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hdtvrocks · Oct 31, 2005, 11:57 PM

Hi All,

I'm trying to replace my old mercury thermostat (from 1979 it seems) with a new 4+1+1 thermostat. I took off the old 'stat and I had 3 wires connected to it brown, green(maybe grey), and black. The brown and green wires were connected to the same terminal while the black one was connected to the other terminal on the old thermostat. My furnace is an old gas furnace with fan I guess. The instructions for the new thermostat says to connect power to R/RC, heater relay to W, and fan relay to G. I assume my green wire is fan relay, but how do I tell which of the black/brown wires is power or heater relay?

TIA.

labman · Nov 1, 2005, 08:08 AM

Check the old thermostat if you can. I think you will find that when it is calling for heat, the 2 terminals are connected together. The new thermostat will connect the R terminal to the W terminal when calling for heat. Connect the black to R and the brown and green to W, and the new thermostat will work just like the old.

The basics:

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors.

If you want to make sure, go to the furnace and see what the wires are connected to there. If any of the terminals are labeled W, that is the gas valve. The fan will be G, and if any provision for A/C, it will be Y. Most furnaces do not need the thermostat to activate the fan. Instead it is controlled by a temperature limit switch, starting the blower after the gas warms the air up, and continuing after the gas quits until the furnace cools down. If you are lucky, there is a wiring diagram somewhere.

hdtvrocks · Nov 1, 2005, 09:44 AM

Originally Posted by labman

Check the old thermostat if you can. I think you will find that when it is calling for heat, the 2 terminals are connected together. The new thermostat will connect the R terminal to the W terminal when calling for heat. Connect the black to R and the brown and green to W, and the new thermostat will work just like the old.

The basics:

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors.

If you want to make sure, go to the furnace and see what the wires are connected to there. If any of the terminals are labeled W, that is the gas valve. The fan will be G, and if any provision for A/C, it will be Y. Most furnaces do not need the thermostat to activate the fan. Instead it is controlled by a temperature limit switch, starting the blower after the gas warms the air up, and continuing after the gas quits until the furnace cools down. If you are lucky, there is a wiring diagram somewhere.

Thanks so much labman! I'll take a look at the furnace and if that fails I'll try black to R and brown/green to W.

hdtvrocks · Nov 1, 2005, 11:36 PM

Checked my furnace, nothing's labelled. I just did what you said labman and it works great! Thanks again.

guelja · Nov 1, 2012, 02:33 PM

I have three wires coming out of the wall - Red, Black, and white. Can you help me with connections?

Also, when my furnace turns on the burners turn on then off after 1 minute. However the fan continues for about 3-5 minutes blowing cold air.

Why would the burners turn off after about 1 minute?

I do see that when I checked the furnace it has not been cleaned in 5 years, could this be the cause?

HELP!