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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I'm trying to install a thermostat that uses a common wire.

Customer Question

I'm trying to install a thermostat that uses a common wire. However, I am using an unused wire which is blue as the common. I connected the blue wire at the furnace, but nothing happened. So, I took my tester and there seems to be no voltage coming from the common at the change board. Any thoughts?
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


Check for 24 volts *between* R and C....tell me what you get.

Expert:  Phil replied 1 year ago.

You should get 19 to 24 volts from R on the thermostat terminal strips either in the furnace or at the thermostat to *ground.

Sometimes C wire will not give a voltage read to ground, in that case the system was designed to have a grounded common (C) wire. So that you need to measure voltage

**between** R and C.

Customer: replied 1 year ago.
Okay, I don't have a meter handy. I will return.
Expert:  Phil replied 1 year ago.


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