As the HVAC industry transitions to refrigerants with lower global warming potential (GWP), A2L refrigerants like R-454B are becoming more common. These refrigerants are classified as “mildly flammable” and require the installation of A2L leak detection sensors in many applications to ensure safety. While these sensors are essential for detecting actual refrigerant leaks, HVAC technicians need to be aware of potential sources of false alarms, particularly from common construction materials.

How A2L Sensors Work 

Most A2L refrigerant sensors operate on principles that detect changes in the surrounding air composition:

The first type of leak detection sensor is the thermal conductivity sensor. Thermal conductivity sensors measure how well a gas conducts heat. A2L refrigerants have different thermal conductivity properties than ambient air. When a refrigerant leaks, it changes the thermal conductivity of the air mixture, and the sensor detects this change, triggering an alarm.

The second type of leak detection sensor is the metal oxide semiconductor (MOS) sensor. These MOS sensors detect gas by measuring changes in electrical resistance across a metal oxide film. When certain gas molecules come into contact with the heated metal oxide, they react, altering the sensor's resistance, which alerts to the presence of gas. 

Both types of sensors are highly sensitive, which is good for detecting tiny refrigerant leaks. However, this sensitivity can also make them prone to reacting to other gases that share similar properties to refrigerants.

For example, some popular construction adhesives or sealants have the unintended side effect of off-gassing. Many of these are for exterior applications, but some people may use these products indoors because of their reliability. As these sealants cure, they release volatile organic compounds (VOCs), including various hydrocarbons and solvents like tetrachloroethylene. These compounds are necessary for the sealant's performance but can become airborne.

Why Certain Construction Adhesives May Be Causing False Alarms

The connection between construction adhesive off-gassing and A2L sensor false alarms lies in the chemical composition of the VOCs it releases.

The hydrocarbons and solvents released by some construction adhesives have different thermal conductivity properties than clean air. When these VOCs are present in the air around a thermal conductivity-based A2L sensor, they can mimic the thermal signature of a refrigerant, causing the sensor to falsely detect a “leak.”

Similarly, the VOCs from the sealant can react with the metal oxide film in MOS sensors, altering their electrical resistance in a way that is indistinguishable from a refrigerant detection.

Essentially, the sensor is doing its job by detecting a change in the air, but it's detecting the sealant's curing byproducts rather than an actual A2L refrigerant leak.

The Critical Importance of Reading Labels 

This scenario highlights a critical point for HVAC technicians: always read product labels and understand their intended use, especially for indoor applications.

Take, for instance, Geocel 2300. (To be perfectly clear, I am only using this product as an example to show the importance of reading manufacturer specifications. I am not taking a stance on whether to use this product or not.) Geocel 2300 is typically recommended for outdoor use or in areas with ample ventilation. Using it in an enclosed indoor space, such as a mechanical room, equipment closet, or even a residential attic where an A2L system might be installed, significantly increases the likelihood of off-gassing interfering with sensitive A2L sensors.

Here is a snippet from the manufacturer’s data sheet : 

You may have noticed that the manufacturer mentions that the product is intended for exterior use due to the odor emitted during cure. This odor is caused by VOCs that could cause false alarms for A2L sensors.

Key Takeaways for Technicians

• Ventilation is Key: When using any sealant or construction adhesive, especially those with strong odors or listed VOCs, ensure that the area is extremely well-ventilated during application and curing.
• Product Selection: For indoor applications, prioritize sealants and adhesives specifically labeled as low-VOC or indoor-air-quality (IAQ) compliant. These products are formulated to minimize off-gassing.
• Awareness of False Alarms: If an A2L sensor alarms after recent construction or sealing work, consider off-gassing as a potential cause before immediately assuming a refrigerant leak or faulty sensor. Investigate the environment for new materials.
• Educate Clients: Inform building owners or general contractors about the potential for false alarms when using certain materials near A2L-equipped systems.

By understanding how A2L sensors work and being mindful of the materials used in and around HVAC installations, technicians can prevent unnecessary service calls, reduce diagnostic time, and maintain the integrity of safety systems.

That said, do not bypass these A2L sensors, even if it seems like it is the only choice you have to get the system up and running. The liability associated with bypassing these safety devices far outweighs a client's immediate comfort. 

—JD Kelly