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How Thermostats Work
Installing a Thermostat


Modern thermostats for heating or air conditioning systems rarely break down. The only maintenance required is an occasional light cleaning. Don't attempt to repair a defective thermostat; instead, replace the entire unit with a new one. Be sure the replacement is the correct voltage and type for your system.

How Thermostats Work

Thermostats are switches that are turned on by a temperature-sensitive device that, in turn, activates the switch controlling the operation of a boiler, furnace, electric heater, air conditioner, or other heating or cooling device. Low-voltage and milivolt thermostats are the most common types. The three principal parts of a thermostat are:

  • Heat Sensor Usually bi-metal coil, the heat sensor contracts as it cools, tripping the switch to "on" and expands as it warms, tripping the switch to "off".
  • Switch The switch may have open contacts (in older models) or a mercury-type contact enclosed in an airtight glass tube.
  • Heat anticipator (found in low-voltage types only) The anticipator prevents the living area from overheating by shutting off the boiler or furnace just before the desired temperature is reached.

Installing a Thermostat

  • After removing the old thermostat, feed the wires through the hole in the new base plate and insert the screws.
  • Level the plate, then tighten the screws.
  • Strip the ends of the wires if needed, or scrape them clean.
  • Wrap the ends clockwise around the terminal screws and tighten the screws.
  • Mount the thermostat on the base plate.
  • Using a pencil, set the anticipator pointer to match the current value marked on the gas valve.
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