SLAAEQ2 September 2024 MSPM0G1506 , MSPM0G3507
This technical article dives into the new A2L standards for residential and commercial HVAC systems and some of the system changes required to meet this standard. A2L refrigerants are refrigerants that have a lower global warming potential or GWP. This paper shares details of the mitigation tasks requirements for new A2L refrigerant HVAC systems as the A2l relate to the UL60335-2-40 standard, as well as give an overview of a basic mitigation board and other relevant content for designers.
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A2L refrigerants get the respective name from the ASHRAE safety classification and are mildly flammable, low toxicity, and low global warming potential, or GWP. These refrigerants also have a zero-ozone depletion potential, or ODP. As far as the chemical makeup of these refrigerants, A2L refrigerants are hydrofluoroolefins (HFO) or a blend of HFOs. Compared to previous HFC and HCFC refrigerants used in HVAC systems, namely R-410A and R-22 among others, the A2L refrigerants are more energy efficient while also conforming to greener energy initiatives worldwide. Table 1-1 shows some common HVAC refrigerants along with the respective GWP/ODP values.
Refrigerant | Type | GWP | ODP |
---|---|---|---|
R-22 | HCFC | 1820 | 0.055 |
R-410A | HFC | 2088 | 0 |
R-134A | HFC | 1430 | 0 |
R-1234YF | HFO | 4 | 0 |
R-1234ZE | HFO | 6 | 0 |
R-32 | HFC | 675 | 0 |
R-454B | HFC | 466 | 0 |
The drawback to A2L refrigerants is the inherent mild flammability associated with the chemical makeup, which means there are equipment manufacturer regulations needed to mitigate any dangerous situations where the refrigerant can combust given an ignition source. Figure 1-1 shows a breakdown of different types of these refrigerants along with the respective ignition combustion data.
A2L-based systems and the additional required electronics to comply with UL 60335-2-40. To make sure safe operation, air conditioning equipment can feature special components, such as refrigerant detection systems (RDS), as per safety standard UL 60335-2-40. RDS detects specific refrigerants and, upon exceeding predefined thresholds, triggers mitigation actions like activating fans to dilute refrigerant concentration via the mitigation board. Requirements for the RDS include:
The deployment of RDS and adherence to safety standards are critical steps in the industry's response to regulatory changes. More information on the requirements of A2L sensor modules can be found in A2L Refrigerant Adoption: Considerations and Requirements.
The second crucial component of this standard is the mitigation board which handles all of the required tasks once the refrigerant concentration threshold has been reached as described above. This includes:
Figure 2-1 shows an example implementation of a mitigation board for HVAC systems. The board interfaces to the sensor module and based on the detection of refrigerant, or lack thereof, the board can carry out mitigation actions as necessary. More information on the sensor module design considerations can be found in A2L Refrigerant Adoption: Considerations and Requirements.
In some cases, the control signals from the mitigation control board are sent to the existing furnace control board to switch relays on or off for the compressor and the indoor blower fan. This can simplify the mitigation board design greatly and allow the preexisting furnace control board to execute the mitigation tasks.
This example design encompasses all of the mitigation tasks outlined in the ensuing section of this article to show a complete design that can be used for reference by the designer. The device selection is mainly driven by the lowest cost options available, allowing for a cost sensitive design that does not compromise the performance of executing the mitigation tasks.
Device Selection
MCU: For the MCU, there is not a need for high performance so the lowest cost MSPM0 was chosen (Figure 2-2). This option provides the necessary peripherals for communicating with the sensor module and other fieldbus devices and also provides PWM and GPIO functionality for relays, the peizo driver, and the indication LEDs.
Relay Control: The relay control for the indoor blower fan and the compressor cutoff is comprised of several devices along with some discrete components as well.
Freeze Detection: For water based systems needing freeze detection, a temperature and humidity sensor can be used to indicate this event. This design leverages the HDC2010 which is a low-cost digital temperature and humidity sensor that can be mounted on the mitigation board, or even off-board near the heat exchanger depending on the use case as shown in Figure 2-6. The HDC2010 is a humidity and temperature sensor that provides high accuracy measurements with very low power consumption. The sensor is factory-calibrated to 0.2°C temperature accuracy and 2% relative humidity accuracy and includes a heating element to burn away condensation and moisture for increased reliability.
Audio Driver: For the audio alarm, the DRV8220 is selected as a low cost piezo driver. Although this device is marketed as a half-bridge motor drive, the device can also be leveraged as a piezo speaker driver for the audio alarm. The DRV8220 supports multiple control interface modes including PWM (IN1/IN2), phase/enable (PH/ EN), independent half-bridge, and parallel half-bridge. Each interface supports a low-power sleep mode to achieve ultra-low quiescent current draw by shutting down most of the internal circuitry. The device can supply up to 1.76A of output current and operates with a supply voltage from 4.5 to 18V as shown in Figure 2-7.
Fieldbus Interface: For the fieldbus communication, there are a variety of communication interfaces that can be chosen based on the target communication protocol. Many sensor modules on the market today use RS-485 as the primary communication protocol between the sensor module and the mitigation control board. For this reason, the THVD1500 is chosen as the communication interface for the example mitigation control board as shown in Figure 2-8. The THVD1500 is a low-cost, low-power half-duplex RS-485 transceiver designed for data transmission up to 500kbps.