You are thinking of using this through the Summer in zone 5? Cooling would be concern unless you can really open it up. IÂm thinking that the tomatoes must be planted in large containers so as to move indoors during September.

Unvented furnaces are used in commercial greenhouses and known as CO2 generators. Oxygen levels can build up so much when lots of plants are grown indoors that simply burning propane or natural gas (thoroughly) and blowing the exhaust around actually benefits the plants.

Charlie's Greenhouse Supply sells vented & unvented gas greenhouse heaters - pricy source. www.charleysgreenhouse.com/

I have a conventional natural gas forced-air furnace - essentially a garage heater that hangs from the greenhouse ceiling. It works fine.

I don't know about ground heating. It sounds reasonable but controlling the temperature of the plants would be critical. You wouldn't want 120 degree soil because it's below zero outside.

Think about putting some type of heater in an adjacent, insulated structure. Move the heat into the greenhouse using ducts and forced air or perhaps an on-demand water heater utilizing pipes. It would take some innovative engineering.

Steve

Here is a link that might be useful: GREENHOUSE CONTROL SYSTEMS

Yes, the whole top comes off and won't be used, except on chillier nights while the fruits are ripening. Hopefully it will prevent the first few frosts, which would have doubled my tomato harvest last year. Had to freeze many green ones. Thanks!

If this is just to protect from the occasional freezes in the fall, I think just having the area completely enclosed will be enough without additional heat. On days when you're expecting it to get down to freezing overnight, just leave it completely closed all day to build up as much heat in the soil as possible and that should be enough to keep it above freezing overnight. As long as you're 1 or 2 degrees above freezing, that's all you need. When it gets that cold around here (Denver) in fall I just cover mine with light weight blankets and sheets, and unless it gets VERY cold, they're fine.

If you still feel you might need some extra heat, I recommend you get one (or two) of the oil filled radiators you use for heating rooms in winter. I have one and it's amazing the amount of heat it puts off if it's turned up high---if used in a medium size bedroom and turned up high, it gets too hot! Just hook it up with a heavy duty extension cord to your nearest power source. They run around $40--cheaper on sale--watch Walmart, etc. That way it's completely portable and can be stored away when you don't want it and gotten out quickly if you do suddenly need it. And you could always use it in the house in winter if you ever need to.

Your tomatoes are planted in the ground, aren't they? Where are you located?

Skybird

It isz easer to heat the ground whre the roots are with hotwater heat than heat the air