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I want to add a relay to my furnace so the attic fan can't run while the furnace burner is firing. Am I correct that the 24V side of the relay should be connected to the C and W terminals on the furnace?

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I'm not sure why you would want to do that, but paralleling the 24V relay coil to the C and W wire and connecting the attic fan to the normally closed contacts on the relay would achieve what you want to do. I would use a RIB (relay in a box) and make sure all wiring above 50V is protected per NEC. Also make sure your control voltage transformer has enough VA to handle the addition.

Good Luck

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  • The fan manufacturer states that if there is a furnace installed in the attic, there has to be an interlock to prevent the exhaust fan from running while the burner is firing. I'm assuming it's to ensure that no combustion fumes are pulled from the exhaust. I did order a RIB; it draws 83ma at 24V. Thanks for your quick answer! – Tim H Jul 5 '17 at 15:22

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