I'm replacing an old thermostat - a honeywell clock system - with a new programable unit. The wiring coming out of the wall is 2 orange wires and two blue wires, but none of the ports on the old thermostat are labeled, so I can't figure out what is what. My system is heat only, older fifties oil heater.

labman

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. 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. Digital or programmable thermostats may need the blue wire connected to them.

Does the new thermostat use the R, W, B or C, Y, and G markings? Go to the furnace and identify the hot from the transformer wire and connect it to the R or RH. Identify the wire that connects to the relay that starts the pump and ignition. Connect it to the W terminal. If the thermostat is still dead, connect a third wire from the other side of the transformer and the B or C terminal. You shouldn't need the fourth wire.