I have two heating systems in my house, oil and wood. I’m currently swapping out old mercury thermostats with new ones. I’ve got the wood one replaced with a newer analog one which I think is okay, but I’m using a digital for the oil. The thing is, the previous thermostats shares the same red wire, so now I’m wondering where the blue and red wire go in the second pic for the digital. Thanks! 
Electrical – Dual Thermostats: Which wire goes where
electricalheating
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Best Answer
You should baptise 'main' and 'auxiliary' heat, so with a single stat, you can control both systems. Using common, 'CallForHeat' and 'AuxiliaryHeat' connections.
For single generator units (Ex: only oil heat), if you have only 2 wires, just use connectors R(h) and Common (W), as Rc is used for cooling. If you have forced air, you may have also G cable (fan). Other wires are used for more advanced setups.