We have a situation where we are using two identical home furnaces. Don't ask why but think of the second one as a ready to go "back up" unit. There is just one thermostat. When it is necessary to make use of the alternate furnace, AC power is switch off of the not working one and the power is switch on to the back up unit. Then there is a switch box which selects which unit will receive the 24VAC from the transformer by choosing "west" or east" to indicate which furnace is which.
If you are not familiar with thermostat wiring, there is a common method as far as color codes of the wires to know which terminal connects to which terminal on the furnace connecting strip. Starting with red, it is the top side of the 24VAC transformer which the thermostat then connects to the other terminals as called for. White is for heating, yellow is for cooling ( AC compressor), green is for fan. That covers most common units but there is one additional connection which is "common" with contact "C". If that is not connected, they will have an internal battery to supply power for the thermostat.
So the thermostat takes the red wire (24VAC) and connects to let say a heating situation.... connects the 24VAC to the heating module and also the blower or fan for circulating the heat. The voltage path is from the red terminal to the item being supplied with the voltage and then from it to "common". to complete the circuit.
I have found that ground and common are not the same so cannot be connected together. I do not quite understand from the wiring I'm looking at, as to how and what are connected so as not to cause a problem or potential difference if I were to combine the frames for ground or if I were to connect the commons between the furnaces.
My goal is to get rid of the internal batteries so as not to forget to change them which results in no heat if they go dead. With the common at the thermostat connected to the common on the furnace hookup strip, this should complete the circuit to supply power for the thermostat from the 24VAC supply.
All I can see on the thermostat switchover box is what appears to be a double pole double throw switch.... 2 screws on each side with those on same side making or braking with switch action. Switch has all red wires except one also has a blue connected with a red... makes no sense as the blue wire from all cables are tied back and not connected anywhere.
Could anyone offer a schematic of how this can be safely done so I can get rid of the battery issue? So far I see that all whites are tied together, all greens the same, all yellow together ....... only the red wires are connected to the switch... the top of a 24VAC transformer.
Thanks so much !
If you are not familiar with thermostat wiring, there is a common method as far as color codes of the wires to know which terminal connects to which terminal on the furnace connecting strip. Starting with red, it is the top side of the 24VAC transformer which the thermostat then connects to the other terminals as called for. White is for heating, yellow is for cooling ( AC compressor), green is for fan. That covers most common units but there is one additional connection which is "common" with contact "C". If that is not connected, they will have an internal battery to supply power for the thermostat.
So the thermostat takes the red wire (24VAC) and connects to let say a heating situation.... connects the 24VAC to the heating module and also the blower or fan for circulating the heat. The voltage path is from the red terminal to the item being supplied with the voltage and then from it to "common". to complete the circuit.
I have found that ground and common are not the same so cannot be connected together. I do not quite understand from the wiring I'm looking at, as to how and what are connected so as not to cause a problem or potential difference if I were to combine the frames for ground or if I were to connect the commons between the furnaces.
My goal is to get rid of the internal batteries so as not to forget to change them which results in no heat if they go dead. With the common at the thermostat connected to the common on the furnace hookup strip, this should complete the circuit to supply power for the thermostat from the 24VAC supply.
All I can see on the thermostat switchover box is what appears to be a double pole double throw switch.... 2 screws on each side with those on same side making or braking with switch action. Switch has all red wires except one also has a blue connected with a red... makes no sense as the blue wire from all cables are tied back and not connected anywhere.
Could anyone offer a schematic of how this can be safely done so I can get rid of the battery issue? So far I see that all whites are tied together, all greens the same, all yellow together ....... only the red wires are connected to the switch... the top of a 24VAC transformer.
Thanks so much !