How to Wire Two Thermostats to One HVAC Unit

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With a home of any appreciable size, there are bound to be areas that are cooler in temperature than others when you want heat, and hotter than others when you want cool. You can help make a more evenly conditioned home by installing a separate thermostat in another part of the house. When two thermostats are wired to a heating or cooling system with a zone control and dampers installed in the ductwork, you have true separate temperature zones in your home.

Things You'll Need

  • Multi-stranded 18-gauge thermostat wire
  • Two thermostats
  • Pointed pliers
  • Screwdrivers
  • Zone dampers
  • Zone control box

Installing Dampers

  • Draw a simple diagram of your HVAC duct system. Identify the rooms that are fed by each of the supply air ducts and mark those ducts with a marker. Find where the duct branches into the two areas of your home that will have separate thermostats. Measure the ducts and buy zone dampers to fit.

  • Cut a hole in the ductwork with tin snips to fit in a zone damper. It should be properly sized for the ductwork. Some zone dampers are drop-in and some are slide-in, so make sure you cut the hole in the proper location. Secure the damper with screws and seal with silicone caulking. Repeat this step for as many dampers as you are installing.

  • Strip the ends and attach 18-gauge thermostat wire to the damper wire holes. Dampers usually have two or three wires. Run the attached wire to the zone control box.

  • Strip the ends of the wires from the damper and attach them to the zone control box in the areas marked Damper 1 and Damper 2. There will be two screws for each damper. Tighten the wires down with a screwdriver.

  • Mount the zone control box near the furnace, either on the ductwork or a close wall. Run an 18-gauge 4-conductor wire from the control board on the furnace to the terminals marked furnace on the zone control box.

  • Strip the ends of the wire and attach them to the furnace control board. They should be attached as follows:

    White to W

    Red to R

    Green to G

    Yellow to Y

    Attach the other end of the wire in the zone control box to the same color coded terminals in the space marked Furnace.

Installing Thermostats

  • Install a thermostat in each of the areas being fed by the zoned ducts, following the instructions for installation that are included with the thermostat. Run a wire from each thermostat to the zone control box.

  • Attach the wire from the thermostat being fed by Damper 1 to the terminals marked Thermostat 1. Repeat for the second thermostat, but attach to the terminals marked Thermostat 2. Again the color coding of the wires will be as follows:

    White to W

    Red to R

    Green to G

    Yellow to Y

  • Test the thermostats in each area to make sure the system responds to a demand for heating or cooling.

References

  • Photo Credit Jonathan Kantor/Lifesize/Getty Images
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