Gravity Furnace wiring question, looking to replace thermostat
#1
03-13-18, 08:54 PM
Gravity Furnace wiring question, looking to replace thermostat
My existing thermostat is a two wire (R and W) cable to the gravity furnace. I've traced the wiring and have come with the attached diagram.
The transformer is an old Honeywell AT72A1K2. Secondary output is 25v/40va.
There's also an old Honeywell PilotStat, part #C418A1 and the Gas Valve is a White Rodgers 25G01.
I'm yet to start anything, just wanted to note the way it is wired now because it seems odd to me that the W wire is going straight to the transformer while the R wire goes to the transformer side of the gas valve, out on the thermostat side of the gas valve, in/out of the PilotStat and then on to the Transformer. But I'm not an HVAC guy so maybe that's the correct way.
In any case, in order to install an ecobee I need a C wire and I'm trying to determine the best way to do it. I intend to run a new wire from the ecobee to the furnace, just not sure what would be the proper way to connect it. Any help is appreciated.
The transformer is an old Honeywell AT72A1K2. Secondary output is 25v/40va.
There's also an old Honeywell PilotStat, part #C418A1 and the Gas Valve is a White Rodgers 25G01.
I'm yet to start anything, just wanted to note the way it is wired now because it seems odd to me that the W wire is going straight to the transformer while the R wire goes to the transformer side of the gas valve, out on the thermostat side of the gas valve, in/out of the PilotStat and then on to the Transformer. But I'm not an HVAC guy so maybe that's the correct way.
In any case, in order to install an ecobee I need a C wire and I'm trying to determine the best way to do it. I intend to run a new wire from the ecobee to the furnace, just not sure what would be the proper way to connect it. Any help is appreciated.
#2
03-13-18, 09:22 PM
Group Moderator
Welcome to the forums.
With a standard two wire thermostat.... it just completes the circuit so there is no attention paid to the terminal designations. Now that you are installing a thermostat that requires a C connection the wiring becomes important. The diagram below illustrates what needs to be done.
You could swap the pilot stat for the gas valve. You could reverse the wiring on the gas valve or the pilot stat. What's important is those two devices are powered by the W and C connections.
With a standard two wire thermostat.... it just completes the circuit so there is no attention paid to the terminal designations. Now that you are installing a thermostat that requires a C connection the wiring becomes important. The diagram below illustrates what needs to be done.
You could swap the pilot stat for the gas valve. You could reverse the wiring on the gas valve or the pilot stat. What's important is those two devices are powered by the W and C connections.
#3
03-13-18, 09:44 PM
Thanks Pete,
One more question if I may. There's nothing on the transformer to tell me with terminal is the common vs the hot. I assume with the rewiring work that becomes important. How can I determine which is which, assuming I get a multimeter?
One more question if I may. There's nothing on the transformer to tell me with terminal is the common vs the hot. I assume with the rewiring work that becomes important. How can I determine which is which, assuming I get a multimeter?
#5
03-13-18, 10:24 PM
Group Moderator
Since this is AC..... there is no polarity.
The red goes on one terminal and the common(s) on the other one.
The red goes on one terminal and the common(s) on the other one.
