Often confused · Type III section
Purge units: why low-pressure chillers need them
Low-pressure chillers need a purge unit because they run below atmospheric pressure: a leak lets air in rather than letting refrigerant out, and that air has to be separated from the refrigerant and removed, which is the purge unit's whole job.
Part of Type III section in the Study guide
What the purge unit does with the air
What the purge run time tells you
Air and other non-condensables take up space the refrigerant needs, which costs capacity, and the moisture that rides in with the air contaminates both the refrigerant and the oil.
A busy purge unit is a leak report
A purge unit that runs and runs is doing its job, and that's the bad news. EPA's Type III test topics tie excessive purging to air leaking into the system (EPA test topics, checked October 7, 2026). It's a gauge that happens to have a motor.
Finding a leak that pulls inward
An inward leak won't show itself while the shell is in a vacuum. The test topics give the order for a leak search: raise the pressure with hot water or a built-in heating device first, then nitrogen if needed, staying under the leak-test ceiling the exam keys. That figure and where it comes from are on the rupture disc.
Why it reaches the leak-rate rules
An R-123 (HCFC) or R-11 (CFC) chiller holding 50 lb or more that cools a building is comfort cooling under 40 CFR 82.157, with a 10% threshold (eCFR as of October 5, 2026). Refrigerant lost through the leaks behind the purge activity shows up as top-offs, and top-offs are what the leak-rate thresholds are measured against.
The machine room around it
Purge discharge and chiller leaks both end up in an equipment room. EPA's Type II and III test topics cite ASHRAE Standard 15 for those rooms: an oxygen-deprivation sensor for all refrigerants, and a refrigerant sensor where the chiller runs on R-123 (EPA test topics, checked October 7, 2026).
A high-pressure leak and a low-pressure leak
How the leak behaves, and how you find it
| Item | High-pressure system (R-410A, R-22) | Low-pressure chiller (R-123, R-11) |
|---|---|---|
| Pressure inside, compared with the room | Above atmosphere | Below atmosphere in the evaporator while running |
| Which way a leak flows | Refrigerant out | Air and moisture in |
| Early sign | Charge drops, cooling suffers | Purge unit runs more |
| Pressurizing for a leak test | Nitrogen, alone or with a trace of R-22 | Heat first (hot water or heating device), then nitrogen |
| Gas you never use | Oxygen or compressed air | Oxygen or compressed air |
Yellow edge: the two columns say different things on this row.
Leak-test order and gases: EPA test topics (Core safety, Type II, Type III), checked October 7, 2026. Pressure classes: 40 CFR 82.152, eCFR as of October 5, 2026.
Read the purge the way the exam does
Separate what the stem says the purge unit is doing from what the chiller is doing before you choose.
0 right · 0 of 7 answered
Which of the following terms refers to the process of removing non-condensable gases from a refrigeration system's refrigerant to improve system performance and decrease pressure?
Why each option is right or wrong
Answer: B. Purge
- AWrong: recycling cleans refrigerant (oil separation, filter-driers) for reuse; it isn't the term for venting off non-condensables.
- BCorrect: purging removes non-condensable gases such as air, which lowers head pressure and restores performance.
- CWrong: pulling a vacuum removes everything from an empty system; it doesn't separate air from a refrigerant charge.
- DWrong: recovering means removing refrigerant and storing it, not removing non-condensables from it.
Purging removes non-condensable gases, which raise condenser pressure and cut the system's efficiency.
What is the correct procedure when the purge unit activates frequently on a low-pressure chiller?
Why each option is right or wrong
Answer: B. Perform a thorough leak check
- AWrong: adding refrigerant doesn't stop air from getting in, and frequent purging points to a leak, not a low charge.
- BCorrect: frequent purging means air is getting in through a leak on the part of the machine that runs in a vacuum, so find and fix the leak.
- CWrong: disabling the purge lets air build up, raising head pressure and causing acid problems.
- DWrong: raising system pressure isn't a fix; find and repair the leak instead.
Frequent purge unit activation indicates air leakage into the system. A thorough leak check should be performed to identify and repair the source of air infiltration.
A factory purge unit fails. A technician manually isolates it and operates the chiller with elevated head pressure. Why is this a problem?
Why each option is right or wrong
Answer: D. It prevents continuous expulsion of noncondensables.
- AWrong: a failed purge leads to air buildup and high head pressure, not liquid flooding the compressor.
- BWrong: a rupture disc relieves to the outdoors through its vent line, and isolating the purge doesn't immediately vent it.
- CWrong: isolating the purge unit doesn't bypass the high-pressure cutout.
- DCorrect: with the purge isolated, air stays in the condenser and head pressure keeps rising, which puts the charge at risk of release through leaks or the rupture disc.
Purge units continuously expel noncondensables. Isolating it allows air to accumulate, elevating head pressure and increasing the likelihood of leaks or eventual rupture disc activation.
A technician is servicing a low-pressure chiller experiencing frequent high head pressure and severe compressor oil acidification. The system logs show the purge unit has been running almost continuously. What is the most likely root cause?
Why each option is right or wrong
Answer: B. Chronic air ingress through low-side leaks
- AWrong: a starved evaporator doesn't make the purge run nonstop or bring in the moisture that forms acid.
- BCorrect: air and moisture leaking into the vacuum side make the purge run constantly, raise head pressure, and form acid in the oil.
- CWrong: an overcharge doesn't bring in air or moisture, so it doesn't explain constant purging or acid.
- DWrong: a miscalibrated bleed valve might affect purge efficiency, but it can't explain moisture and acid in the oil.
Continuous purge unit operation indicates a severe low-side leak. Air ingress introduces moisture, which reacts with refrigerant and oil to create acids, leading to severe system contamination.
A low-pressure chiller's purge controller indicates a bleed rate of 1.5 minutes of purge per hour of operation. If the manufacturer specifies a normal maximum allowable continuous purge time of 0.2 minutes per hour, what does this indicate?
Why each option is right or wrong
Answer: D. A severe air leak is causing excessive refrigerant venting
- AWrong: liquid stacking in the condenser wouldn't make the purge run more often.
- BWrong: an expansion valve restriction doesn't create non-condensables for the purge to remove.
- CWrong: purge time far above the manufacturer's limit signals a problem, not good operation.
- DCorrect: purging far above the limit means air is leaking in, and every purge cycle also releases some refrigerant along with the air.
Purge unit operation highly exceeding manufacturer limits indicates noncondensables (air) are rapidly entering the system. This severe leak causes the purge unit to unintentionally vent excess refrigerant.
In a low-pressure chiller, where do non-condensable gases such as air collect?
Why each option is right or wrong
Answer: B. At the top of the condenser
- AWrong: the compressor moves gas through; air doesn't settle there.
- BCorrect: air can't condense, so it gathers in the top of the condenser above the refrigerant; the purge unit draws its vapor from there.
- CWrong: the bottom of the condenser fills with condensed liquid refrigerant; the air sits above it.
- DWrong: air in the evaporator is swept to the compressor and on to the condenser, where it collects.
Air and other non-condensables can't condense at condenser pressure, so they collect in the top of the condenser, above the liquid refrigerant. That is where a purge unit draws vapor from to separate the air out.
A factory purge unit on a low-pressure centrifugal chiller is running continuously but failing to remove noncondensables effectively. Which troubleshooting sequence addresses the most likely causes of this malfunction?
Why each option is right or wrong
Answer: D. Check purge controller settings and sensors, then verify purge valve seating and seals.
- AWrong: an oil filter and cooling tower setpoints have nothing to do with how well the purge removes air.
- BWrong: bypassing a rupture disc removes overpressure protection and is never acceptable.
- CWrong: recovering the whole charge is a drastic step before simple purge checks have been done.
- DCorrect: check the purge controller settings and sensors first, then make sure the purge valves seat and seal properly.
When a factory purge runs continuously but fails, the primary maintenance steps are checking controller settings and sensors, then verifying purge valve seating and seals.
More practice across all four sections: Practice test · Timed mock
Key takeaways
- A low-pressure chiller runs below atmosphere, so leaks pull air and moisture in.
- The purge unit separates that air from the refrigerant, returns the refrigerant and discharges the air.
- Leak search on a chiller: heat first, then nitrogen, under the exam-key leak-test ceiling.
- Chiller rooms need an oxygen-deprivation sensor, plus a refrigerant sensor for R-123 (ASHRAE 15, per the EPA test topics).
- The whole low-pressure section is on Type III low-pressure chillers.