending commodity.

A masonary stove or stove with water jacket and storage might work - a short, hot fire is more efficient, less smokey and produces less CO and creosote. You could combine this with solar technologies quite nicely. The trick is to have thermal storage enough for a day or two.

So I've changed my mind somewhat.

I use wood in combination with propane. If it starts to get too cold the propane comes on. I think it lowers the heating expense significnatly.
I have it installed just about the same way as bmoser.

If your heating system is water based, as in a storage tank of warm/hot water then indirect heating of your greenhouse is possible using solar, wood, oil, waste veggie oil, propane and electricity (or the decay of radioactive isotopes)

Use the fuel to heat the water that is most cost effective at that moment. However that takes energy (electricity) to move the water (heat) around.

Lots of water, and moving the water. That is the key.

Even better if you can extract all the useful work from the energy first. For example: running a diesel generator in your greenhouse, connected to the grid. You can pick up a self-starting diesel generator for under $500 now, with remote start. You can extract heat from the water jacket and from the exhaust, and CO2 from the exhaust for improved plant growth.

There are simple ways to directly connect the generator to the grid, but they require some understanding of synchronisation and AC grids, and would require someone knowledgable to wire them safely.

With current diesel prices and electricity prices, this is generally a reasonable way to make money.

I find that propane is a good backup and I'm using it now also when temps are hovering around freezing at night. In the spring when it is more important to keep inside temps above 50F it almost requires a 200K BTU furnace to maintain those temps in my larger GHs. That is why I think we need to communicate on the same scale.

The Penn State specialists I've asked claim that the passive heat systems are too unpredictable and usually insufficient in quanities of gained heat. As night temps fluctuate in a roller coater fashion, sometimes dipping into freezing, we need to monitor temperatures and act accordingly. Often the weather forecast is wrong and we find temps getting dangerously low at 2:00 AM. My first action is to light up the propane heaters and start circulating inside air. Then I decide whether it is worth the effort to start a wood burner for the duration of the night.

If you are heating a water bank you may not need it for weeks at a time and then when you do need it there is hardly enough reserve heat stored to handle temperature demand. I long for a system that doesn't rely heavily on fossil fuels but for now my crop is too valuable and fuels are too easy to work with.

What about using a forced air wood furnace? Would put it in a separate shed close by and pipe the heat exchanged air to the greenhouse. I'm thinking about adding heat to the main greenhouse with gas backup and also adding heat to the winter crops in the hoop house. I should also be able to get the early toms out earlier. Not to worried about fluctuating temps for that. Any comments?

I had the same question as the the orginal post. I have wood burning stove that I want to only have in the very early time of the year until shortly after the 1st frost. I have a 12'x24' greenhouse to heat. I have it all framed up and waiting for the weather to get better to put on the polycarbonate sheeting. I am trying to deceide to come up high inside and out with the flue, so I could get most of the radient heat or go straight out and up the side? In the summer I will remove & store the stove and part of the flue for the summer months. I am going to try to use Passive Solar Heating by filling 10 black painted 55 gallon drums with water on the south side. I never done this before but I could not see the drums working in the winter months, maybe in the spring. Has anyone done passive solar with drums before? I have pictures of the framed greenhouse if you like to see.

Post your pics!

I to have just completed a 12x20 ready for spring plant. I'm using eight 55 metal drums along the north wall. In the process of building the raised beds. I'm lining the bottom with radiant tubing, and will be building a small shed with hot water solar collector as roof. It will be a drainback system with storage tank below frost line. I also have used wood bruner for those really cold nights. I'm thinking of pumping the warm solar water through a heat exchanger place above the stove. THat way I can heat the incoming water on cold nights, or if the sun hasen't shone in a while. I've been told that you can allow the GH to get close to freezing, as long as the soil stays around sixty degrees.

So I'm thinking to heat the beds as the most cost efficient?

Celtic, for the sake of your polycarbonate you need to maintain a safe distance (~2') between the exiting flue pipe and the poly roof. I always have a cheap box fan blowing air accross the pipe and stove to prevent a hot air zone just above the pipe.

You could just move the stove further inward or to the center a little so as to have a longer inside pipe but you'll find that a damper in the pipe will work better if you don't have a air inlet damper on your woodstove. I realize that you want to get the maximum heat from the flue gases but if they cool too much before exiting you will have excess creosote buildup at that point of condensation (~250F).

An Exit hole should usually be taylored to the type of stove. If you have a 8" pipe coming from the top of your stove then keep the 8" flue and make your wall opening around 2 feet above that to allow for upward pitch of the pipe.

To create the exit hole I frame up a 2x4" window (my end studs are all 2x4"). Then I fasten a sheet of thin sheeting on both inside and outside surface of that frame (I've used galvanized sheeting, aluminum storm door bottoms or whatever cheap sheeting was available). Then just trace a circle onto one side and cut it out with a shears, match the opening on the opposide side and cut an oposing opening. The horizontal exit pipe should be tightly balanced through both sheets. I usually pack some fiberglass insulation into the cavity between metal sheets before inserting the exit pipe for the final time. I have 3 GHs set up this way and there is never a problem with the exit wall getting too hot.

Since you want the flue top outside you GH a few feet above the top of your structure a reasonably good support is needed. Try cementing a 1" pipe into a 5 gal. bucket (filled to the top with concrete). Then you can wire the flue at various positions to the pipe for support. Keep it simple so that you can disassemble and lower the outside pipe regularly for cleanout.