How to Replace a Honeywell 2-Wire Thermostat

If your heating system has only two thermostat wires, the job of the thermostat is simple. All it has to do is turn the heat or the cooling on and off. There's no auxiliary fan to worry about, no heat pump system with its compressor and reversing valve, and no common wire (C-wire). A C-wire provides power to a digital or programmable thermostat; most two-wire thermostats are mechanical and do not need a power source.

The Honeywell T87F and CT87A thermostats are good choices for such a setup. Both of these models have mechanical temperature sensors and don't need a C-wire or batteries for power, and they're easy to mount. Because there are only two wires to connect, this type of thermostat wiring is easy, and there isn't much that can go wrong.

When you remove the backplate of a Honeywell thermostat, you'll find the connection terminals; the number depends on the sophistication of the model. Whether there are two, five, or more, however, they all bear generic labeling that is the same on any thermostat. The terminal labels correspond to wire colors and have specific functions. The thermostat wire color code is as follows:

• ‌R terminal:‌ This is for the red wire (R-wire) that carries power from the system transformer. When it operates a heater, it's called the Rh-wire, and when it operates a cooling system, it's the Rc-wire. Sometimes the Rh and Rc terminals are bonded, and the R-wire performs both functions.
• ‌W terminal:‌ This is for the white wire, which forms the return path back to the transformer to complete the circuit. Depending on the complexity of the HVAC system, the white wire might be used for a different purpose, such as second-stage heat pump heating.

When doing thermostat wiring with only two terminals, the red and white wires are the only two you need to worry about. However, since you can adapt any Honeywell thermostat to work with only two wires, it's good to know what the other terminals are for.

• ‌G terminal:‌ The green wire (G-wire) controls the system blower, which is a separate component of the HVAC system located in the air handler along with the furnace or air conditioning unit.
• ‌C terminal:‌ The C-wire (common wire) is a blue wire that forms a return circuit for the thermostat itself. A thermostat with batteries doesn't need a C-wire, so if you have a battery-powered model, the blue wire is probably controlling the reversing valve for a heat pump.
• ‌Y terminal:‌ "Y" stands for yellow, and when there is a yellow wire, it controls a primary cooling system.

Most multifunction thermostats have these wires. A four-wire thermostat doesn't have a yellow wire and is usually battery-powered, which frees up the blue wire to control a heat pump reversing valve. In a battery-powered five-wire thermostat, the yellow wire controls the cooling system while the white wire controls the heating system.

There are wires of other colors you might find on a more sophisticated model, such as a smart thermostat, like a Nest thermostat. For example, you might find an O terminal, which corresponds to an orange wire for controlling a heat pump reversing valve, or there may be a terminal for a brown thermostat wire — or a wire of a different color — to control second-stage cooling. You don't have to worry about any of these when doing two-wire thermostat wiring.

Replacing a low-voltage HVAC thermostat is not a hazardous DIY task, but a line-voltage thermostat is a different matter. If your thermostat controls a baseboard heater, the wires carry 120 VAC power, so it's best to have the job done by an electrician. A homeowner can safely replace a thermostat that controls a furnace, propane heater, or air conditioner, however, because it operates at 24 volts, which isn't enough voltage to deliver a serious shock. Nevertheless, it's a safe practice to turn off the power for the system before you start. Here's how to do it step by step:

Locate the circuit breaker in your home's service panel (breaker box) that controls the heating or cooling system. Switch the breaker to the OFF position.

Unsnap the cover from the old thermostat or, if there are screws, remove them using a screwdriver. After exposing the wall plate, loosen the screws holding the red and white wires to their respective terminals and remove the wires. Unscrew the wall plate and remove it carefully from the wall, being careful not to let the wires fall back into the wall.

The new thermostat has to be mounted level for the temperature sensor to work properly. The mounting plate has two leveling posts on which you can rest a torpedo level. Position the plate so the bubble on the level is centered between the lines, then mark through the plate's screw holes onto the wall with a pencil. Remove the plate and install a screw anchor at each mark. Feed the wires through the wire opening in the plate, position the plate over the anchors, and secure it with screws driven into the anchors.

The new thermostat may have different wiring terminals than the old one, or the configurations may be different, but since there are only two wires, you shouldn't need to consult a wiring diagram for terminal designations.

Start by connecting the red wire to the Rc or Rh terminal. If your Honeywell thermostat controls a heater, connect the other wire to the W terminal. For thermostats that control air conditioning systems, use the Y terminal instead of the W terminal. The setup is the same for Honeywell Wi-Fi thermostats in the Honeywell Home line.

The final step in thermostat installation is to snap the cover onto the wall plate. If your new thermostat needs batteries, place these in the battery compartment first. The LED screen should illuminate.

Switch on the circuit breaker to restore power. Set the thermostat to a high temperature (if you have a heating system) or to a low temperature (if you have a cooling system), and wait for the heating or cooling system to come on. Check the wiring if nothing happens. If you're replacing the thermostat because of a problem with the operation of your HVAC system — and the problem continues with the new thermostat — you may need professional help with troubleshooting the problem.