Duct Fan/Limit Switch Product Development

Recently, a manufacture reached out to us with a temperature monitoring application where they would like to have a thermostat switch that controlled the fan and limit. It got us thinking, maybe the creation of duct fan limit switches would be an appropriate market to jump into. Since we already produce multiple limit thermostats with fixed and adjustable temperatures, it fits in perfectly with our product line. We are currently working on the Research and Development for other duct control possibilities. Our current product offering for the TC Series only opens on rise. We are looking at a close on temperature rise option; and a DPDT switch, which will open and close two sets of contacts when there is a change in temperature. Click to take a look at our Firestat Limit Switch options!

In the meantime, since we didn’t have a drop-in option for this manufacture using our TC Series, we came up with a solution using our snap disc thermostats. Senasys was able to achieve the temperature range that the manufacture was looking for. Senasys did this by using one snap disc to control the fan and the other snap disc as a high limit. Click on the diagram for further information.

Bi-Metal Snap Disc ThermostatsDuct Fan Limit Switches

Senasys manufactures a wide variety of bimetal, fixed temperature, snap disc thermostat switches. Available as a 3/4″ for a larger surface area giving you a more accurate response or 1/2″ surface area for those small spaces. Quality-engineered thermal sensors specifically designed for a variety of major appliances, including clothes dryers, hot liquid and food-dispensing machines, room unit and space heaters, air conditioning systems, copy machines, engine heaters, and wood stoves.

Senasys bimetal snap disc thermostat switches are available in four different options: automatic reset, hermetically epoxy sealed, manual reset, or single shot.

Duct Fan Limit SwitchesFirestat duct fan limit switches that we currently offer are installed in air ducts to shut down heating, air conditioning and ventilation equipment when air temperature inside the duct exceeds a pre-set limit. 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, including apartment buildings. The adjustable duct limit switch can be set by the operator to sense 100°F to 250°F, and our standard fixed temperatures include, 125°F, 165°F or 200°F.

A dual fan/limit switch works the same way the limit switch does, it simply has the ability to not only turn off the fan on the limit side, but it has the ability to turn the fan on as well. When the air temperature rises inside the duct where the unit is mounted, (meaning the furnace is on and running) it triggers the unit at a certain set temperature to close the contacts and turn on the fan to disperse the hot air through the ducts.  As soon as the temperature in the duct begins to fall back down to normal inside the duct, (meaning the furnace is no longer running) it triggers the unit at a certain set temperature to open the contacts and turn the fan back off, as to not blow cold air through the ducts after the furnace is done running. (Note: If the temperature continues to rise past the set “fan on” temperature, and reaches that high limit pre-set temperature, this is when it triggers the contacts to break and the fan to turn off to ensure no smoke or fire gets blown and spread through the ducts. At this point it would have to be manually reset.)


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CEMCO Duct Limit Switches

The Firestat Duct Fan and Limit Thermostat Switches operate using the same physics to ensure HVAC safety. A helix bi-metal element senses a rapid temperature change. When the temperature changes the bi-metal expands and triggers a snap action switch and either opens or closes a circuit. These switches are commonly installed in air ducts designed to shut down heating, air conditioning and ventilation equipment in apartment complexes and other multiple dwelling units when air temperature exceeds the set point.

UL & cUL Approved
Made in the U.S.A.