Metering Device Troubleshooting: Subcooling, Sight Glasses & Restrictions

Getting an accurate subcooling reading is only useful if you also know what's happening on the liquid line, and that's where this video starts. We look at why a metering device needs pure liquid refrigerant — never a vapor mixture — and how a simple sight tube or glass installed before the device lets you visually confirm what your numbers are showing you. A big chunk of this troubleshooting conversation centers on what happens when subcooling looks normal but bubbles show up where you're viewing the line — a classic clue that something is restricting flow, even when a temperature drop test alone won't catch it. We also dig into long line set scenarios, like a liquid line running across a hot attic, where the line can reach saturation and lose subcooling entirely along the way, something every tech chasing a charge problem should understand. You'll also see why adding a service port near your viewing glass on long runs gives you a real-time, accurate read instead of relying solely on readings at the condenser. We cover liquid line sizing, refrigerant velocity, oil return, and why some manufacturers shrink that line down rather than oversize it. Filter dryer placement (never behind the valves!) gets its own rant too — and yes, we mention sight glasses again before we're done, because they really are one of the most underrated diagnostic tools out there. Whether you're hunting down a true restriction or just want a faster way to confirm liquid ahead of the device, this one's full of practical field tips for your next service call. Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/ or the HVAC School Mobile App on the Google Play Store (https://hvacrschool.com/play-store) or App Store (https://hvacrschool.com/app-store).