What is the size of your exhaust vent opening, and what is the size of your air intake opening? If the intake is too small vs. the amount of air being exhausted, you will not get good ventilation and could damage your fan.

You need a better passive intake. A dryer vent turned backwards might work. Even several dryer vents would probably be cheaper than a louvered window.

RE: Cole, "A dryer vent turned backwards might work."

Very useful idea. I had been thinking of putting in a louvered window placed backward so it would open from the suction as one solution or using a second thermo switch to open the window before the exhaust fan.

Right now IF I am in the greenhouse (I cannot leave it this way in our sub zero weather) I can open partially a storm door window/screen to relieve some of the suction pressure and then the window will open OK so something of this sort should work.

you can use a two stage thermostat the first stage will open the motorized luver and after a time the temperature is rising the second stage will turn the exhaust fan on

Thanks for that suggestion as well. I will certainly check it out. I had thought of installing a separate thermostat to do that but if I could get a two step thermostat to replace the one I have it might utilize my current wiring.

To tell the truth I thought to myself that the thermostat should work that way for a greenhouse since I am sure this is not an isolated problem, but I had not heard of them before personally and did not know whether it was an option.

P.S. I just took a look and see they are readily available, even the same brand as my equipment. Wish I had purchased this originally. I have spent a lot of money replacing motors, etc., not to mention wasted time and unfortunately some lost plants.

This post was edited by James-MG on Mon, Feb 10, 14 at 22:40

I use a ranco thermal controller 2 stage good for greenhouse because moisture proof. I think Grainger handle them

Carl

Here is a link that might be useful: ranco

How do you hang your shade tarp inside your greenhouse?

The 2-stage thermostat would work but if you want more control over when your intake vent opens just be sure to set that at a much lower temp. I have both setups and prefer the second thermostat b/c it "Mellows" the inside temp more in extreme cold by allowing heat to escape thru the intake louvers without drawing cold air in over tender plants. You didn't mention GH size but your problem is more of an issue with smaller structures.
My 2-stage has a fixed 4*F differential setting, I suspect most are setup that way. Also,if you set you intake thermostat to 10*F under the exhaust fan thermostat you'll find that the exhaust fan doesn't need to work as often and it could save a little on the electric bill.
Another option which I use for my smallest Gh is to manually turn the exhaust fan off until there is make-up air opening. This is more risky but over the past few years I've been overriding the exhaust thermostats frequently to help retain heat in the later afternoon. These thermostatically controled systems always seem to open late (higher temps inside) and close late (lower temps inside).

Thanks for posting, bmoser. I have already committed to a course of action and your note supports my decision. I was uncertain of some of the things you mentioned but I think I guessed right.

I could not find enough information about the two stage thermostat to know about the adjustment options but I feared just what you confirmed, that there would be a set temperature differential. As a result, I have ordered a second thermostat.

Depending on the season I think it would be helpful to use different settings and in any case I think it would often be an advantage to space the two more than three or four degrees.

I have a couple of circulation fans in the greenhouse so with that in mind it would probably save a lot of running time on my exhaust fan if the window opened at a lower temperature as well.