The short answer
Low-pressure work runs backwards from the rest of the exam. A low-pressure appliance uses a refrigerant whose liquid-phase saturation pressure is below 45 psia at 104 °F — R-11, R-123, R-113, R-245fa (§82.152). Because those refrigerants boil above ordinary room temperature at atmospheric pressure, the low side of the machine runs below atmospheric pressure. So a leak pulls air and moisture in instead of pushing refrigerant out — and the opening rules flip direction: for non-major work you pressurize up to 0 psig instead of pulling a vacuum down to a Table 1 level.
Major vs. non-major: the line that decides everything
Whether you evacuate to Table 1 or pressurize to 0 psig depends entirely on whether the work is major maintenance, service, or repair as defined in §82.152. It is major if it involves:
- Removing the compressor, condenser, evaporator, or auxiliary heat exchange coil; or
- Uncovering an opening of more than 4 square inches of flow area for more than 15 minutes.
Anything else — a control swap, a sensor, an oil change handled under its own rule below — is non-major, and the non-major path in §82.156(a)(1) applies only when the refrigerant will not be evacuated to the atmosphere after the work. If either condition fails (the work is major, or the charge will be vented to atmosphere afterward), you are back on the full Table 1 evacuation.
Major work: evacuate to 25 mm Hg absolute — and verify
Before a low-pressure appliance (or the part being serviced, if its refrigerant can be isolated to a system receiver) is opened for major work, it must be evacuated — including all the liquid refrigerant — to 25 mm Hg absolute, using recovery and/or recycling equipment certified under §82.158, and the technician must verify that level has been reached before opening (§82.156(a)). Low-pressure is the one Table 1 row that does not split by equipment era: 25 mm Hg absolute with pre-November 15, 1993 equipment and with post-1993 equipment alike.
Non-major work: pressurize, don't evacuate
- Low-pressure, non-major: pressurize to a pressure no higher than 0 psig before opening. When isolation is not possible, cover all openings while the work is performed (§82.156(a)(1)(ii)).
- Medium-, high-, and very-high-pressure, non-major: the mirror rule — evacuate to a pressure no higher than 0 psig before opening (§82.156(a)(1)(i)). Same number, opposite direction.
- Oil changes (low-pressure): their own line — evacuate or pressurize to no higher than 5 psig before opening, or drain the oil into a system receiver that is itself evacuated or pressurized to no higher than 5 psig (§82.156(a)(1)(iii)). The 5 psig figure belongs to oil changes only; it is not a general opening pressure.
- Leaking components: isolate leaking from non-leaking components wherever possible; non-leaking components still hit Table 1; leaking components go to the lowest level attainable without substantially contaminating the recovered refrigerant, capped at 0 psig (§82.156(a)(2)).
Raising the pressure: heat first, nitrogen last
For low-pressure refrigerants with a boiling point above 85 °F at standard atmospheric pressure (29.9 in Hg), §82.156(a)(1)(ii) requires using heat to raise the internal pressure as much as possible; nitrogen may be used only to raise the pressure from the level heat can reach up to atmospheric pressure. For refrigerants boiling at or below 85 °F, the rule bars methods such as nitrogen that would require subsequent purging. The practical reason is the same in both cases: gas pushed in beyond what the job needs has to come back out again, mixed with refrigerant.
Purge units: what they do, and the one CFR number that governs them
Because a low-pressure machine's low side runs below atmospheric pressure, air and moisture leak in and collect as noncondensable gas. A purge unit removes that noncondensable gas from the machine so it does not blanket the condenser and wreck efficiency. That is the job the exam expects you to recognize — and the CFR's own treatment of purge devices is narrow and specific:
- Under §82.158(d)(6), recovery/recycling equipment equipped with a noncondensables purge device must not release more than 3 percent of the quantity of refrigerant being recycled through noncondensables purging, under the applicable appendix B test conditions. That 3% cap is the purge number the regulation actually sets.
- Purge-unit operating details — purge rates, setpoints, and machinery-room monitor requirements — are not in Section 608's text, and this page does not invent them. If a study guide quotes a purge rate as a regulation, ask for its citation.
Deliberately omitted: the Type III leak-test pressure and rupture-disc pressure figures that circulate in exam-prep materials are not stated here. No primary EPA/CFR source for them was located in CoolBooked's verification passes (see SOURCES.md). Do not memorize a number for them from this page — and treat any guide that prints one without a citation with suspicion.
Why chillers sit in the leak-repair rules
§82.152's comfort-cooling definition expressly includes chillers, so a building chiller is comfort cooling with a 10% annual leak-rate trigger — not the 30% industrial-process rate. ODS chillers at 50 lb or more fall under §82.157; HFC appliances at 15 lb or more fall under §84.106, whose requirements apply as of January 1, 2026. A chiller that exceeds its trigger is repaired within 30 days (120 days where an industrial process shutdown is required), or a retrofit/retire plan is developed within 30 days and completed within 1 year.
Drill it
The 0 psig / 5 psig / 25 mm Hg absolute trio is the most-tested Type III distinction. Run the Type III bin, then the scenario quiz.