Type III: Low-Pressure Appliances
Type III certification covers low-pressure appliances, the large centrifugal chillers that use refrigerants like R-11, R-123, and R-1233zd. Because these machines run in a vacuum, they demand special recovery levels, careful pressurization limits, and equipment such as purge units. This chapter walks through the facts unique to low-pressure work.
Low-Pressure Appliances Defined
A low-pressure appliance uses a refrigerant with a boiling point above roughly 50°F at atmospheric pressure, so the machine operates at or below atmospheric pressure — in other words, in a partial vacuum on the low side. Classic low-pressure refrigerants are CFC-11, HCFC-123, and the newer low-GWP HFO R-1233zd, all used in large centrifugal chillers that cool commercial buildings. Because the evaporator runs in a vacuum, any leak draws air and moisture inward rather than pushing refrigerant out, which shapes almost every Type III procedure. Servicing these chillers is specialized, high-value work, and the recovery, pressurization, and purge rules below apply specifically to low-pressure machines.
Recovery from Low-Pressure Systems
The required recovery level for a low-pressure appliance is 25 mm Hg absolute, and this figure is the same whether your recovery equipment was made before or after November 15, 1993. Note the units: 25 millimeters of mercury absolute is a deep vacuum, roughly 29 inches of mercury vacuum on a compound gauge, far deeper than the levels used for high-pressure systems. Because low-pressure refrigerant boils near room temperature, recovery is helped by keeping the refrigerant warm and the recovery cylinder cool so vapor migrates toward the cylinder. You may circulate warm water through the chiller tubes or use the machine's own heaters to raise the refrigerant temperature and speed recovery, but never apply a torch. Recover liquid first, then draw the remaining vapor down to 25 mm Hg absolute.
Pressurization and Leak Testing
Because a low-pressure chiller normally runs in a vacuum, you must add pressure to test for leaks, and doing this safely is critical. Never pressurize a low-pressure appliance above 10 psig. Low-pressure chillers are protected by a rupture disk that is typically set to relieve at 15 psig, so exceeding 10 psig risks bursting the disk and venting the charge. To raise pressure for a leak test you may add a small amount of refrigerant vapor and then nitrogen, or use controlled heat, but you must watch the gauge closely and stay under the limit. Some technicians pressurize just enough to bring the refrigerant above atmospheric so an electronic detector or bubble test can find the leak. Never use oxygen or compressed air, and never let the pressure approach the rupture-disk setting.
Purge Units and Non-Condensables
Because a low-pressure chiller runs under vacuum, air and moisture leak inward and collect as non-condensable gas at the top of the condenser, where they raise head pressure and cut efficiency. A purge unit removes these non-condensables from the high point of the system and vents them, ideally recovering refrigerant vapor that would otherwise escape with the purged air. High-efficiency purge units greatly reduce refrigerant loss compared with old designs, releasing very little refrigerant per pound of air purged. Frequent purging is a warning sign that the machine has a leak drawing air in, so a rising purge rate should prompt leak repair rather than just more purging. Keeping the purge unit maintained protects both the charge and the chiller's cooling capacity.
Water, Rupture Disks, and Chiller Service
Water is a special hazard in low-pressure chillers because a tube leak lets cooling water enter the refrigerant side while the machine is under vacuum. Water causes corrosion and acid formation, can freeze and damage tubes, and contaminates the refrigerant. If the low-pressure limit control or a high-pressure limit device trips, investigate rather than simply resetting it, because it may be signaling water intrusion or non-condensables. The rupture disk must be kept in good condition and never bypassed. When servicing, isolate and recover the charge before opening the machine, protect the rupture disk from over-pressure, and check the purge and float systems. Handling the large low-pressure recovery cylinders carefully and evacuating to 25 mm Hg absolute round out safe, code-compliant chiller service.
Last updated: July 2026