Frequently Asked Questions

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.

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A duct high limit switch protects HVAC equipment and building occupants by shutting down heating equipment if excessive duct temperatures occur due to equipment malfunction or fire conditions.

Visit our applications page to learn more about where these products are commonly found.

Yes.

Firestat offers:

• Multiple probe lengths
• Adjustable or fixed temperature settings
• Private labeling
• Custom configurations for OEM applications.

Contact a product specialist today with your requirements.

Typical applications include:

• Apartment buildings
• Commercial HVAC systems
• Schools
• Hospitals
• Office buildings
• Industrial facilities
• Electric duct heaters
• Building automation systems

Visit our applications page for more information.

In most applications, the Firestat sensing probe should be installed downstream (discharge side) of the heating equipment, where it can accurately monitor the temperature of the heated air entering the duct system.

Installing the probe downstream allows the Firestat to detect abnormal temperature conditions caused by equipment malfunctions, restricted airflow, or overheating before excessive heat can spread through the ductwork.

For best performance:

• Install the sensing probe in the center of the air stream whenever possible.
• Avoid mounting the probe too close to the heating element or heat exchanger, as localized hot spots can cause nuisance trips.
• Follow the minimum installation distances recommended by the equipment manufacturer.
• Install the probe where airflow is representative of the overall duct temperature, avoiding areas with excessive turbulence or dead air.

Some specialized HVAC systems or equipment manufacturers may specify an alternate mounting location. Always follow the installation instructions provided with the heating equipment and the Firestat installation guide for your specific application.

Yes. Firestat duct high limit controls can typically be mounted in either a horizontal or vertical orientation, provided the sensing probe is fully inserted into the ductwork and positioned within the airflow.

For reliable operation:

• Mount the control securely to the duct surface.
• Ensure the sensing probe extends into the airstream as specified in the installation instructions.
• Avoid locations where the probe may contact duct walls, heating elements, or other obstructions.
• Follow the recommended installation location relative to the heating equipment for accurate temperature sensing.

While the mounting orientation of the control is generally flexible, proper probe placement within the duct is the most important factor for accurate temperature monitoring and dependable operation. Always follow the installation instructions provided with your Firestat model and the HVAC equipment manufacturer’s recommendations.

When the preset temperature is reached:

• The internal bi-metal sensing element activates.
• The electrical contacts change state.
• The HVAC system shuts down (or performs the connected safety function).
• Manual reset is required before restarting.

No.

Firestat duct controls require manual reset after tripping. This ensures the cause of the over-temperature condition is investigated before the HVAC system is restarted.

To ensure the potential fire does not spread to the rest of the building. The manual switch ensures that human interaction is needed in order to reset that switch to allow the furnace/fans to turn back on. If an automatic switch was used and switched the furnace/fans back on too soon, it could cause the potential fire to be spread throughout the building.

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Before resetting a Firestat, identify and correct the cause of the over-temperature condition. The Firestat is designed to require a manual reset to help ensure the HVAC system is inspected before being returned to service.

To manually reset the Firestat:

1. Allow the duct temperature to return to a safe operating level.
2. Inspect the HVAC system for the cause of the high-temperature condition, such as restricted airflow, a dirty air filter, blower failure, or a malfunctioning heating component.
3. Once the issue has been corrected and the system has cooled, press the manual reset button located on the Firestat control until it clicks into place.
4. Restore power to the HVAC system and verify that it operates normally.

If the Firestat trips again after being reset, do not continue resetting the control. Repeated tripping indicates an underlying problem that should be diagnosed and repaired before the system is returned to service.

Always follow the  installation instructions provided with your Firestat model and the HVAC equipment manufacturer’s service recommendations.

If your Firestat will not reset, it usually means the underlying over-temperature condition has not been corrected or the control has not cooled below its reset temperature.

Common reasons a Firestat won’t reset include:

• The duct temperature is still too high. Allow the HVAC system to cool completely before attempting to reset the control.
• The cause of the overheating has not been resolved. Common issues include restricted airflow, clogged air filters, blocked ducts, blower motor failure, or a malfunctioning heating element or heat exchanger.
• The reset button has not been fully engaged. Press the manual reset button firmly until it clicks into place.
• The Firestat has reached the end of its service life or has been damaged. If the control will not reset after the system has cooled and the cause of the trip has been corrected, the switch may need to be replaced.

If the Firestat repeatedly trips or will not reset, do not bypass the control or continue attempting to reset it. Frequent tripping is a safety warning that indicates an HVAC system problem requiring inspection and repair.

If you need assistance troubleshooting your Firestat or selecting a replacement, our technical support team is available to help.  Contact us today.

Firestat duct high limit controls should be tested during routine HVAC preventive maintenance to help ensure they are operating properly. For most commercial and industrial HVAC systems, annual testing is recommended. However, facilities with critical environments or equipment that operates continuously may require more frequent inspections based on the equipment manufacturer’s maintenance schedule or local codes.

A typical inspection should include:

• Verifying the Firestat is securely mounted and the sensing probe is properly positioned in the duct.
• Inspecting wiring and electrical connections for signs of damage or corrosion.
• Confirming the manual reset mechanism operates correctly (if applicable).
• Testing the control’s ability to interrupt the circuit when the preset temperature is reached, following the manufacturer’s recommended test procedure.
• Returning the system to normal operation after testing.

If a Firestat trips unexpectedly or repeatedly, it should be inspected immediately rather than waiting for the next scheduled maintenance. Frequent tripping often indicates an HVAC system issue, such as restricted airflow, blower failure, or overheating, that should be corrected before the system is placed back into service.

Always follow the maintenance recommendations for your HVAC equipment and the  installation instructions  for your specific Firestat model.

Adjustable models

• Temperature can be changed in the field
• Ideal when system temperatures vary

Fixed models

• Factory calibrated
• No field adjustment
• Excellent for standardized installations

Visit our product page to learn more about available options.

Standard sensing element lengths include:

• 5″
• 7.5″
• 11″

Additional custom options may be available. Visit our product page to learn more about available options.

Yes. Firestat offers a variety of customization options to meet the needs of OEMs, equipment manufacturers, and specialized HVAC applications.

Available customization options may include:

• Fixed or adjustable temperature settings
• Multiple probe lengths to accommodate different duct sizes and installation requirements
• Custom temperature calibrations for specific applications
• Private labeling and custom branding
• OEM-specific configurations
• Custom packaging and labeling
• Application-specific electrical and mounting options

Our engineering and manufacturing teams work closely with customers to develop solutions that meet their performance, installation, and regulatory requirements.

If you have unique application requirements or need a custom duct high limit control, contact our team to discuss your specifications. We’ll help determine the best configuration for your project.

Yes.

Firestat controls can be wired in both low-voltage and line-voltage control circuits depending on the application.

Yes.

When multiple switches protect the same heating source, they are typically wired in series so any activated switch shuts down the equipment.

Our duct limit switches can be a direct replacement option to many other models in the marketplace today.  Click here to reference your part number to ours!

Yes. Firestat offers a variety of customization options to meet the needs of OEMs, equipment manufacturers, and specialized HVAC applications.

Available customization options may include:

• Fixed or adjustable temperature settings
• Multiple probe lengths to accommodate different duct sizes and installation requirements
• Custom temperature calibrations for specific applications
• Private labeling and custom branding
• OEM-specific configurations
• Custom packaging and labeling
• Application-specific electrical and mounting options

Our engineering and manufacturing teams work closely with customers to develop solutions that meet their performance, installation, and regulatory requirements.

If you have unique application requirements or need a custom duct high limit control, contact our team to discuss your specifications. We’ll help determine the best configuration for your project.

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.

Check out this article to learn more!

Normally Closed (NC) contacts that open when the preset temperature is reached

Most Firestat controls are designed to interrupt the heating circuit when an over-temperature condition is detected, but specific wiring requirements vary by application.

If you’re replacing an existing duct high limit control or specifying a Firestat for a new installation, our technical support team can help you select the correct contact configuration for your system.

Firestat duct high limit controls are available in a range of temperature settings to suit a variety of HVAC safety applications.

• Adjustable models can be field set from 100°F to 250°F (38°C to 121°C) and are factory preset to 125°F (52°C).
• Fixed-temperature models are factory calibrated to trip at a specific temperature and are available in a variety of preset temperature ratings.

The appropriate operating temperature depends on your HVAC equipment, normal duct air temperature, and the level of over-temperature protection required. As a general guideline, the trip temperature should be set approximately 40°F above the normal operating air temperature in the duct, unless otherwise specified by the equipment manufacturer.

If you’re unsure which temperature range is appropriate for your application, our technical support team can help you select the correct Firestat model based on your system’s operating conditions and safety requirements.

Firestat duct high limit controls are available with a range of electrical ratings to accommodate both low-voltage and line-voltage HVAC control applications. Electrical ratings vary by model and are designed to meet the requirements of different heating equipment and control circuits.

When selecting a Firestat, consider:

• Control circuit voltage
• Current (amperage) requirements
• Contact configuration (NO, NC, or SPDT)
• Type of electrical load being switched
• Applicable agency approvals and certifications

For detailed voltage, current, and switching capacity ratings, refer to the specifications on the individual product page or the product datasheet for your Firestat model.

If you’re replacing an existing duct high limit control or specifying a Firestat for a new application, our technical support team can help you select a model with the appropriate electrical ratings for your system.

Yes.

Firestat controls are UL recognized and manufactured in the USA. Refer to the individual product pages for specific certifications and ratings.

Firestat strives to ship orders as quickly as possible. Many standard products are in stock and can ship within 1–2 business days, while lead times for larger quantities, custom configurations, or OEM orders may vary.

Shipping times depend on several factors, including:

• Product availability
• Order quantity
• Standard or custom configuration
• Shipping method selected
• Delivery destination

Once your order has shipped, you’ll receive a shipping confirmation and tracking information (when applicable).

If you have an urgent requirement or need expedited shipping, please contact our sales team before placing your order. We’ll do our best to accommodate your schedule and provide the fastest available delivery.

Yes. Firestat offers OEM pricing for equipment manufacturers, distributors, and customers purchasing production quantities.

OEM pricing is based on factors such as:

• Annual usage or forecasted volume
• Product configuration
• Standard or custom specifications
• Packaging and labeling requirements
• Long-term supply agreements

In addition to competitive pricing, Firestat supports OEM customers with custom product configurations, private labeling, engineering assistance, and reliable manufacturing in the USA.

If you’re interested in OEM pricing, contact our sales team with your application details, estimated annual volume, and product requirements. We’ll review your project and provide a customized quotation based on your needs.

Yes. Firestat provides engineering and technical support to help customers select, integrate, and apply the right duct high limit control for their HVAC system or OEM application.

Our team can assist with:

• Product selection and application guidance
• Temperature setting recommendations
• Probe length selection
• Electrical contact configuration selection
• Installation and wiring questions
• Troubleshooting and technical support
• Custom product development for OEM applications
• Private labeling and custom configurations

Whether you’re replacing an existing duct high limit control, designing new HVAC equipment, or developing a custom safety solution, our experienced team can help you identify the best Firestat product for your requirements.

Contact us with your application details, equipment specifications, or existing part number, and we’ll work with you to recommend the right solution.

Basic installation includes:

• Select the proper sensing location.
• Mount the probe in the duct.
• Wire according to the supplied wiring diagram.
• Set the desired temperature (adjustable models).
• Test system operation.

Always follow the installation instructions included with the product. For more details,  click here!

Firestat is always interested in partnering with qualified distributors who serve the HVAC, electrical, industrial, and OEM markets.

As a Firestat distributor, you may benefit from:

• High-quality duct high limit controls manufactured in the USA
• Competitive distributor pricing
• Technical product support and application assistance
• Reliable product availability
• Custom and OEM solutions
• Responsive customer service

To become a distributor, contact our sales team with the following information:

• Company name and location
• Markets and territories you serve
• Product lines you currently represent
• Customer base and industry focus
• Estimated annual sales volume or business opportunity

Our team will review your information and discuss available partnership opportunities, pricing, and support programs to determine the best fit for your business.

We look forward to helping you grow your business by offering dependable HVAC safety controls backed by knowledgeable technical support.

Yes.

Firestat products can be purchased directly through our website or by contacting our sales team for larger projects.

CEMCO firestats are now manufactured by Senasys in Eau Claire, Wisconsin, U.S.A.

CEMCO firestat duct limit switches can be found worldwide. We can ship directly to you, or you can work with one of our distributors. 

Click here to contact us today!

Yes. Senasys gained ISO 9001 certification in 2026. Please contact us for certificate.