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The Mercury Switch Thermostat | HVAC Control - Mercury switch thermostats are not available for purchase in many states as they have been banned because they contain mercury and it is important to properly dispose of these thermostats that contain mercury if they are changed out for newer digital thermostats. There are many homes and businesses that still have these thermostats in use so we will offer a brief of how these thermostats work including the components inside the thermostat.

  • Two metal contacts inside the mercury bulb indicate the thermostat is either a heat only or an air conditioning only thermostat.
  • Three metal contacts inside the mercury bulb indicate the thermostat is an air conditioning and heating thermostat.
  • Two mercury bulbs with two contacts each indicate a dual control thermostat. The thermostat is fully automatic and does not require the user to change a manual selection switch for either air conditioning or heating. The user can set the switch to automatic and the thermostat will maintain user selectable set points based on the setting of the temperature selectors. There is a built in dead band between heating and cooling to prevent the air conditioning and heating from operating at the same time.

The Mercury Switch Thermostat

The Mercury Switch Thermostat - mechanical thermostatOther uses for mercury bulb thermostats include uses for zone valves and boiler controls. These thermostats operate in much the same manner as air conditioning and heating thermostats but control different functions for heating applications.

The Mercury Switch Thermostat - The Thermostat Bi-metal Strip

The mercury bulb and contacts in a mercury thermostat are simply the switch mechanism which closes contacts to complete a circuit. The mercury bulb or switch mechanism is connected to a bi-metal strip. The bi-metal strip in a thermostat is made of two dissimilar metals which are bonded together and have a distinct and calculated reaction to temperature.

The bi-metal strip is wound in a loop or spiral and when it gets hot and/or cold it expands and contracts according to the temperature. This causes the mercury bulb to move. This tilts the mercury bulb inside the thermostat in one direction or the other depending on the temperature. When the thermostat is set to heat and the temperature gets cold the bimetal moves and the bulb tilts causing the mercury to slide in the bulb to complete the circuit and call for heat.

As the temperature warms the bi-metal reacts to the warming and when the bi-metal reaches the user selected set point it causes the mercury bulb to tilt and the mercury to slide breaking the circuit on a call for heat. The same sequence happens on a call for air conditioning except the mercury in bulb completes the circuit on a different set of contacts which control the air conditioning.

The Mercury Switch Thermostat - Mercury Thermostat Disposal

It is important that when the mercury thermostat is replaced by another thermostat that the thermostat is disposed of properly. Mercury is a highly toxic metal and care is not taken in disposal of the mercury the environment suffers. This directly affects things like water purity, wildlife, and even the health of humans. Call your local sanitation department or consult with a thermostat supplier. Typically they have programs set up to ensure that these thermostats are disposed of properly.

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The Mercury Switch Thermostat

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