Refrigerant detection systems and leak mitigation

Why A2L equipment needs detection

An A2L only burns when a leak builds up to its lower flammability limit (LFL) and meets an ignition source. In a small room with no air movement, a large enough leak from an indoor coil could reach that concentration. The safety standards prevent this in two ways: they limit the charge for a given room area, and above that limit they require mitigation that keeps the leaked refrigerant diluted.

In an ASHRAE Journal roundtable on the flammable refrigerant standards, UL’s Brian Rodgers summed up the concept as “keep it below the lower flammability limit (LFL).” He described the m1 charge limit as “the charge limit when only room area is used to determine the charge limit: M1 ≤ 6 × LFL. Above that we have to look at mitigation.” He listed mitigation methods such as turning on safety shutoff valves, turning on fans, and shutting down compressors (or leaving them on if that reduces the leak rate).

Craig Grider of Intertek named the common forms of mitigation: a refrigerant detection system, safety shutoff valves to control the releasable charge, or continuous operation of a circulation fan. The panel also noted that the installation side of these requirements is moving into ASHRAE Standard 15 and Standard 15.2P (residential, then in development). Grider added that UL 60335-2-40 and UL 60335-2-89 require manufacturers to provide installation manuals with tables covering releasable charge, connected spaces and ventilation.

When a system needs a sensor

The manufacturer’s installation instructions decide it, using the system charge and the room area. Under IRC M1411.2 in the ICC required changes to the 2021 IRC, A2L systems must be listed to UL/CSA 60335-2-40 and installed in accordance with the listing, so those tables are part of the code requirement.

Rheem’s RP17AY R-454B instructions show how this works in a residential split system:

• A table lists, for each total system charge, the minimum indoor airflow and the area of the smallest conditioned room or space where the indoor unit is installed. The room area depends on the height of the lowest vent opening or the bottom of the indoor unit (2 ft or less, 6 ft, 9 ft or more).
• “If the Smallest Conditioned Space Room Area is below the specified value in the table for the Total System Charge, a Refrigerant Leak Detection Sensor will need to be installed.”
• For systems with a factory-installed sensor, the installer verifies that the total conditioned area and minimum indoor airflow meet the table minimums.
• At the larger charges in the table, the without-sensor column reads “not permitted - sensor required.”
• Room area values are corrected for altitude with an adjustment factor (1.050 at 1,000 ft in Rheem’s table).

So the total charge, including the line-set adder, has to be known before deciding about the sensor. Weigh in the charge and record it. Rheem also says R-410A or R-22 indoor coils have not been evaluated for R-454B and “may lack a refrigerant detection system necessary for safe operation,” which is one reason an old coil cannot be reused with a new A2L condenser.

What the system does on a leak: one manufacturer’s sequence

Behaviour differs by manufacturer and model. The sequence below is from Carrier’s 50FE R-454B packaged unit instructions. Use it as an example, not as a description of every RDS.

Hardware. Carrier calls it a leak dissipation system: an A2L sensor and a Refrigerant Dissipation Board (RDB), both in the indoor coil section. The sensor sits between the indoor coil and the air filters and connects to the RDB by a harness. The harness drain wire must be grounded at the RDB; Carrier warns that poor sensor harness grounding “can lead to false dissipation events.”

On detection. “If the refrigerant detection sensor sends a signal indicating a refrigerant leak, the control board will prevent heating and cooling operation and begin dissipating the sensed refrigerant with a blower request.” The status LED flashes a code identifying the sensor. Carrier’s troubleshooting table defines the leak code as a sensor reading of 20% of LFL or more.

Operational checks (Carrier Table 6):

Recovery. When the sensor shows the refrigerant has dissipated, the board shows flash code 3, returns to its normal state and allows unit operation after a 5-minute delay.

Faults. An open sensor, a sensor fault, or no voltage on the blower output also put the board in dissipation mode, according to Carrier’s flash code table.

Building integration. With zoning, VAV or VVT dampers, the zone dampers must open to meet the unit’s minimum dissipation circulation rate. Electric reheat in zone boxes and duct heaters must be off during dissipation mode. The RDB has a dry-contact alarm relay, and SystemVu units expose a BACnet point, to tell other controls that dissipation is active.

Ignition sources. Carrier: “Do not install an ignition source or potential ignition source in a space where the total conditioned area is below the minimum total conditioned area (TAmin), unless a flame arresting device has been installed.”

Service implications

• Test it at commissioning. Carrier’s self-test: with no faults present, press the Test button on the RDB and the system enters dissipation mode for 60 seconds. Holding the button for 10 seconds shows the alarm history (last 7 codes from 1 week), and three presses clear it. Check each row of the operational table above.
• Check airflow. The dissipation mode depends on the blower meeting a minimum airflow, which Carrier lists by unit and Rheem lists by charge. The airflow (CFM) calculator helps with the field check.
• A nuisance trip is still a call to investigate. Before assuming a bad sensor, leak check with an A2L-rated detector. Then check the harness grounding and the sensor connection. Carrier says to try power cycling first for any flash code.
• Keep the sensor as shipped. The sensor and board are part of the listed equipment, and IRC M1411.2 requires installation in accordance with the listing. Rheem’s A2L service section says “replacement parts shall be in accordance with the manufacturer’s specifications.”
• Duct accessories. IRC M1411.4 (ICC change document) bans accessories in the ductwork of A2L systems that contain electric heating elements, open flames or devices switching more than 2.5 kVA. Check this before adding a duct heater or electronic air cleaner.
• Labels. IRC M1411.5 requires a label with the installer’s contact information, the system charge and the refrigerant number.

Machinery rooms

Larger commercial systems in machinery rooms fall under the IMC instead. The ICC 2024 A2L change document shows IMC 1106.4 provisions for Group A2L and B2L machinery rooms, including:

• detectors that activate at or below 25% of the LFL and start the emergency ventilation, with supervised detection, signaling and control circuits;
• a detector activation table: at or below the OEL, a 300-second maximum response, ventilation level 1, automatic reset, trouble alarm; at or below the Table 1103.1 refrigerant concentration level, a 15-second response, ventilation level 2, manual reset, emergency alarm;
• no permanently installed open-flame devices or continuously operating hot surfaces over 1,290°F (700°C) in the room.

The emergency ventilation rate is the minimum in ASHRAE 15 or the IMC exhaust table, and shutdown of the emergency ventilation is by manual means.

Where the 2L limits come from

The LFL that all these calculations use is the one measured for the ASHRAE 34 classification. A 2L must have an LFL above 0.10 kg/m³ under Addendum g to ASHRAE 34-2016. See flammability safety classes for the full criteria.