How to wire a 24v thermostat

Written by eric jonas
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How to wire a 24v thermostat
Replace your old thermostat with a modern model with digital controls. (temperature rising image by Keith Frith from

Most homeowners never have contact with their own heating and cooling systems beyond adjusting the temperature controls on that good, old-fashioned thermostat. Despite this limited experience, however, wiring a new thermostat is not beyond the capabilities of the do-it-yourself individual. Naturally, if any concerns do happen to arise during the installation, it is important to check with a local licensed HVAC technician.

Skill level:

Things you need

  • Multimeter
  • Screwdriver
  • Wire cutters/strippers

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  1. 1

    Turn off all power to the furnace. Service switches are usually found around the furnace, but turning the power off at the circuit breaker is usually the safest way to ensure power is off. As an added precaution, never assume the power is off and always use a multimeter at the circuit board to verify.

  2. 2

    Read the furnace instruction manual and check the wiring diagram to identify where each wire connects. Crossing wires or placing 24-volt wires where 110-volt wires are meant to go can cause serious damage to the equipment. The wires used to connect to a thermostat can range from simple two-wire units to very complex units, depending on the heating and cooling system run by thermostat.

  3. 3

    Remove the furnace faceplate and open the furnace access door. Locate the circuit board and the 24-volt wiring connection on the board.

  4. 4

    Connect each coloured wire according to the letter that the colour belongs to on both the furnace and thermostat. While most manufacturers use the same letter-to-colour format, not all do and any changes will be clearly noted in the furnace instruction manual.

  5. 5

    Put the access door and front panel of the furnace back on and mount the thermostat to the wall. Place the thermostat front cover on.

  6. 6

    Make sure the thermostat is off and then turn on the power to the furnace.

  7. 7

    Program the thermostat for heating and cooling according to the manufacturer's instructions.

  8. 8

    Turn the thermostat on. Lower the temperature on the thermostat and listen for a click indicating that the call for heat has ended. Wait a couple of minutes for the blower to remove any remaining heat and check that the furnace turned off.

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