Challenging assumptions about “nonflammable” labels and the risks they may obscure.

Last month’s column took a light-hearted look at the complicated and serious issue of toxicity. This time around the spotlight is on the other side of the safety classification coin: flammability.

With flammability, dictionary definitions are not much help when it comes to the specifics of refrigeration. They generally talk about substances that “can burn easily” or are “capable of being easily ignited,” but we need to turn to refrigeration standards to put this in context. Flammability class 1 is often described colloquially as “nonflammable,” but ASHRAE Standard 34-2022 doesn’t define flammability explicitly and the definition of class 1 in the standard is actually “no flame propagation.” The standard does define flame propagation as “any combustion that moves upwards and outward from the point of ignition.”

This leads us further down the rabbit hole: combustion is defined in the dictionary as “the act or instance of burning” and burning is defined as “consuming fuel and giving off heat, light and gases.” My take on this is that anything that is being oxidized in a process that produces heat, light and oxides is being burned; however, this includes a lot of substances classified as having “no flame propagation” by Standard 34. In fact, I only found seven substances listed in the standard that could truly be described as “nonflammable.” They were helium (R-704), water (R-718), neon (R-720), argon (R-740), carbon dioxide (R-744), nitrous oxide (R-744A) and sulphur dioxide (R-764). These are all either products of combustion that have already been oxidized (for example R-718, R-744, R-744A and R-764) or are noble gases (R-704, R-720 and R-740), which, like the nobility, don’t mingle with anything.

All the other flammability class 1 substances, even the perfluorinated ones like R-125 and R-218, are capable of being oxidized given an ignition source that is hot enough, so in that sense, even they can be burned.

Of course, the hazards that various refrigerants present when they are oxidized vary enormously. Some, like R-152a, R-290 or R-600 will ignite easily and burn so rapidly that they cause a very rapid rise in air pressure, producing shock waves that can blow down walls and send debris flying for long distances. Others, like R-32, R-1234yf and R-717 are more difficult to ignite and the flame front spreads more slowly. This often means that the pressure rise is more gradual and so the destructive power is much less, but given the right fuel to air mixture the flame front will still spread once ignited and typically has the power to blow out windows and dislodge ceiling tiles.

Refrigerants that burn but don’t show flame propagation under laboratory test conditions (those in flammability class 1) present a different sort of hazard. The combustion does not usually produce rapid, destructive pressure rises (although it can in some circumstances), but the oxidation frequently produces highly toxic products of combustion. These substances were frequently the cause of illness suffered by technicians using halide leak detection torches, and they have also been linked to sick building syndrome.

The hard-pressed designer of the system needs to do a competent risk assessment of all foreseeable hazards in order to complete a demonstrably safe design. In this case it is necessary to remember another favorite saying of Professor Joachim “Joe” Paul, whom I quoted last month on toxicity. Joe was fond of citing Murphy’s Law: “If it can go wrong, it will go wrong,” adding what he called Joe’s Law—“Murphy was an optimist.”