The relay has 3 terminals - C, NO and NC - and a coil.



C= Common or wiper

NO = Normally Open with relay coil de-energized

NC = Normally Closed with relay coil de-energized



When the Ranco is in heat mode and the temperature sensor registers a temp LOWER than the setpoint temp (the temp you set for the Ranco to trip at), the relay coil is energized and the C becomes connected to the NO contact. This happens inside the relay. A little metal arm that the C contact is connected to is pulled to the NO contact by the electromagnetic force of the energized relay coil.



When the Ranco senses the setpoint temp, C swings back to the NC contact by a spring as the coil is de-energized.



Coil energized (below setpoint) - C connects to NO

Coil de-energized (at or above setpoint) - C connects to NC



Some power relays (relays for carrying a good amount of current) may have one contact heavier-rated than the other. A contact that stays closed can carry alot more current than one which is opened often, because there is an arc on an opening contact if the load is a motor (like an AC unit.) The Ranco may have a higher rated NO or NC contact for that reason, whichever one is used in Cool mode for an AC. I forgot which one you said it was.

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