Trying to find C wire

[fquogue]

I have an installation with two zones (1st floor and 2nd floor) oil burner and HVAC. I replaced both thermostats with wifi filtrete thermostats recently. I don't have a C wire available at any of the thermostats.

I am using an external transformer to power the thermostat on the 1st floor and it is working fine. It is not convenient to do the same on the second floor and I am trying to use the G wire instead: I am still trying to locate the proper C port by the furnace.

The furnace has an Aquastat Relay (L8124A,C) which is controlling the ground floor thermostat (both T ports connected to the RH and W ports of the first floor thermostat). There is also a Switching relay ARM 842DP from Arco Industries which seems to be controlled by the second floor thermostat: (both T ports of the priority zone on the ARM 842DP go to RH and W upstairs). On the same ARM 842DP, there is one 24VAC (1VA) port and one Common port. I thought that one of them would be fine to power the second floor thermostat, being on the same board as RH and W. It doesn't work though... Without batteries, no power

What do you suggest?

[blaine]

Measure the voltage at the HVAC between the RH and C terminals for 24VAC (not DC). Then measure between the same two wires at the thermostat.