Heating multiple greenhouses

[vtfarmer]

Hello all,

I am new to this site -- what a great resource. I am hoping some of you can send me in the right direction.

I operate three heated greenhouses for our farm here in Northern Vermont. They currently run on three propane unit heaters (2 @ 200K BTU, 1 @ 300K BTU). I'd like to scrap the old propane heaters and switch to a centralised wood gassifier setup running hydronic unit heaters.

The houses are only heated from the end of Feb. through spring (would need to prevent system freezing rest of winter). They generally only call for heat at night and on really overcast days. However, when the sun goes down they really don't hold any heat at all and the unit heaters pretty much run nonstop with very brief cycles off. So on those cold nights, I would need to be able to heat through the night or automatically switch to a backup source (we do occasionally go away, and a backup heat source would be essential).

Any help or random thoughts would be greatly appreciated!

BTW, I currently run a Tarm with a 1000 gal storage tank and radiant heat for my house and I love the system.

thanks, eric

 

[Eric Johnson]

Welcome to the Boiler Room, Eric.

You may have noticed a couple of other threads seeking information on high-capacity gasification systems. I'd recommend checking out http://www.newhorizoncorp.com and look over their small industrial gasification systems. That sounds to me what you're looking for. Either that, or a big Garn.

 

[BrownianHeatingTech]

I'm here in Northern Vermont. They currently run on three propane unit heaters (2 @ 200K BTU, 1 @ 300K BTU). I'd like to scrap the old propane heaters and switch to a centralised wood gassifier setup running hydronic unit heaters.

The houses are only heated from the end of Feb. through spring (would need to prevent system freezing rest of winter). They generally only call for heat at night and on really overcast days. However, when the sun goes down they really don't hold any heat at all and the unit heaters pretty much run nonstop with very brief cycles off. So on those cold nights, I would need to be able to heat through the night or automatically switch to a backup source (we do occasionally go away, and a backup heat source would be essential).

Any help or random thoughts would be greatly appreciated!

Well, if we figure that your propane units are 70% efficient, you're actually using about 500k btu per hour during the night. To run for an eight-hour night, you need to have 4 million btu's stored.

If we were to oversize the hydronic heaters, we could run them on variable water temperature all the way down to 160-degrees. So call it a 30-degree difference (190->160) from the max tank temperature to the minimum at which the heaters can keep up.

That comes out to 16,000 gallons of water storage.

Not saying it can't be done, but just want to make sure you're aware of what would be involved in a system to handle that.

I'd also suggest looking into radiant heat instead of unit heaters. It can use a wider temperature differential and, since it heats the plants' roots directly, it doesn't heat the greenhouse air as much, which means less heat loss through the wall of the greenhouse. A radiant system would likely need 5000 gallons of water storage, or maybe a bit less.

Joe