System and Duct Design January 13, 2026, 5:51 am

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System and Duct Design January 13, 2026, 5:51 am

January 13, 2026

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How to Evacuate an AC system, Full Vacuum Procedure

Our latest Podcast

HVAC School – For Techs, By Techs

Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

In this short podcast, Roman Baugh takes over with his new series, “The Deep End,” where he shares tech support conversations. This time, Roman is on the other end of a live tech support call about a grounded compressor with an existing Mitsubishi mini-split (Mr. Slim heat pump). The unit in this case had an unclear service history due to the homeowner moving in recently.

In this issue, the compressor kicked on and kicked off immediately. After the technician in this scenario told tech support that he had no continuity from the compressor to ground, tech support asked him to check the resistance (which was 43 ohms from black to ground and 43 from white to ground). Tech support concluded that the compressor was grounded, and it needed to be replaced (along with the PCB). However, the resistance disappeared mid-call, and intermittent resistance readings don’t always indicate a failed winding; it’s worth noting that cool ambient temperatures that enable liquid refrigerant to migrate into the compressor could potentially skew electrical readings.

Roman recommended shutting the system down and trying to get the system to run. His rationale was that if the unit ran and didn’t keep stopping, the system may have been overcharged or lacking a crankcase heater. However, that did not show any improvement in operation; the compressor rattled for about a second and shut off. A resistance test confirmed that the compressor was grounded, pointing to a mechanical issue with the compressor and not just a one-off instance of refrigerant migration. This type of failure can happen when there are repeated instances of liquid refrigerant in the crankcase over time.

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

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