Thermostat C wire to RobertShaw 780-715 ICU
#1
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Thermostat C wire to RobertShaw 780-715 ICU
I have a gas furnace (no A/C) with a RobertShaw 780-715 U Ignition Control Unit. I have a 4-wire bundle coming into the furnace from the wall thermostat. Only 2 wires are being used (red and white). I'd like to use one of the extra wires in the bundle to use as a common wire (c-wire) in order to power one of those new thermostats (Echo, Nest, etc). Below is my crude drawing. Where do I connect the third wire to create a C-wire?
#4
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PJ is showing the safest way to do it.
It appears that the ground is connected internally to the TR and MV/PV terminals on the unit. I can't verify it but if so you are correct that you could connect C to the TR terminal. Is there a wiring diagram for the furnace? I would expect it to show that the transformer be attached to TR with the gas valve connected to MV/PV. And a wire from the ground terminal to the furnace chassis.
It appears that the ground is connected internally to the TR and MV/PV terminals on the unit. I can't verify it but if so you are correct that you could connect C to the TR terminal. Is there a wiring diagram for the furnace? I would expect it to show that the transformer be attached to TR with the gas valve connected to MV/PV. And a wire from the ground terminal to the furnace chassis.
#5
Astuff is correct. All three terminals are connected together inside the module.
We "could" use that connection or you could even connect your C wire to chassis.
I like connecting C right at the source for the thermostat.
We "could" use that connection or you could even connect your C wire to chassis.
I like connecting C right at the source for the thermostat.
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Thank you again.
I finally did the wiring adjustment to create a "C" wire, as recommended in PJ's mark'ed up diagram. I then hooked up a cool , Echo Bee smart thermostat. Works great! Just wanted to thank you again. No way I could have figured this out on my own even with the wiring diagram in the furnace.
-James
-James