Are you able to have 220 Volt electricity to the green house?
If so it would be much more economical to do so.
If you are going to want to auto vent it you might think of putting in a vent fan as well.
This way when the temps got too hot the fan would kick on and the vent would open up at the same time.
In my experience with these setups when using the vent fan and gravity dampers the dampers dont work worth a hoot.
The best ones work off a motor drive with limit switches.
So you would have a fan at the top pulling air out and another vent at the bottom letting air in.
How far is the green house from the electrical source and is the power running in conduit.
If so do you know what gauge wire it is and what size conduit it is.
The minimum to go 220 would be 10 gauge wire depending on the load and the distance of the wire run.
Plus if done correctly you could put in a sub panel and have 220 and 120 volt ac at the green house.
Here would be a typical panel you would install at the greenhouse.
It already comes with about everything you need.
The 50 amp breaker would be for the heater the 30 amp breaker would be for the fan motor and so forth.
Then you would have 6 more 120 20 amp breakers to do what you wanted to do.
http://www.homedepot.com/p/GE-PowerM...0?N=5yc1vZbm2w
So far the cheapest I have found tempered green house glass it would cost about 3,000 dollars or more to replace everything.
I would rather go with polycarbonate than non tempered glass.
The first hail storm would beat all of the glass panels out again.
I know this is way more than you asked for but I thought I would put it out there.
Worth