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If the idea is to turn on an exhaust fan when the temperature rises in the greenhouse, you might want to check out your local hardware store. They sell a bi-metal contact in a 1/2" x 2" case. When the temp rises, the two metal strips contact and hold until the temp falls 10 degrees F. The "turn-on" temperature is adjustable from 95 - 120 degrees F. They are made for attic fans, cost under $5, and will handle a couple of Amps.