How does a vapor sensor work?

What Is A Flammable Vapor Sensor?

Flammable vapor sensors are likely to ring a bell if you’ve ever encountered gasoline-based water heaters. Their role in ensuring security makes them an integral component of water heating systems.

Understanding these sensors becomes vital to undertake water heater maintenance independently.

Within this article, we delve into a comprehensive exploration of flammable vapor sensors. In addition to shedding light on their operational mechanisms, we provide insights into installation and maintenance procedures.


Rheem SP20172 Flammable Vapor FV Sensor Kit White, 6.7X2.7X9.4 inches


Rheem SP20172 Flammable Vapor FV Sensor Kit White, 6.7X2.7X9.4 inches

What is a flammable vapor sensor?

Modern equipment incorporates cutting-edge safety technology to mitigate the risk of unforeseen and catastrophic incidents. Among these safety measures are flammable vapor sensors specifically designed for installation in water heaters.

Within a water heater system, a flammable vapor sensor functions as a surveillance mechanism, diligently detecting the presence of gasoline components in the surrounding air. Promptly initiating a troubleshooting protocol is a crucial safety feature, preventing potential harm or damage.

In the event of a gas leak, the sensor promptly identifies the threat and triggers a system shutdown. This action effectively interrupts the circuit, preventing the combustion of flammable vapors. Typically, the combustible vapor sensor is strategically positioned at the bottom of the heater.

Top-quality sensors possess remarkable sensitivity, allowing them to discern specific threats without being influenced by other potentially dangerous substances, such as paint fumes. Consequently, water heaters with superior flammable vapor sensors can eliminate false positives. With heightened accuracy, end-users can enjoy security and peace of mind.


Bradford White OEM Flammable Vapor Sensor 239-45560-00 Water Heater Replacement Part 2394556000


Bradford White OEM Flammable Vapor Sensor 239-45560-00 Water Heater Replacement Part 2394556000

How does a flammable Vapor sensor work?

The combustible vapor sensor is a remarkable design innovation, serving the critical purpose of detecting volatile elements within the environment and initiating a troubleshooting mechanism primarily to prevent gasoline spills. Its installation in water heaters is imperative, as these devices are susceptible to fire if flammable vapors permeate their gas chambers.

To avert such unforeseen incidents, the combustible vapor sensor plays a crucial role by promptly identifying the presence of such elements early, ceasing the gas flow, and shutting down the water heater. Employing a chemical absorption-based mechanism, this sensor detects volatile substances in the surroundings, triggering a lockout mode.

Crafted with a silicon-based material, the sensor comprises carbon particles in a solid-state configuration. When gasoline vapors are present near the sensor, the silicon expands, causing a corresponding expansion of the carbon particles. The magnitude of silicon film expansion correlates with the concentration of vapor. Higher vapor concentrations lead to more excellent resistance.

Each sensor device possesses its unique tipping point, representing the threshold at which the sensor indicates exposure to flammable substances. Once this tipping point is reached, the entire unit shuts down, interrupting the chain of electrical transmission. Consequently, the heating unit in a water heater ceases operation, resulting in a halt to the water heating process.

It is important to note that the sensors are not sensitive to low concentrations but instead focus on elements within an optimal range. Each sensor has a Lower Flammability Limit (LFL) that denotes the minimum concentration susceptible to ignition, assuming other conditions are standard. Occasionally, a “Nuisance Trip” may occur, indicating the sensor has tripped despite the absence of flammable vapor.

The occurrence of a “Nuisance Trip” can be attributed to the presence of other materials in the environment that trigger the sensor. These materials may include paints, adhesives, mainly oil-based paints, or even excessive dust from a renovation or other activities.

Specific flammable vapor sensors possess an automatic reset feature after sensing and tripping off the unit. However, in most cases, it is necessary to replace the sensor apparatus.

Rheem AP13447-3 Water Heater OEM Replacement Flammable Vapor Sensor

Rheem AP13447-3 Water Heater OEM Replacement Flammable Vapor Sensor

How to Properly Install A Flammable Vapor Sensor?

During the installation of the flammable vapor sensor, ensuring that the surrounding environment remains devoid of any explosive or contaminated vapor sources is crucial. Even the presence of these elements in minimal quantities can result in false positives, compromising the sensor’s accuracy.

Commence the installation process by securely attaching the connectors to the tank. Connect the sensor to the wiring connectors at the water heater’s front center, just below the outer door. Gently push the sensor into place, ensuring a perfect fit within the designated space and the sensor cap.

25VS – Rheem OEM Upgraded Replacement Flammable Vapor Sensor

25VS - Rheem OEM Upgraded Replacement Flammable Vapor Sensor

How do you reset a flammable vapor sensor?

When dealing with an explosive vapor event, it is paramount to exercise utmost caution when handling any electrical equipment near the water heater. Begin by carefully sensing any gas odors and promptly identifying the source location. Gasoline traces or other combustible materials could trigger such incidents.

Under no circumstances should direct contact be made with electric switches, plugs, or wires. Adequate ventilation plays a crucial role in minimizing potential damage. Refrain from turning on any devices until the issue has been identified and resolved. If you lack experience handling such situations, you should seek assistance from trained professionals.

Engaging the services of a skilled technician is ideal for effectively managing the condition. With their expertise, the technician can dismantle the product under the supervision and inspect various heater components for any signs of damage, burning, deformation, melting, or soot deposits.

Based on their assessment, the technician will guide whether specific parts need replacement or if the entire heater requires replacement.

Since technicians often possess specialized equipment, the chances of accurate detection are significantly higher.

Many sensors are equipped with a resetting mechanism. Following the detection of volatile constituents, the system would have transitioned into a lockout mode to prevent the water heater from restarting.

To resume the regular operation of the heater, the sensor system must be reset. This can be achieved through various methods. Some plans offer a convenient “reset button” for quick resetting. In other cases, manually closing the gas valve and waiting a few moments before reopening it can reset the unit’s normal state.  

Rheem SP20172 Flammable Vapor FV Sensor Kit White, 6.7X2.7X9.4 inches


Rheem SP20172 Flammable Vapor FV Sensor Kit White, 6.7X2.7X9.4 inches


How do you clean a vapor sensor?

Flammable vapor sensors are commonly positioned at the bottom or in front of water heaters. Due to their compassionate nature, these sensors are housed within protective plastic cases to safeguard against exposure to dust or other substances.

Furthermore, the flammable vapor sensor may enter lockout mode following an explosive vapor event. Inspecting the sensor unit for any signs of contamination or dirt buildup is essential when resetting the system. Consulting the maintenance literature at this stage is advisable for the cleaning procedure. However, since each brand may have specific steps, adhering to the guidelines outlined in the user manual for optimal results is recommended.  


Flammable vapor sensor testing

A multimeter is recommended to measure resistance for an explosive vapor sensor test. Begin by disconnecting the sensor from the water heater.

Ensure that the wiring to the water heater is properly disconnected. Next, detach the wiring from the sensor terminals. Take measurements of the resistance and make a note of the readings. Many ohmmeters are equipped with an advanced auto-range feature, simplifying the process.

If your meter lacks this feature, perform the test twice to ensure accuracy. The first round of testing should be conducted with the ohmmeter set to a scale above 170,000 ohms. The second set of readings should be taken with a scale above 5,000 ohms. The obtained readings will determine whether the sensor requires replacement. As a general guideline, if the resistance measurement falls below 5,000 ohms, it is advisable to replace the sensor.

Similarly, if the resistance exceeds 1,700,000 ohms, returning is considered the safer and more optimal choice. Cross-check the obtained results with the manufacturer’s official website for additional reference and confirmation.  



The flammable vapor sensor must be addressed during an explosive vapor event. The sensor triggers a system-wide shutdown by detecting explosive elements in the surroundings, effectively preventing volatile components from entering the chamber and averting catastrophic incidents. Therefore, the installation of a high-quality flammable vapor sensor is not only ideal but also crucial for ensuring a safe environment.

Additionally, the operation of these sensors is generally straightforward, although there may be occasional occurrences of “Nuisance Trips.” Resetting a combustible vapor sensor can be accomplished by carefully following the provided instructions manual or seeking the guidance of a qualified technician. It is important to remember that you need not tackle these tasks alone. If you are unfamiliar with handling scenarios involving flammable vapors, we strongly recommend seeking the assistance of a technician.

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