Smart Thermostat on Old Two-Wire Wiring: Reusing the Wall Wires

Richard Trethewey of Ask This Old House replaces a thermostat in a house he says was built in 1962, using only the two wires already in the wall. The homeowner, Michael, had an energy audit that suggested a programmable thermostat, but found his wiring was not compatible with a new unit.

The problem with two wires

The old thermostat has no heat/cool switch, so Richard assumes the system is heating only. It works as a simple switch: heat needed or not. A programmable model with a battery could power a clock, but would have the same on and off periods each day and could not be reached from the internet. A modern internet-connected thermostat needs at least a third wire to power it, and a system with heating and AC needs at least five. Running or fishing new wires down to the furnace is not always easy.

Removing the old unit

The furnace service switch is turned off first. Richard explains that older thermostats sensed temperature in different ways: sometimes a mercury-filled bulb that tipped to make or break the circuit, and in this case a bimetallic sensor with a spring. A couple of screws hold it, and then the base plate comes off. He notes the wall might need touch-up or a new base plate.

A two-piece smart thermostat

In the basement is an oil-fired furnace with ductwork out and a return coming in. A typical smart thermostat has a sensor inside the home, a Wi-Fi connection and generally five wires. The unit used here splits those functions into two pieces: a touch-screen user interface with sensor and Wi-Fi upstairs, and a base module at the furnace. The two wires become a digital link between them, and the base module supplies power upstairs. The module also allows later additions such as air conditioning, ventilation or humidity control.

Wiring and mounting

Richard mounts the module on a pegboard in a central location. The two thermostat wires go to the terminal marked for the thermostat, and he says it does not matter which is which because the connection is not polarity sensitive. At the furnace, the connections are R, W and C: red to R, white to W, and the blue wire to common, which powers the whole rig. Upstairs, the new wall plate covers the patch left by the old unit. After a pilot hole is drilled and the plate fastened, the unit is pushed onto its connections and the wires are fed into the wall.

Power on

With the furnace power back on, the thermostat runs. Settings can be changed on the screen and from a phone, which allows remote adjustment, a check on operation and a look at how often the system came on during a month.

Sensor location and wireless

The sensor and control remain in the hallway. For a different location such as the master bedroom, a separate sensor disc exists, but two wires would have to be run down to the module. Richard says wireless indoor thermostats exist, but frequent transmission wears down batteries and radio frequency interference is a concern, so he prefers a wire to carry the digital signal.