Thermostat Plate

• Make certain the power to the furnace system is off. Newer furnaces have a shutoff switch positioned near the furnace itself and it usually looks like a standard wall switch. Older homes and older systems may not have a power cutoff switch near the furnace, but power can also be turned off at at the home's circuit breaker panel.
  
  Start by removing the round plastic dust cover from around the thermostat. This piece will pop off easily by simply prying it with your fingers. Once removed, both the thermostat and mounting screws are visible.
  
  Three mounting screws hold the thermostat in place and they are located near the perimeter of the plate. Using a narrow bladed standard screwdriver, loosen all three screws. Be very careful not to damage the small glass vial near the top of the thermostat plate. It contains mercury which can cause serious contamination if released.
  
  Once the screws are removed, the thermostat mounting plate will come off easily. There are no wires connecting it to the mounting plate.
  
  

Wiring

• There are usually four wires connected to four separate terminals on the plate. These wires are color coded, typically green, yellow, red and white. Use a screwdriver to loosen the four terminal screws holding the wires in place and pull the wires free of the terminals. You may need to use needlenose pliers to gently pry the wires loose.
  
  

Mounting Plate

• Two screws secure the mounting plate to the wall. Using a screwdriver, remove both screws. The mounting plate will now come free of the wall. Be sure to remove it gently so that the four wires do not accidentally get pushed inside the wall. If the wires were cut short by the installer, you may need to hold them together between your fingers as you remove the mounting plate.