I am building a greenhouse in North Carolina, 20' x 8' , one side built against a building so 3 glass walls (2 panel glass).
Can someone recommend what type/size of ventilation system and where a good online place to buy is?
I assume I need an intake vent, an outtake vent, and a controller which turns them on when it gets too hot.
I highly recommend "greenhouse vent openers". They are totally passive, no electricity required. They open and close windows based on temperature. (I get mine at Harbor Freight). Simple as can be. My 10' X 12' greenhouse has 16 square feet of windows in the roof, and during warm weather, 35 square feet of door that I also leave open.
Passive greenhouse vent openers are manufactured in lots of different configurations. Even for horizontal walls. Vents in the roof are very appropriate for greenhouses, because the hottest air is in the highest point of the greenhouse. I can't trust electricity to provide ventilation for me, and I can't trust myself to open vents. Passive greenhouse vent openers have never failed me.
Probably we are missing on the construction details that limit the ventilation. Does the greenhouse have any openings or is entry only from the house?
Is this a manufactured unit? or something you are constructing yourself? General expectation is that a manufacturer will have designs for ventilation.
There are many examples in this forum of storing the excess heat for later use. If you are going to use a vent fan it can be set up to blow the hot air down a pipe and under the floor of the greenhouse then exit on the other end while another underground pipe brings cool air into the greenhouse. Consider also an insulating cover that can be rolled out over the top for shade when there is too much sun and all the way over the front to keep heat in when there is no sun.
I use two large vents low (one on each end) and 4 roof line vents with openers like Kola Lofthouse showed, they work very well and as he mentioned you can find many configurations for how you need the window to open.
Joseph has is right, the automatic openers are great. I had one in my GH and I could tell from the house when it was warm in the greenhouse by watching it open. Another automated albeit expensive alternative is a solar powered ventilator. One of the best uses for solar is heat ventilation. The fan will only have power when the sun shines and if the panel is oriented the same direction as your greenhouse will come on slowly and increase to maximum speed as the sun becomes more direct. It only runs when you need it to run.