Hooking up mercury thermostat


Old 10-16-18, 12:24 AM
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Hooking up mercury thermostat

out of wall red, white, and green wire.
theromstat says hook up red, white and yellow
i hooked up red and white. but boiler furance won't kick on
should i hook up the green wire to the y terminal?
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Old 10-16-18, 12:26 AM
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Welcome to the forums.

Are you asking us for a best guess ?
Without knowing what you have and what the thermostat is we can only guess.
Wire colors mean nothing.
Old 10-16-18, 11:59 AM
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With the very limited info and no pics of the system it's difficult to say but this is just a shot.

My guess is that your t-stat is connected to an old 3 wire stack relay that is in your smoke pipe.

These were used before the cad cell relay, which is now used. Your furnace is probably at least 60 years old as is the stat.

Pics would be helpful but you need the 3 wires on that stat. As PJ said, colors mean nothing. Proper terminal is everything.

Actually only 2 of those wires will run the furnace even though you must hook up the 3.

What you can do is jump 2 wires, say red & white to see if the furnace starts, If not try another pair until you get 2 that work. Attach those 2 to your red and white terminals and then attach the other wire, or you can remove the cover of the control that those wires go to and jump 2 terminals at a time to see when the furnace comes on and see what color the wires are and do the same upstairs at the stat.

If you have a meter and know how to use it you can check for 24V at different pairs of wires at the stat. Only the right pair of wires will read 24V.

If you buy a new basic heat only stat it will require only 2 wires. In that case you would just tape off the third wire after you locate the right pair.

Hope this isn't too confusing and it helps a little.
Old 10-22-18, 03:57 AM
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got it, red ad gree wire were twisted together, in the junction box, above the boiler.

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