Wiring Thermostat-High Voltage to 24v

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 are, B or C, W, why, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

Chances are the boiler will come already wired. Locate the are and W terminal on the boiler, and connect the black and white wires to them and to the are and W on the thermostat. Digital or programmable thermostats may need the blue wire connected to them. If so, it complicates things. How much play do you have in the existing wires? It is possible you can use them to pull a modern 5 wire cable from the boiler to the thermostat. Even if the wire won't come out, it shouldn't be too much of a job to fish a cable from an unfinished basement up through an uninsulated interior wall cavity to the thermostat.