Thermostat C wire to RobertShaw 780-715 ICU

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  #1  
Old 04-17-17, 04:40 PM
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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?

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Old 04-17-17, 04:54 PM
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Welcome to the forums.

I remarked your diagram for you.

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Old 04-18-17, 08:34 AM
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Thanks Pete! Just curious... why wouldn't it be "TR" transformer common? (It seems that pin is specifically designed for running a common wire? )
 
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Old 04-18-17, 09:56 AM
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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.
 
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Old 04-18-17, 10:31 AM
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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.

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Old 04-20-17, 09:09 AM
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Thanks guys! Much appreciated.
 
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Old 01-11-18, 04:54 PM
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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
 
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Old 01-11-18, 07:35 PM
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Good job. Thanks for stopping back and letting us know how you made out.
 
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