How to Wire a Furnace Thermostat With 2 Wires

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The simplest setup for a heating system is a two-wire thermostat. If the unit requires only two wires, the system is only able to heat the space. If any cooling, like an air conditioner or fan control, is required, more wires are needed. Most homeowners with moderate home improvement skills can wire a heat-only thermostat. However, basic safety procedures must be followed whenever working with electricity.

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Electricity Safety Basics

It's critical that you read all instructions and the thermostat's manufacturer instructions before attempting any work on the unit. If you have any confusion or questions about the process, you should invoke the help of a professional.

Additionally, prior to getting started on any work that involves electricity, tell those around you what you'll be doing. This is for their own safety as well as yours. Unplug or disconnect all components of the system before beginning work, and turn power off to the area at the circuit breaker box. Whenever possible, use high-voltage gloves and tools with insulated handles while doing electrical work.

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2-Wire Thermostat Basics

Turn off power to the furnace by turning the switch on the side of the furnace to the "off" position. Failure to disconnect power to the furnace before wiring the thermostat can result in damage to the furnace. Follow all other safety instructions as outlined above.

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Run the thermostat wire from your furnace to the location where your thermostat will hang. Drill a hole in the drywall, and use a fish tape to feed the wire from the furnace through the hole in the wall. The wire will run through a dedicated opening on the side of the furnace into the control panel. The opening should have a rubber sleeve to protect the wire from being cut. Next, mount the thermostat wall plate to the wall. Use drywall anchors to fasten the plate.

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Thermostat Color Coding

Most two-wire thermostats wire have a red and white wire encased in a brown insulated coating. Strip the red and white wires back about 1/4 inch at both the thermostat and the furnace ends. Connect the white wire to the "W" terminal on the furnace and thermostat. Repeat this for the red wire, connecting it to the "R" terminal on both the furnace and thermostat.

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The c wire, or common wire, enables the continuous flow of 24 VAC power to the thermostat. Not all thermostat systems have a c wire, which is typically blue. In many instances, a two-wire thermostat does not have a c wire.

Completing the Rewiring Project

Once you have connected the colored wires to the proper terminals, snap the thermostat to the wall plate. Turn the power back on to the furnace by flipping the switch on the side of the furnace to the "on" position.

Turn the thermostat to "heat" mode and program it to the desired setting. If the temperature in the room is colder than the setting, the furnace should cycle on and begin heating the space. The use of a programmable thermostat can help you to save on heating costs. Programmable thermostats automatically lower the temperature when you are asleep or not home. Knowing how to do your own furnace and thermostat maintenance can also go a long way toward helping you to stay on budget.