Source
r/DIY•
Generacy

[HELP] Replacing old mercury thermostat with new digital programmable

help

I took out the old plate, and there is a yellow and white wire connected to the two terminals of the old mercury thermostat. There's also a blue wire that was just taped up, connected to nothing. I'm trying to connect it to a new digital programmable thermostat, so I connected blue to B, yellow to Y, and white to W. But nothing's happening...

I have a gas furnace, no AC.

Thanks in advance for the help!

Posted May 7, 2015, 5:31 AM UTC

87
Imaelectrician•

Hook the yellow wire to the r or rh terminal and the white wire to w. Leave the blue how you found it.

2
GeneracyOP•

Thanks, that worked!

1
danauns•

From my limited experience (Installed 2 thermostats. Lifetime) I would go more by how the wires are connected to the furnace, and the furnace requirements. Both times I changed mine, all of the colours were borked - none of them used properly. The only way I could get it to work was to figure out how they connected to the furnace, and then echo that upstairs on the new thermostat. Good luck.

2
cyb3rphr33k•

pt100 wont do anythong without a controller the old mercury has a switch inside it. the pt100 is just a sensor.

-1
snakeoiljoe•

I hate to be this guy, but did you read the instructions? There should be a wire map with it. It will point out what color should be in what terminal.

-1
GeneracyOP•

Yup, those letters correspond to the colors. Thanks though.

1
sharkmonkeyzero•

The mercury switch is just that, a switch. It needs no power to operate. Your fancy new programmable does need power. Chances are that blue wire is not hooked up on the other end. You need to figure out what the wiring looks like on the other end, then maybe we can help more.

-1