The HVAC industry's ongoing shift towards eco-friendly solutions continually introduces new challenges and learning opportunities. A2L refrigerants are at the forefront of this transformation, offering a lower global warming potential (GWP) than many of the refrigerants we’ve been using. They’re also mildly flammable, meaning they require some enhanced safety measures. One of these is the use of dissipation boards and thermal conductivity sensors for leak mitigation while the unit is running.

Dissipation control boards work with refrigerant leak sensors to detect leaks and prompt a series of safety measures, such as shutting down the compressor and activating the blower fan to dissipate or dilute the refrigerant. This emergency response keeps refrigerant from accumulating to unsafe levels.

A Deeper Look at “Unsafe Levels”

An “unsafe level” is defined by the lower flammability limit (LFL) of a gas, specifically R-454B in this context. The LFL indicates the concentration of gas in a given volume of air that is necessary for ignition to occur.

R-454B has an LFL of 0.303 kg/m³ (or 11.8% by volume); if you have a hollow 1 meter x 1 meter x 1 meter cube with a cloud of R-454B inside it, that refrigerant can’t ignite unless there’s at least 300 grams (think 300 postage stamps’ weight) in that entire space. Most manufacturers design sensors to detect and respond to concentrations that are 25% of the LFL (so, they’ll go off at <0.01 kg, closer to 3% by volume). 

As you can see, that 25% needed to set off the sensor is still far less than the concentration needed for a fire to start. The sensor just provides a very early warning signal that kicks into action before the concentration of refrigerant is anywhere near the LFL.

How Dissipation Systems Work

Dissipation systems rely on communicating sensors, which are the typical A2L refrigerant detection sensors (usually a 4-wire sensor type). These sensors continuously monitor the air for the presence of refrigerant. You’ll usually find them factory-installed in the evaporator coil section of an air handler.

A sensor communicates its readings to the dissipation control board through a wiring harness. Most manufacturers require the dissipation control board to be installed externally, near the air handler, within 8 feet of the sensor.

Dissipation Mode Activation

Dissipation mode activates when the refrigerant concentration surpasses the LFL threshold. While the UL standard is 25% LFL, manufacturers are now targeting 20% for an increased safety margin.

Here's a basic scenario for a unit that has reached 20% LFL:

1. The dissipation sensor detects the gas and transmits a signal to the dissipation control board.
2. The control board then signals the fan controller, initiating fan operation to circulate air and prevent refrigerant pooling.
3. Concurrently, the dissipation control board intercepts the thermostat's Y call, shutting down the compressor to halt further refrigerant circulation. The W signal is also blocked to disable auxiliary heat strips or gas furnaces, preventing them from becoming an ignition source.
4. If the structure has a building management system, a signal is also sent to that, alerting the owner to the situation.

Essentially, the dissipation control board can override the thermostat and control the equipment even if the thermostat isn’t calling. This sounds more complicated than it actually is. 

Consider this an override system: the HVAC unit operates routinely until the gas's LFL reaches a preset limit. At that point, the dissipation control board activates the blower fan to disperse or dilute the gas, simultaneously preventing the compressor or any heating source from operating.

—JD Kelly