I dont have more than one rack though so I don't know for sure, but with the spread in just one rack id be nervous to put multiple racks on one thermostat unless it had multiple probes like a herpstat 2. I interpreted the OP was trying to take a herpstat 1 or something and run multiple racks off of it which I wouldn't personally do although if conditions are similar between the racks it could work perfectly fine.

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