I have a new custom built greenhouse I was talking about a while back, made with twinwall plastic. It's working well, in winter with ambient temperatures during the day of around 10-15 degrees it gets up to 20-28 degrees, at night I've seen the outside get down to -1 but the greenhouse rarely goes below 3-5 degrees. So that works pretty well.

I'm thinking heat is going to be a pretty big problem in summer, it wouldn't surprise me if it hit 40 degrees, and photosynthesis is impaired over around 28 degrees. I have doors at each end, but I don't want to have to open and close them manually every day, plus that will let pests and birds in. I have self opening windows, but they're not huge. I also have a panel that can be removed and a grill put in for summer. I have no power in there but I could get it in there reasonably easily.

I think I'm going to need mechanical ventilation. I've read you need to change the air in the greenhouse once per minute in summer, which means a decent sized fan. Ideally I'd like to find a shuttered fan, so I can have it there summer and winter. Without shutters it will let a lot of heat out, which isn't great for winter. Initially I thought about solar, but I don't think a solar fan can generate the airflow I need. I'd need a thermostat as well.

I'm having trouble finding anywhere in NZ that sells a suitable fan, my google-fu is failing me. Does anyone know of anywhere good that sells this stuff?