How Does A Firestat Work?


These Firestat duct fan and limit switches are installed in air ducts to shut down heating, air conditioning and ventilation equipment when air temperature exceeds a pre-set limit. Our CEMCO Firestat switch division commonly are installed in apartment complexes or equipment that require several units.

You may be wondering though, how do these work? The Duct High Temperature Switches feature a helix bimetal element that senses rapid increases in duct temperature, such as those caused by fire. The high temperature limit then operates an SPST switch that opens with a rise in temperature. A manual reset button reactivates the temperature limit after the high-temperature condition has cleared. These duct high temperature limits are indispensable in specialized HVAC applications and Building Automation Systems.

 

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Now, let’s talk about how the internal components in these switches work!

 

HOW DOES BIMETAL WORK?

If you’ve looked into these kinds of temperature switches before, you’ve most likely heard the term “Bimetal”. But what does that mean? The way Bimetal works is that two different types of metal are fused together. The two metals have different coefficients of thermal expansion, which basically means that they expand at different rates when they heat up. In the case of our switches, the bimetal is in a coil, and the expansion and contraction of the coil is what causes the switch to trigger and open the contacts.

The Duct High Temperature Switches feature a helix bimetal element that senses rapid increases in duct temperature, such as those caused by fire. The high temperature limit then operates an SPST switch that opens with a rise in temperature. A manual reset button reactivates the temperature limit after the high-temperature condition has cleared. These duct high temperature limits are indispensable in specialized HVAC applications and Building Automation Systems.

Bimetal thermostats are commonly used because of how versatile and robust they are. They can sense a wide range of temperatures and last a long time. They are also entirely physics based, so they do not draw power or require any sort of software.

If you want to learn more about bimetal and the specifics on how it works, check out this great website.

SWITCH AND MANUAL RESET INSIDE A FIRESTAT

As we said above, our Firestat switches use a bimetal coil to open a set of contacts when they heat up. However, you may be wondering what these contacts actually look like, and how they are reset after the switch has been triggered. When placed in an HVAC duct system, it will sense the air temperature. Based on what temperature the device is set at, it will open the contacts whenever that temp is reached, and stop the fan from running and spreading the potential fire or smoke it detected to more areas of the building.

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When the switch heats up, the contacts open as seen in the picture below:

Once the contacts are opened, the switch needs to be manually reset. However, the switch can only be reset once the temperature of the bimetal goes below whatever the switch is set to. This manual reset button reactivates the temperature limit after the high-temperature condition has cleared. It is a manual reset as to make sure someone has to physically check the area to make sure the danger has cleared before the device can be reset back to normal. This prevents any further damage from happening if it were an auto reset and did not require human intervention.

Once the switch cools down, it is reset with the level on the top of the switch, as seen below. You simply pick up on the metal flap and let it “snap” back down, resetting the switch inside the device:

That’s all there is to it! Firestat switches are easy to install and use. If you have any questions, be sure to contact us here.

 

Click here to purchase one of our Firestat units today!

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