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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have an old mercury switch Robert shaw 4 wire. I want to

Customer Question

I have an old mercury switch Robert shaw 4 wire. I want to install a new programmable model but not sure which wire is common or c. The terminals on the Robert shaw are w1, G,R,Y. Do you know?
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.
Hello,C is the terminal opposite + on the units transformer... thermostats that require both sides of the transformer (+ Red. and C - white at the transformer) do not need to be powered by batteries.. you need 5 thermostat wires pulled to the thermostat for that... the bundle of wire the installer pulled very often has a few spare wires, one of which you can use for C. If you only have 4 wires, you can buy a battery powered programmable thermostat to do the same job. Let me know what you think, and what your preferences are, we can go from there. .