I’m in Zone 4, and I also need a fan for the greenhouse even with two vents and the double doors open. Right now, a 60 degree day means a 90 degree greenhouse!
It’s really nice for thing starting in March, but after mid-May it’s useless - then again it’s useful in the fall and into the late fall for keeping potted tomatoes & peppers going, or to do salad greens late into the season.
At my old farm I had fresh tomatoes at Christmas one year. But just that once out of 25 years, LOL!
I’m reading books on year-round use. We’ll see what I can accomplish. ;)