R-600a (isobutane) PT Chart and Refrigerant Profile

Where you will find R-600a

R-600a is the refrigerant in most newer household refrigerators and freezers, and in some small stand-alone commercial cases and vending machines. EPA’s SNAP program lists isobutane as acceptable subject to use conditions in household refrigerators and freezers since December 20, 2011, with the listing updated on August 8, 2018 to reference UL 60335-2-24. Stand-alone retail units and vending machines are also listed end uses.

It took over from R-134a in domestic refrigeration, and before that R-134a had replaced R-12. Check the rating plate inside the cabinet or on the compressor: it names the refrigerant and the factory charge, usually in grams or ounces.

The pressures are much lower than R-134a. At 40°F saturation R-600a reads 11.9 psig, against 35.1 psig for R-134a and 36.9 psig for R-12. Below 10.9°F (its boiling point at sea level) it runs in a vacuum: at 0°F evaporating it reads 6.4 in Hg vacuum. A freezer low side reading inches of vacuum is therefore normal on R-600a, and a leak on that side can pull air in.

Charging R-600a

Charge by weight only, with a scale that resolves grams. Charges are small and the system is sensitive to small deviations, so the nameplate number is the target. Linde, which sells R-600a as CARE 10, states it is not a retrofit refrigerant: it needs equipment and a compressor designed for it. Do not top off an R-134a or R-12 box with isobutane.

R-600a is a single-component refrigerant, so there is no glide and one PT column serves both sides. If the unit has access ports and you take readings for diagnosis, convert suction and liquid pressures with the chart and use the superheat calculator or subcooling calculator, but defer to the manufacturer’s service data for targets. Because low-side readings are often in a vacuum, make sure your gauge reads in Hg correctly; the pressure converter handles psig, psia and in Hg. For evacuation targets before charging, see the evacuation microns guide.

Oil and compatibility

Danfoss states R-600a is mainly used with mineral compressor oils, with alkylbenzene or POE also possible. BITZER’s refrigerant table lists mineral oil, PAO, POE and alkylbenzene. In practice the compressor comes factory-charged with the oil its maker approved; replace a compressor with one specified for R-600a and do not mix in oil from another system. The refrigerant oil types guide covers the families.

Safety, handling and recovery

A3 is the highly flammable class in ASHRAE 34 (LFL at or below 0.10 kg/m³ or heat of combustion of 19,000 kJ/kg or more), with lower toxicity. Work with ventilation, keep ignition sources away, and do not braze on tubing that still contains refrigerant. The flammability classes page explains the difference between A3 and A2L.

Section 608: at 104°F R-600a saturates at 62.4 psig (77.0 psia), inside the 45 to 170 psia medium-pressure band of 40 CFR 82.152. A household refrigerator is a small appliance (factory-sealed, 5 lb or less), so Type I certification applies; larger commercial equipment falls under Type II.

Isobutane is exempt from the venting prohibition under 40 CFR 82.154(a)(1), but only in the listed end uses: stand-alone retail units, household refrigerators and freezers, and vending machines. In any other use it is a non-exempt substitute. There venting is prohibited, recovery levels apply (small appliances 90% with the compressor running, 80% not running, or 4 in Hg vacuum; medium-pressure appliances under 200 lb to 10 in Hg), and sales have been restricted to certified technicians since January 1, 2018. Any recovery machine used must be certified for flammable refrigerants under Appendix B4 of 40 CFR 82.158, which adds UL 1963 Supplement SB. See recovery machine compatibility.

Cylinders: new containers are light green-gray RAL 7044 under AHRI Guideline N-2017, with a red band on the shoulder or top marking a flammable refrigerant. EPA SNAP use conditions specify red (PMS #185) markings for flammable refrigerants. More on the cylinder colors page.

Status and replacements

R-600a is not an ozone-depleting substance and not a regulated HFC, so the AIM Act phasedown and Technology Transitions limits do not apply. What controls its use is SNAP: acceptable with use conditions in the listed end uses, with charge limits set through UL 60335-2-24 for household products and UL 60335-2-89 for commercial units. GWP has no IPCC AR4 value; EPA uses 1 in 40 CFR 84.64, while its SNAP pages list 3. For how other refrigerants compare, see the GWP chart.

Common questions

Is isobutane refrigerant dangerous?

It is classed A3: lower toxicity but highly flammable. That is why EPA allows it only in specific self-contained end uses with charge limits and markings, and why recovery equipment must be rated for flammable refrigerants.

Can R-600a replace R-134a in a refrigerator?

Not as a field conversion. Linde states R-600a is not a retrofit refrigerant and requires purpose-designed equipment; its pressures are also far lower than R-134a at the same temperature, as the chart shows. Replace an R-600a compressor only with an R-600a compressor.