How to Fix a Low-Voltage Thermostat

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While low-voltage thermostats are very reliable and rarely defective, thermostat-related problems, such as faulty wires or loose connections, may cause a failure in your heating or cooling system. The problems are easy to identify, and just as easy to fix by making the appropriate repair or by replacing a faulty thermostat.

Things You'll Need

  • Artist's Brush
  • Neon Tester
  • Batteries
  • Replacement Thermostat
  • Tape And Pen
  • Level Or Plumb Line
  • Screwdriver
  • Wire Cutters/strippers
  • Turn off the power to heating and/or cooling systems at the main service panel or system shutoff switch.

  • Remove the thermostat. On some models, remove the cover plate and then unscrew the thermostat body from a wired base plate secured to the wall. Other models plug into the base plate without screws.

  • Remove any dust from the thermostat and base plate with a soft artist's brush.

  • Check for broken, frayed or corroded wires and loose wire connections. Tighten any loose connections. Use wire cutters/strippers to cut damaged wires and strip off about 1/2 inch (12 mm) of insulation, then reconnect the wires.

  • Restore the power to test the thermostat. Consult the manual for your thermostat and disconnect the power wire (usually red), then touch it to the terminal for the heat (the terminal marked as W with white wire from heater's transformer).

  • Similarly bypass the thermostat to check its cooling function by touching the disconnected power wire (red) to the terminal for the cooling system (typically yellow).

  • If one or both systems fail to start, turn off their power, then identify (see step 8) and replace any faulty wires between the thermostat and the low-voltage transformer(s) at the heating or air-conditioning units.

  • One way to test thermostat wires--on your heating system, for example--is to disconnect the wires from the thermostat and transformer, wire them together at one end, and test for continuity at the other end by clipping a continuity tester to one wire and probing the other wire. If the tester fails to light, replace the wires.

  • Check for loose low-voltage wire connections at the transformer or loose line-voltage connections to the transformer (see How to Fix a Doorbell).

  • If both systems activate, replace the faulty thermostat (see below).

Replace a thermostat

  • Choose a thermostat suitable for your particular system. Read the thermostat labeling and/or consult with a knowledgeable salesperson or HVAC (heating, ventilating and air-conditioning) service professional.

  • Shut off the power to your heating and/or cooling systems at the main service panel or system shutoff switch.

  • Remove the old thermostat as described above.

  • Disconnect all wires from the base plate one at a time and tape a label on each that identifies its terminal connections (R, Y, W and so forth).

  • If the new thermostat base does not cover the old mounting holes, either patch them and touch up the paint (see How to Repair Minor Drywall Damage) or install the cover ring that the new unit may provide.

  • Mount the new thermostat to the wall, leveling it according to the manufacturer's instructions. Typically, you can align notches with a level or plumb line on the wall, or place a small level on leveling posts at the base or its edge.

  • Use the wiring chart provided in the installation instructions to connect the labeled wires to the appropriate terminals, removing the tape as you make the connections.

  • If applicable, install batteries and program the thermostat.

  • Attach the thermostat to the base and restore power.

Tips & Warnings

  • If you don't have the installation instructions for your thermostat, go online to the manufacturer's Web site and download them, or call to have them faxed or mailed.
  • Make sure disconnected thermostat wires don't slip inside the wall. Pull them out a bit and either tape them to the wall, bend them or loop them around a pencil.
  • If you're replacing a thermostat, choose a programmable model that allows nighttime setback for heating or daytime setback for cooling. This could save up to a third of your energy bill.
  • Unless power is required for low-voltage testing, always shut the power off and follow the manufacturer's wiring instructions. Accidental wire contact or improper connections may damage the thermostat.
  • While low-voltage wiring cannot hurt you, it can startle you, and that may cause an injury. Work with the power off whenever possible.

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