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Change is coming to the HVACR industry as state and federal regulations are being enacted that will lead to a phase down of high-GWP refrigerants such as R-410A. Unlike the R-22 phaseout, which saw the transition from one A1 refrigerant to another, some of the new refrigerants being proposed to replace R-410A in air conditioning equipment are lower flammability (A2L) refrigerants.

Because of this A2L designation, special features will be incorporated into the air conditioning equipment itself to ensure safe operation, per safety standard UL 60335-2-40, third edition. One of those features is the refrigerant detection system (RDS), which will be required for all systems in an occupied space that exceed a prescribed refrigerant charge limit. In a recent webinar (below), AHRI invited several RDS manufacturers to discuss these devices in more detail.

The Basics

Carl Bengtsson, technical director at Senseair North America, Air and Gas Business Unit, started off the webinar by discussing how RDSs work.

“The RDS is a stationary device that detects the presence of a specific refrigerant at a specific threshold concentration and then signals initiation of one or more mitigation actions by the HVAC equipment,” he said.

While there are a variety of RDSs available, the way in which they all work is similar. As Bengtsson explained, the RDS contains a sensing element, or uses an intermediary sensing media (e.g., infrared light), that is sensitive to the refrigerant gas or vapor being measured. If there is an A2L leak, the signal from the sensor setup is conveyed to the mitigation control board. If the refrigerant concentration passes the predefined threshold, the RDS will communicate that the HVAC equipment should initiate mitigation actions, such as turning on fans and blowers to dilute the refrigerant concentration and maintain a concentration well below the flammability limit.

“Depending on the HVAC equipment, there may be one or several sensing parts,” he said. “Also, the specific communication or the control interfaces for how these RDSs actually talk to the HVAC equipment depends on the HVAC OEM. RDSs can also come with additional features and different communication interfaces.”

The RDS will be factory installed, and the installation of the sensor will be in a location that is determined by the equipment manufacturer as well as the equipment in which it's installed, said Ted Sunderland, market technology engineering lead at Parker Hannifin, Sporlan Division.

“The size of the equipment, configuration of coils, the installation itself — all these things affect where the most opportune place for the sensor is to be in order to ensure the safety of the system, so that the refrigerant concentration level can be detected before it becomes a hazard,” he said. “The equipment is tested in accordance with Annex MM of UL 60335-2-40 to demonstrate that the sensor adequately detects the leaked refrigerant in the location specified by the equipment manufacturer.”

The maximum concentration thresholds at which the RDS must initiate mitigating actions is defined in the UL standard and its Annex LL. The actual threshold for detection in the application can be less than that defined as the maximum but no greater, said Sunderland. This provides assurance that an A2L refrigerant leak will be detected at concentrations well below those that would pose a hazardous condition.

“Just to expand on that a bit, the standard currently states 25% lower flammability limit (LFL) or below,” he said. “The ‘or below’ covers if there’s a leak event and refrigerant begins to pool. The standard includes tests that prescribe where the biggest leak event may occur that would most likely create a hazard — that may not necessarily be where the sensor is located. In order to compensate for that, by lowering that detection threshold, there may be 25% LFL in this area and only 19% in another area. These are diffusion sensors that rely on the diffusion mechanism to trigger that threshold and may be set lower than 25% to ensure safe operation.”

The RDS will also be installed with a preset detection threshold and will be calibrated for the refrigerant listed on the equipment nameplate, said Sunderland. “A pre-calibrated and installed RDS not only will provide the safe operation of the A2L charged HVAC equipment, but it will also prevent false nuisance trips from having been supplied with too low of a detection threshold by a faulted or failed sensor.”

Sunderland explained that the RDS will also be subjected to vibration testing to ensure its ability to reliably operate when exposed to vibration that would occur when mounted in the system. The RDS will include software as well, which will initiate and perform a sensor self-test at least once per hour. If a failure of the RDS detected, the mitigation action will be initiated.

Active Monitoring

Even though the hope is that an RDS will never detect a potentially harmful refrigerant leak, it will actively and continually monitor for the presence of refrigerant, said Bob Christensen, senior director of business development at NevadaNano. As stated above, at least once an hour, the RDS will confirm its own state of readiness to the control board. Other than that, the RDS will sit quietly, waiting to signal if a potentially harmful refrigerant is present.

“The RDS will employ what is called or referred to as active mitigation,” he said. “This methodology will detect leaking refrigerant, and the HVAC equipment will take some sort of mitigating action, whether that's turning on a fan, closing the valve, etc., but the main objective is to continually monitor for potentially harmful leak at the desired setpoint. The set points are dependent upon the amount of refrigerant that's present, as well as the volume within the area that it is installed.”

The RDS is expected to last 10 to 15 years or more, said Christensen, and after reaching the expiration date, the sensor will need to be replaced by OEM-approved parts. If the RDS has a pre-determined lifetime, the UL standard requires that it self-indicate its end of life, so it can be replaced.

As for maintaining the RDS, care should be taken to keep these components in good working order during service calls, said Christensen. “Technicians should avoid direct spraying of a sensor when cleaning the coils and also avoid positioning the sensors or tampering with sensors while servicing unit.”

If an RDS should fail, the fan or blower will run all the time as a mitigation action, but the HVAC equipment can continue to operate safely until the RDS is replaced, said Kazuaki Watanabe, CEO of Figaro USA. If a leak is detected, the HVAC equipment may safely continue limited operations until the system shuts down or is repaired.

“When a refrigerant leak occurs, the fan or blower runs continuously as long as the refrigerant concentration is above the threshold and the HVAC compressor will be shut down,” he said. “If the leak is not below the threshold, heaters and other devices are turned off. If the leak goes below the threshold after five minutes of fan or blower activation, the HVAC compressor will be allowed to run.”

Because the RDS plays such an important role in preventing leaked refrigerant from accumulating, it is important not to bypass or turn it off, said Watanabe. “OEMs are actually designing systems that will automatically enter mitigation if the sensor is bypassed or disabled; therefore, it is quite important to ensure that the RDS is not disabled.

Other Considerations

Gary Maus, senior mechanical design engineer in the industrial products division at Sensata Technologies, discussed the number of RDSs that any given piece of HVAC equipment may be required to have.

“It’s not an easy question to answer, because the number of sensors in any given system will depend on the size and the configuration of the system,” he said. “But in some cases, as per UL 60335-2-40, if the system is small enough (less than 4 pounds of refrigerant), it actually won't have any RDS at all. Systems with a larger refrigerant charge, such as most residential and commercial applications, will require an RDS. If a system is small enough, it is possible that only one sensor will be required; in larger systems, proximity to possible leak points would become an issue, so more sensors would be necessary to be deployed throughout system. OEMs will determine how many sensors are needed in a particular system.”

OEMs will also determine where the RDSs will be installed, as they need to be located in specific areas within the HVAC system to ensure detection and mitigation in the event of a refrigerant leak, said Maus. Regardless of the number of sensors required, the OEM will define the location within their equipment.

“Each system has to demonstrate compliance in order to be put out into the field,” he said. “For systems that have multiple possible configurations, the sensor location for each possible configuration will be determined through testing by the OEMs. In every case, it's required that the locations selected by the OEM for a given system can detect refrigerant within a defined time. This is so that the RDS can initiate mitigation, well before the concentrations increase to a potentially unsafe level. To confirm the OEM system is designed correctly and is in compliance with the UL standards, a national nationally recognized test lab will be used to confirm each system.”

Each RDS will also be designed and certified for a specific refrigerant, as well as a specific piece of equipment or HVAC system, to ensure that it operates and functions properly, said Maus. In addition, the RDS will intentionally be nonconfigurable in the field to avoid the possibility of being incorrectly configured or used with the incorrect refrigerant.

“In the event that any portion of the RDS requires field service, maybe due to a failure of an RDS, UL standard 60335-2-40 defines that replacement has to be with original equipment parts,” he said. “That’s the only option. The RDS is matched and tested to specific HVAC systems by the OEMs, so deviating from what's been defined to something that's close is not an option. The only option will be to be replaced with original equipment.”

To conclude the webinar Mary Koban, senior director of regulatory affairs at AHRI noted, “The RDS will ensure safe handling of a leak in the event that it ever occurs. This is achieved by the RDS continuously monitoring a single or multiple sensors within the HVAC system, and that will depend on the size of the system. It's always looking for the presence of refrigerant. Due to the work of the OEMs to ensure the proper number and the proper location of the RDS sensors in each system, mitigation actions — whether they occur due to a leak or if they occur due to a system fault of some sort — will keep the concentration of A2L refrigerant well below the lower flammability limit.”

Sensing a difference

There are various types of sensing technology that will be available for A2L systems, and listed here are just a few of the possible options:

A2L DETECTION: The compact metal oxide sensor (MOS) from Figaro USA is designed to detect A2L refrigerants. (Courtesy of Figaro USA)

Figaro USA: The compact metal oxide sensor (MOS) offers proven technology as a residential gas alarm application, having been available for over 50 years, said Kazuaki Watanabe, CEO. The sensor technology has already been used for A2L gas leak detection in the Japanese market since 2017, he added, and it is cost effective and responds quickly. The way this sensor works is as follows:

1. In clean air, donor electrons in tin dioxide are attracted toward oxygen, which is adsorbed on the surface of the sensing material, preventing electric current flow.
2. In the presence of certain gases (e.g., A2Ls and other combustible gases), the surface density of adsorbed oxygen decreases as it reacts with the gases. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor.

Essentially, MOS-type gas sensors change resistance as a result of a change in adsorbed oxygen concentration, allowing them to not only detect gases like A2L refrigerants, but also estimate their concentration. The sensor requires no maintenance and its expected lifespan is 15 years, said Watanabe.

FAST AND ACCURATE: The MPS A2L gas sensor from Nevada Nano is a MEMS-based sensor designed to deliver fast and accurate detection. (Courtesy of NevadaNano)

NevadaNano: The Molecular Property Spectrometer™ (MPS) A2L gas sensor is a micro-electromechanical system (MEMS)-based sensor that integrates multiple, complementary chemical sensors on a single silicon chip. It delivers fast and accurate detection but is also very robust and hardened to perform within an HVAC system for over a decade, said Bob Christensen, senior director of business development. This means it is impervious to poisons and challenging environmental conditions and requires no maintenance or servicing once it leaves the factory.

Parker Hannifin: Parker’s infrared (IR), A2L refrigerant sensor operates by using the NDIR principle to measure the presence of the target refrigerant gas, said Ted Sunderland, market technology engineering lead.

The sensor contains a long-life tungsten filament infrared light source; an optical cavity into which gas diffuses; a temperature compensated pyroelectric infrared detector; an integral semiconductor temperature sensor; and electronics to process the signals from the pyroelectric detector. The A2L sensor utilizes a direct light path from the IR lamp to the detector, which enhances performance in HVAC applications where small particulates and moisture are present, he said.

Parker’s A2L sensor is housed in an enclosure designed specifically to mitigate the harsh environmental conditions that exist inside HVAC equipment while allowing refrigerant vapor to easily enter the sensing cavity. Sunderland added that this maintenance-free A2L refrigerant sensing solution provides long operational life (15-plus years) and failsafe operation in HVAC applications.

Sensata Technologies: Sensata’s sensor solution uses patented Speed of Sound (SoS) technology to detect the presence and concentration of refrigerant in the air. It should not be confused with acoustic sensing technology, which simply listens for an audible leak, said Gary Maus, senior design engineer.

SoS technology uses send and receive transducers to establish the resonant frequency of the gas within a sensing cavity then compensates the speed of sound determined by the resonant frequency for environmental conditions such as temperature and humidity. Using a standard gas as a reference — air for example — the speed of sound is altered as the gas mixture is altered. As an A2L refrigerant mixes with the air in the sensor cavity, the speed of sound changes predictably, making it an ideal, robust method of detection, explained Maus. The greater the concentration of refrigerant present, the greater the speed of sound is altered, enabling the refrigerant concentration to be determined.

Sensata’s SoS sensor does not require any regular maintenance, said Maus. It is calibrated at the factory for a specific refrigerant and never needs recalibration or re-zeroing for the life of the HVAC unit.

Senseair: The NDIR-based RDS for A2L refrigerant leak detection uses gas absorption spectroscopy in the IR-light spectrum. This robust technology for detecting and measuring gases has been used reliably in the gas detection industry for many decades, said Carl Bengtsson, technical director.

Selectivity for a certain gas’s absorption is created by an optical interference bandpass filter. The IR transmission signal is quantified into gas and LFL concentrations by advanced factory calibration and compensation models in an integrated microprocessor, he said. The specific interface and communication after this microcontroller depends on each HVACR OEM and the integration into their mitigation system.

The sensing components use irradiation energy levels of IR-light as an intermediary signal for the presence and quantity of refrigerant gas. As a result, they do not depend on any physically reacting substrate that would be affected by the various content in the continuously sampled air mixtures directly, which makes the NDIR-based RDS sensor immune to typical poisoning and environmental disturbances, said Bengtsson. The expected lifetime of the sensing system can last more than 15 years with maintained detection accuracy; however, there are NDIR sensors from Senseair still operational in building HVAC systems for more than 25 years, fully maintenance-free, he added.