I have installed in my greenhouse 2 x 24" exhaust fans. I have shutters+wet wall on the other end of the greenhouse. The fans are these ones: http://www.greenhousemegastore.com/Shutter-Fan/productinfo/FA-VSF-/ It turned out that 2 fans was overkill and I only use one now. The other one is like a backup. I tried to slow them down with a speed control but they were getting overheated, since the fan is designed to cool off with the air going through it. Problem I am having is that this fan is VERY NOISY!!! I need to replace it with a much quieter fan. I am not sure if installing 2 smaller fans would help with the noise or it would be just as noisy. Or alternatively I would completely eliminate the exhaust fan + wet wall and install an evaporative cooler. But then I don't know how much noise the evaporative cooler would make, and it would be more expensive to buy it. Any suggestions would be greatly appreciated.