How to Exchange a Round Thermostat for a Programmable One


Programmable digital thermostats help save energy (and money) by automatically controlling your home’s heating and cooling needs according to your daily schedules. With a programmable thermostat, you don’t have to worry about lowering the temperature setting before leaving the house for the day or when going to bed at night—the thermostat does it for you. Most programmable thermostats allow for four daily settings, and some let you program different settings for the weekdays and weekends. Swapping an old round-style thermostat with a new digital model is an easy project, provided the old thermostat is low-voltage, identified by four thin colored wires connected to the thermostat screw terminals. If your old thermostat is mounted to an electrical box and connected to a standard (120-volt) household circuit cable, you have a line-voltage thermostat; hire an electrician to replace the thermostat.

Things You'll Need

  • Screwdrivers
  • Masking tape
  • Shut off the electrical power to your furnace by switching off the breaker at the home’s service panel (breaker box). Pull off the decorative ring or face from the old thermostat. Unscrew the body of the thermostat from the base and carefully place the body in a safe location.

  • Disconnect each low-voltage wire from its screw terminal on the thermostat base, and label it with masking tape before moving to the next wire. On the tape label, write the letter or symbol next to the screw terminal the wire was attached to. Tape the low-voltage cable to the wall to prevent it from slipping into the wall cavity. Remove the thermostat base from the wall.

  • Disassemble the new thermostat as needed. Thread the low-voltage cable through the new base, and mount the base to the wall. Use hollow-wall anchors to secure the mounting screws if necessary.

  • Connect the low-voltage wires to the terminals on the new base, following the manufacturer’s wiring diagram. Install a backup battery into the thermostat body/cover, as applicable, then fit the body/cover over the base. Program the thermostat as directed. Restore power to the furnace.

Tips & Warnings

  • Mercury-type thermostats contain one or more glass vials filled with liquid mercury, a highly toxic material. Dispose of an old thermostat body at a local hazardous waste or mercury recycling site.

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  • Photo Credit Ryan McVay/Photodisc/Getty Images
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