In my experience, it's always better to run an exhaust fan in the South end of a greenhouse roof as the intake air from the North side of the greenhouse is always a few degrees cooler. In the Northern Hemisphere of course! Using a Ludvig-Svensson shade cloth as a barrier between the plant area and the upper reaches of the greenhouse roof will also greatly improve the effectiveness of an exhaust fan. You do not need any mixing of the air in this situation as the cooler air is closest to your plants and the heat arising from solar conversion is higher up in the exhaust zone.
Andy Easton
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