How to Install and Connect a 2-Wire Furnace Thermostat

Quck answer

To install and wire a furnace thermostat with 2 wires, follow these steps:

1. Turn off the power to the furnace.

2. Remove the old thermostat from the wall.

3. Identify the two wires coming from the furnace – typically labeled “R” and “W”.

4. Connect the “R” wire to the “R” terminal on the new thermostat.

5. Connect the “W” wire to the “W” terminal on the new thermostat.

6. Mount the new thermostat onto the wall.

7. Turn on the power to the furnace and test the thermostat.

By following these steps, you can easily install and wire a furnace thermostat with 2 wires.

If you want to regulate a furnace, wall heater, or electric baseboard heater, a two-wire thermostat is all you need. Although limited in comparison to four- and five-wire models, it’s simple to replace an analog White-Rodgers, Emerson, or Honeywell thermostat with a digital or smart thermostat with Wi-Fi capability. However, ensure that your new programmable thermostat has its own power source, typically batteries, as it can’t draw power from the HVAC system and requires an additional wire called a C wire, or common wire.

Before beginning, determine whether your old thermostat is low-voltage or line-voltage. If it controls an electric baseboard heater, it’s probably a line-voltage model, which means that the thermostat wires carry 120-volt electricity or, in some cases, 240-volt. If the thermostat controls a central heating or cooling system, it’s probably a low-voltage model that gets its power from a transformer, and it operates at 24 volts. These two types of thermostats aren’t cross-compatible, so you have to know which one you have.

Here are the basics of installing a low-voltage two-wire thermostat.

How Does a 2-Wire Thermostat Work?

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A thermostat controls temperature by responding to a temperature-sensing device and is essentially a switch. In electronic models, the sensor is typically a thermistor, which responds electrically to changes in temperature. The sensing device in analog models is usually a bimetal strip that responds mechanically to temperature changes, but some models contain a small vial of mercury that tips one way or the other in response to a moving coil. If your old thermostat contains mercury, contact the Thermostat Recycling Corporation for the nearest drop-off and recycling center.

Like a light switch, a two-wire thermostat has one “line” terminal for a hot wire and one “load” terminal to power a single appliance. There are no terminals to control fans, compressors, or heat pump reversing valves, which are common components of more sophisticated HVAC systems. You can use a thermostat with more than two wire terminals as a two-wire thermostat, but the reverse isn’t true. If your HVAC system includes both heating and cooling systems, you require a thermostat capable of accepting more than two wires. You also require more than two wires to control a Wi-Fi thermostat, a smart home programmable model, or a learning thermostat that needs a common wire for power.

Using a Multiwire Thermostat for 2 Wires

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Using a four- or five-wire thermostat to control a two-wire system isn’t an issue; you simply won’t use the thermostat’s full capabilities. A four-wire thermostat has the following terminals:

The R terminal is for a red wire that supplies power, while the W terminal is for a white wire that controls the heating system. The G terminal controls the blower and the Y terminal controls the compressor for the cooling system, with the possibility of a blue wire being connected to it. A five-wire terminal has an additional C terminal for a common wire that powers features in the thermostat like the LED screen or Wi-Fi function. The common wire is typically blue, but it can also be black if the blue wire is used for cooling.

When wiring a thermostat with two wires, connect the red wire to the R terminal and the other wire to its respective terminal based on its color and function. White wires likely control the heater and go on the W terminal, while yellow wires control air conditioning and should go on the Y terminal. If the system only consists of a fan, the green wire can be connected to the G terminal.

To replace an old two-wire thermostat with a new one, turn off the power and remove the existing thermostat by disconnecting the wires and unscrewing the base plate from the wall. It’s recommended to take a picture of the wire configuration before disconnecting them. Tools needed for the job include a Phillips screwdriver, mobile device, torpedo level, drill, drill bits, conical screw anchors, and batteries.

Step 3: Mount the New Thermostat Base Plate on the Wall

Place the base plate against the wall and level it. If your model doesn’t have a built-in level, use a torpedo level. Mark the wall at the screw holes, remove the base plate, and drill a hole at each mark large enough for a conical wall anchor. Insert the anchors, feed the thermostat wires through the hole in the base plate, replace the plate, and secure it with the screws provided. Check the level again before tightening the screws fully.

Step 4: Connect the Wires

Attach the red wire to the R terminal. Most thermostats have an RH terminal for heating and an RC terminal for cooling, with a bonding strap connecting the two. Since you can’t use the thermostat for both, it’s best to remove the strap and connect to either the RH (for heating) or RC (for cooling) terminal depending on your needs.

The connector may be a screw that you turn with a #2 Phillips screwdriver or a plastic spring tab that you hold while inserting the wire end. Connect the other wire, usually white, to the W terminal for controlling a heater or the Y terminal for an air conditioner.

Step 5: Insert the Batteries and Install the Cover

If the thermostat requires batteries, insert them now. The battery compartment may be on the front of the base plate or the back of the cover. Snap the cover onto the base plate. The LED screen should light up if the thermostat has one, and the touch screen should be active if it’s a touch screen model.