Duct High Limit Controls

The appropriate temperature setting depends on the HVAC system and its normal operating conditions.

For an adjustable Firestat, one general starting point is to measure the normal operating air temperature near the control and set the limit approximately 40°F above the normal operating temperature, unless the equipment manufacturer, applicable code, engineering requirements, or system design specifies otherwise.

The high limit should be high enough to prevent nuisance trips during normal operation while still providing the desired protection against abnormal temperature increases.

If you’re replacing an existing duct limit switch, use the original equipment specifications and application requirements when determining the appropriate replacement.

Probe length and temperature setting are two separate considerations.

First, select the correct sensing element length for the duct. Then determine the appropriate temperature setting based on the HVAC equipment, normal operating temperature, applicable requirements, and the intended safety function.

Firestat adjustable duct limit controls provide a 100°F to 250°F adjustment range, allowing the temperature limit to be selected for the specific application.

When selecting a temperature setting, the goal is generally to provide enough separation from normal operating temperatures to avoid nuisance trips while still responding appropriately to an abnormal high-temperature condition.

Always follow the equipment manufacturer’s specifications and applicable installation requirements.

Choosing the correct bimetal probe length is an important part of selecting a duct limit switch. The sensing element must extend far enough into the duct to accurately measure the temperature of the moving air. Temperature is not necessarily uniform throughout an air duct. Air flowing through a duct can be affected by:

• Duct size
• Air velocity
• Pressure changes
• Turbulence
• Turns and bends
• Heating equipment
• Obstructions within the duct
• The distance from the duct wall

The air near the center of a duct can have a different temperature than the air near the duct wall.

For this reason, the goal when installing a duct limit switch is generally to position the sensing element as close to the center of the air stream as practical.

A properly positioned probe gives the bimetal element better exposure to the air being monitored.

If the probe is too short, the sensing element may be positioned too close to the duct wall. Because air temperature can vary throughout the duct, this can result in an inaccurate temperature reading and may cause the limit switch to trip too early—or fail to respond as expected when temperatures rise.

The correct probe length depends primarily on the size and configuration of the duct.

Firestat adjustable duct limit switches are available with 5-inch, 7.5-inch, and 11-inch bimetal sensing elements , making it possible to select a configuration appropriate for different duct sizes.

• The 5-inch option  is well suited to smaller or shallower duct applications where a longer probe is unnecessary. It can also be useful when installation space is limited.
• The 7.5-inch option  provides additional reach for medium-sized duct applications. It can be a good choice when a 5-inch element does not extend far enough into the airflow.
• The 11-inch element  is designed for larger or deeper duct applications where additional sensing-element length is required. Before selecting an 11-inch element, verify that sufficient clearance is available inside the duct.

These lengths should be considered a starting point rather than a universal duct-size chart. The actual installation location, duct dimensions, airflow pattern, and equipment design should also be considered.

A simple rule to follow is: Choose a probe length that allows the sensing element to extend close to the center of the duct without contacting the opposite wall or interfering with other components.

A duct limit control, also called a duct high limit switch or duct temperature limit switch, is a temperature-activated safety device used in HVAC ductwork. Its primary purpose is to detect excessive duct temperatures and shut down connected heating or ventilation equipment before an abnormal temperature condition becomes more serious.

Firestat duct high limit controls use a helix bi-metal sensing element to monitor air temperature inside the duct. When the temperature reaches the control’s preset limit, the bi-metal element activates a snap-action switch and opens the electrical circuit. This can shut down the connected heating equipment, blower, or other HVAC components.

Unlike an automatic-reset thermostat, Firestat controls require a manual reset after the high-temperature condition has cleared. This provides an additional layer of safety by requiring someone to verify that the abnormal condition has been addressed before the HVAC equipment can restart.

The process works in several steps:

1. The sensing element monitors duct temperature

The Firestat’s sensing element extends into the air stream inside the duct. As heated air passes over the element, the bi-metal coil responds to changes in temperature.

Proper placement of the sensing element is important because it needs to be exposed to representative duct airflow for reliable temperature detection.

2. The bi-metal element responds to rising temperature

A bi-metal consists of two different metals that expand at different rates when heated.

When the temperature increases, the two metals expand differently. Because they are bonded together, this differential expansion causes the material to move.

Firestat uses this principle in a coiled, or helix, bi-metal element. As the temperature rises, the coil moves and eventually activates the switch mechanism.

3. The switch contacts change state

When the duct temperature reaches the control’s set point, the movement of the bi-metal element activates a snap-action switch.

For Firestat duct high limit controls, the normally closed contacts open when the temperature rises to the specified limit. This interrupts the connected electrical circuit.

4. The HVAC equipment shuts down

The Firestat can be wired into the appropriate HVAC safety or control circuit. When the high-temperature limit is reached and the contacts open, the connected equipment can be shut down.

Depending on the system design, the control may interrupt the heating equipment, blower, or another circuit associated with the HVAC system.

The specific wiring configuration should always follow the equipment manufacturer’s requirements and the Firestat installation instructions.

5. The control must be manually reset

After the high-temperature condition has been resolved and the sensing element has cooled below the reset temperature, the Firestat can be manually reset.

The manual reset feature is an important part of the safety function. Rather than automatically restarting after the temperature falls, the control requires human intervention before the circuit can return to its normal operating condition.