The easiest & most reliable means of Greenhouse Vent Fan avoiding this is to install automatic greenhouse vent openers.
The last thing you wish to have happen is for your plants to perish because they literally were cooked.
The sun is a powerful source of energy and a small greenhouse can reach blistering temperatures in just a few hours.
After all that's how they are designed. To capture solar energy.
greenhouses have lightweight 
roof vents and could require a stronger vent opener to maintain them closed or prevent them getting blown away - check before you buy.
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Should you have a greenhouse, you know how quickly it could heat up on a sunshiney day, even in winter.
You can make sure your greenhouse ventilation is continually regulated during hot summer months, whether you’re at home or not, by installing an automated vent opener. It contains a wax that expands when heated, starting the vent. When the temperature falls, the wax agreements and the vent closes.