I searched for a pic of your greenhouse and posted the link below. If that's the right picture, it might help other people answer your question.

wall mounted exhaust fans with louvers to keep the greenhouse cool during winter.

Cooling is a lot easier in winter than in summer. The roof vents alone should be enough to cool your greenhouse on a sunny winter or early spring day. Above 80 degrees or so, you would definitely want cross-ventilation through open windows or doors. If you can't have that, you'd need to use blowers or exhaust fans to exchange the air. Wal-Mart sells a 250 cfm Stanley Blower for $40 that would work for you. If you use an exhaust blower, don't put it right next to an open vent. Make it draw its intake air from the center of the room, or more ideally you put it at one end, seal everything up, and make passive louver vents at the other end of the greenhouse as an air intake. Then you get the cross-ventilation you want; air comes in one side and out the other.

Here is a link that might be useful: {{gwi:309326}}

Yes that is my greenhouse, so will the fan I posted not work to exchange a sufficient amount of air? Could I position it so most of the air blows out one vent so air from outside can enter from the other three? I will be able to open the doors during the spring and summer, but I may leave them closed on some spring days if the nighttime temperature dips below 32.

with 4 roof vents and a door that opens as wide as the one in the picture shows you should have pretty good ventilation as is. what will you be growing in it?

Thank you for the answer, I will be growing carnivorous plants.

If you can open the doors, you're fine. The fan is not a bad idea, but it won't make a big difference in internal temperature if you have the door closed on a sunny day. It's better than nothing, but when it gets hot and sunny, you will want the door to be open.

I could leave the door open while I am away. I just fear of animals like racoons, cats, and squirrels wreaking havoc in the greenhouse as there are quite a few of them wandering around during the day and night. I think I may build a fence made of chicken wire to go in front of the door to discourage anything from going inside.

I also have one more question, although this has to do with an air gap in winter. Almost everything in the greenhouse is sealed with caulk, but the outer edges of the doors are not sealed properly and have long air gaps that are the length of the door. The gaps are about 1/3 of an inch wide. The doors do have rubber gaskets on them, but with the way they close only one side of one door can have somewhat of a seal (the bottom of that door will still even have a large air gap near the bottom). I was wondering, should I close these gaps down? The only reason I can think of not closing them would for fresh air exchange, is this necessary?

Has anyone put any kind of insulation on the inside of the sliding doors for the 10x12 newer model? This is hard because there is no room. I have put bubble rap on the inside of my greenhouse but the doors are the only thing that does not have any insulation. I might be able to put thin window plastic but I remember somewhere on the greenhouse forum I thought I saw sliding doors with bubble wrap.Any suggestions? thanks, Ken

Maybe just a "flap" of bubble wrap over the inside of the doorway in winter would help?

I pull off the front wall of polycarbonate and open all the windows in the glass house and grow inside all summer in Oklahoma...without any fans, shading or electrical use. These are the only houses around here that can claim that, the others are all either shaded, shut down, or using tons of electrical power.