How to Adjust a Mercury Thermostat


Mercury thermostats are outdated and relatively unreliable compared to modern, digital thermostats. The age of a mercury thermostat or general wear and tear causes the units to become largely inaccurate and in need of adjustment. However, adjusting the unit is much simpler than replacing. Mercury thermostats have two options for adjustment; leveling the unit and adjusting the heat anticipator, which is what allows a heating or cooling unit to activate before the house becomes too cold or too hot.

Things You'll Need

  • Level
  • Screwdriver
  • Remove the face of the unit; it should just pop straight off.

  • Loosen the retaining screws located along the back of the device with a screwdriver. They will be visible once the face is removed.

  • Set a level on top of the unit and adjust the position as needed until the thermostat is aligned. Tighten the retaining screws.

  • Slide the heat anticipator lever, which is visible just above or to the side of the mercury vial and is very small, left or right to increase or decrease, respectively, the time it takes for the furnace to activate when the specific temperature is reached.

Tips & Warnings

  • Replacing the unit may be preferred as digital thermostats do not require any adjustments. Most digital thermostats come with explicit installation instructions allowing any homeowner to properly install the units.


  • Photo Credit Jupiterimages/Creatas/Getty Images
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