Refrigerant Decomposition and Oxygen Displacement

Why this matters

Refrigerant is handled every day by techs who think of it as an environmental and paperwork problem: recover it, log it, do not vent it. In the air of a basement it is a body problem, three different ways. It is heavier than air and collects where the tech is working. When it reaches a flame or a glowing element it turns into acids that burn the eyes and lungs. And the newer A2L refrigerants that replace R-410A can burn. A leak into a mechanical room is a short-lived event most of the time, and the techs who get hurt are the ones who treat it as a slow one.

Where it goes: heavier than air

Every common residential refrigerant vapor is denser than air. Using molar mass against air's 29 (ideal gas, room temperature):

A leak in still air does not spread evenly through the room. It slumps to the floor, runs downhill, and fills low places first: the bottom foot of a basement, a sump crock, a floor drain pit, a crawlspace below a living space, an elevator or equipment pit, a trench. Air movement mixes it eventually; in a closed basement with the air handler off, eventually can be hours.

The consequence for where you put your face is direct. A tech kneeling at a basement air handler, or lying on a crawlspace floor reading a nameplate, has their head in the layer where the concentration is highest, while a detector clipped to a shirt pocket or set on a shelf reads the cleaner air above.

The three ways it hurts you

Displacing oxygen. Enough of any gas in a space lowers the oxygen fraction. OSHA treats air below 19.5 percent oxygen as oxygen-deficient. See related: Nitrogen Purging and the Air You Cannot Smell, which owns the oxygen line and what happens to a person below it. The point here is a comparison that site meters hide: to pull room air from 20.9 down to 19.5 percent oxygen, about 1 minus 19.5 over 20.9, or 6.7 percent of the air, has to be something else. A refrigerant at several percent is a serious exposure in its own right well before an oxygen monitor alarms. An oxygen meter is a late warning for a refrigerant leak, not a refrigerant detector.

Acting on the body directly. At high concentrations halocarbon refrigerants are narcotic (dizziness, poor coordination, confusion) and can sensitize the heart to adrenaline, which can trigger an abnormal heart rhythm in a person who is exerting or startled. That is the mechanism behind deaths from deliberate inhalation, and it is why ASHRAE Standard 34 assigns each refrigerant a refrigerant concentration limit for occupied spaces built from its toxicity, its oxygen-deprivation effect and, for flammable ones, its flammability.

Decomposing. When refrigerant vapor passes through a flame or across a hot surface, it breaks down. Fluorinated refrigerants (R-410A, R-32, R-454B, R-134a and the rest) form hydrogen fluoride (HF) and carbonyl fluoride. Chlorinated ones such as R-22 also form hydrogen chloride and small amounts of phosgene. HF in air burns the eyes, nose and throat and the deep lung, and lung injury from these acids can appear hours after the exposure. The cue is a sharp, acrid, stinging smell nobody can mistake for refrigerant itself, which is close to odorless. Where it happens in houses:

• a gas water heater, furnace or boiler in the same basement drawing leaked refrigerant in with its combustion air;
• a brazing torch on a line that still holds refrigerant;
• an electric heat strip or a space heater element in the airstream.

The same chemistry corrodes. Acid from a burner that has been drawing refrigerant attacks heat exchangers and venting, and acid in a burned-out compressor's oil attacks skin. Handle burnout oil with gloves and eye protection.

A2L refrigerants burn

The replacements for R-410A in new residential equipment, R-32 and R-454B, are classed A2L under ASHRAE Standard 34: lower toxicity (the A) and lower flammability (the 2L). "Lower" means a minimum concentration in air below which they will not ignite, the lower flammable limit (LFL), and a burning velocity of 10 cm/s or less, slow enough that ignition in a leak usually produces a flash and decomposition products rather than a fireball. It does not mean it will not ignite. R-32's LFL is about 14 percent by volume.

Equipment and installations using A2Ls are governed by UL 60335-2-40 for the equipment and ASHRAE 15 for the system, which bind you through the mechanical code your jurisdiction has adopted (the IMC, UMC or IRC mechanical chapters, in the edition adopted locally), plus EPA's approval conditions for each refrigerant under its SNAP program. Those set charge limits for the room, leak detection and mitigation ventilation on larger charges, and labeling. EPA's Technology Transitions rule (40 CFR Part 84) is what moved new residential equipment off R-410A, on dates EPA has since adjusted; re-check them before you quote a changeover. The handling side (recovery, cylinders, tools rated for A2Ls) is its own card. See related: Refrigerant Transition - R-410A to A2L Reference.

A worked example

A heat pump in a finished basement has an R-32 charge of 6 lb on its nameplate (illustrative). The homeowner calls about poor heating. The tech arrives to find the indoor coil's suction line cracked at a vibration point and the system flat: the whole 6 lb has gone into the basement overnight. The basement is 30 ft by 40 ft with a 7.5 ft ceiling, 9,000 cubic feet, closed up, with the air handler off on a pressure fault. A gas water heater stands 15 feet away. There is a sump crock in the floor 18 inches across and 24 inches deep.

How much vapor. R-32 vapor at room temperature weighs about 0.135 lb per cubic foot (52 g/mol over 24.1 liters per mole). 6 lb divided by 0.135 is about 44 cubic feet of refrigerant vapor.

If it mixed evenly. 44 over 9,000 is about 0.5 percent. Far below the LFL of about 14 percent, and the oxygen would drop from 20.9 to about 20.8. That number is why people relax.

If it settled in the bottom foot. The bottom foot of the room is 30 by 40 by 1, or 1,200 cubic feet. 44 over 1,200 is about 3.7 percent: still under the LFL, but about a quarter of the way there, at kneeling-face height, and a substantial halocarbon exposure.

The sump crock. Its volume is pi times 0.75 ft squared times 2 ft, about 3.5 cubic feet. To fill it to 14 percent takes about 0.5 cubic feet of vapor, roughly 1 percent of what leaked. A crock at the low point of a floor that has been draining refrigerant vapor all night can hold an ignitable mix, and so can a floor drain pit.

The water heater. Its burner sits at floor level and draws combustion air from the bottom of the room. Overnight it has been pulling the refrigerant-rich layer through its flame. An A2L at a few percent passing through a burner decomposes; it does not need to reach its LFL to make HF.

So the tech's order is:

1. Before diagnosing, stop the flame and move the air. On arrival with the system found flat in a closed basement, the tech turns the water heater gas control to off, not pilot (a standing pilot is still a flame), or closes the appliance's gas shutoff valve if the control has no off position, and does not light, braze or plug anything in. They prop the basement door, open the egress window and stand a fan at the window blowing out, so the vapor leaves at floor level rather than being stirred upward. Light switches and the existing fan circuits are not worked at floor level near the crock, and the portable fan is plugged in at a receptacle above the low layer.
2. Check the air, low. A refrigerant leak detector rated for A2Ls is held at floor level and lowered into the sump crock; a reading at the low point is the one that matters. A sharp acrid smell at the water heater means decomposition products: everyone out, ventilate from the outside, and nobody re-enters until the smell is gone.
3. Only then the repair. The crack is cut out and brazed only after the system is confirmed empty of refrigerant (recovery of whatever remains, then a nitrogen purge flowing through the line while brazing), with the basement air checked clean at the floor first. The ventilation runs throughout.
4. Resume the appliance last. The water heater is relit only after the floor-level reading is clean and the refrigerant system has been pressure tested and found tight. If the appliance had been drawing decomposition products for a night, its burner and vent are inspected for acid damage before it goes back in service.

What changes the call

• R-410A instead of R-32. No flammability, so the crock is not an ignition question, but steps 1 and 2 stand unchanged: the water heater will still turn the leak into HF, and the low layer is still the exposure.
• A crawlspace or pit, not a basement. The tech's whole body is in the low layer. Treat a recent large leak into a crawl as a reason not to enter until it is ventilated and checked from the access, and use the shop's confined-space judgment for pits. See related: Confined Space Entry (OSHA 1910.146).
• Symptoms. A tech or occupant who has dizziness, a racing or irregular heartbeat, or a burning throat or chest after a leak or a decomposition smell leaves the space now. Burning throat, coughing or chest tightness after an acrid exposure warrants medical evaluation the same day, because acid lung injury can be delayed; trouble breathing, chest pain or collapse is a 911 call.

References

• ASHRAE Standard 34, refrigerant designation and safety classification, and ASHRAE Standard 15, in the editions your code adopts
• UL 60335-2-40, safety requirements for heat pumps, air conditioners and dehumidifiers (A2L provisions)
• EPA Section 608 rules, 40 CFR Part 82 Subpart F (venting prohibition and recovery), and SNAP approvals for A2L refrigerants
• See related: Nitrogen Purging and the Air You Cannot Smell; Refrigerant Recovery Procedures Reference; Refrigerant Tank Handling Reference