Flammable Refrigerants Under UL 60335-2-40 Testing

UL 60335-2-40 sets the safety requirements and test procedures that verify household appliances using flammable refrigerants — R32, R1234yf, R290 and R600a — do not create unacceptable fire or explosion hazards. This article follows an accredited laboratory's test sequence, from sample receipt to the final compliance determination.

UL 60335-2-40 Testing: Scope & Test Objects

UL 60335-2-40 applies to electrically operated heat pumps, air conditioners and dehumidifiers for household and similar use. It covers the flammable refrigerant provisions within the household appliance safety framework. Refrigerant hazard classification follows ANSI/ASHRAE 34. This standard assigns Class 2L to mildly flammable fluids such as R32 and R1234yf. Class 3 covers higher-flammability fluids such as R290 and R600a. The test objects are complete appliances charged with their designated refrigerant. They include the pressure-retaining circuit — compressors, heat exchangers, service valves and internal piping — plus electrical controls and any leakage detection or mitigation devices. Assessment addresses the appliance as installed. It also covers the interaction between the refrigerant system and potential ignition sources such as relays, fan motors and electric heaters. Larger-capacity commercial equipment and industrial process chillers fall outside this scope. They are evaluated under separate product standards.

Sample

Preparation & Refrigerant Charging
Testing begins with a representative production sample installed per the manufacturer's instructions. Room geometry and installation height affect measured concentrations. The laboratory verifies the nameplate data: refrigerant designation, rated charge mass, allowable refrigerant class and electrical ratings. Charging is performed by weight on a calibrated charging scale. The technician evacuates the circuit, then introduces refrigerant from a cylinder until the specified mass is reached. The measured charge is recorded. It determines the applicable charge limits and test room dimensions in later stages. Before measurements, the appliance is operated until steady-state conditions are established. Condensing and evaporating temperatures must stabilize within the tolerances of the test program. Instrumentation — leak detectors, concentration sensors, thermocouples and pressure transducers — is positioned and documented before the sequence starts.

Leak

Tightness & Concentration Measurement — NDIR / GC
Leak tightness is verified in two complementary ways. Circuit integrity is checked first by pressure-decay measurement or refrigerant-specific leak detection. Heated-diode and infrared sniffers are swept across brazed joints, valve stems and access ports. This localizes leaks at the gram-per-year level. Concentration behavior is then evaluated by releasing refrigerant at a defined leak rate from a designated failure point. The appliance operates in a sealed test room during the release. Nondispersive infrared (NDIR) analyzers sample the room air continuously at fixed positions. They log concentration against the lower flammability limit (LFL) of the refrigerant. gas chromatography (GC), with flame ionization or thermal conductivity detection, serves for calibration and confirmatory analysis of sampled gas bags. Where an appliance relies on a refrigerant detector as a mitigation measure, its alarm threshold and response time are verified against the measured concentration curve.

Charge Limit & Ignition Risk Tests

The maximum permissible charge is calculated from the refrigerant's lower flammability limit and the smallest room area permitted by the installation instructions. Larger charges restrict installation to larger rooms. They may also require mitigation such as ventilation or distributed piping. The laboratory checks the declared charge against these limits. It also confirms that the marking and instructions state the minimum floor area. Ignition risk assessment then addresses every electrical component within the zone where a flammable mixture can occur. Spark-producing contacts, switching relays and heaters are handled in one of three ways. They are relocated outside the zone, enclosed in sealed housings, or verified by test to be incapable of igniting the refrigerant-air mixture. Control circuits that operate safety devices are assessed for functional behavior. This includes detector response and shutdown logic. A short practical demonstration of several of these verifications appears in Assurance In Action: Episode 40 Testing on this site.

Pressure Strength & Burst Testing

Pressure-retaining parts of the refrigerant circuit must withstand pressures well above normal operating values. A proof pressure test is applied first. Components such as condensers, evaporators and piping assemblies are held at a pressure defined relative to the maximum operating pressure. No rupture or permanent deformation may occur. A burst test follows on sample parts. These are pressurized until failure. This demonstrates that the burst pressure exceeds the required multiple of the design pressure. High-side and low-side circuits are tested separately because their operating pressures differ. Hermetically sealed motor-compressors and assembled systems may be qualified by equivalent system-level tests where component-level testing is impractical. Pressure testing is conducted behind protective barriers with remote pressurization. Test fluids, temperature and pressurization rate are documented. These parameters influence the measured strength.

Acceptance

Criteria & Compliance Judgement
Compliance is judged element by element against the criteria of the standard. During refrigerant release tests, no ignition may occur from any source on the appliance. Concentrations at the specified sampling positions must remain below the limits derived from the LFL. Static leak measurements must not exceed the permitted leak rates. Pressure parts must survive the proof pressure without rupture or visible deformation. Burst pressures must exceed the required minimum. Detector-based mitigation is accepted only when the alarm and shutdown sequence operates within the verified response time. The laboratory also reviews markings, instructions and the declared minimum floor area for consistency with the tested configuration. A pass in every element results in a compliance determination. A single failure is documented with photographs, measured data and the clause reference. Corrective action and retest then follow.