What it is

There are two separate questions with a refrigerant restriction, and most techs only answer the first: is there a restriction, and where is it. Confirming a restriction is the easy part — high superheat and high subcooling at the same time, with low suction, is the classic signature (liquid stacks behind the choke, evaporator starves downstream). The hard, money-saving part is localizing it so you replace the right component instead of throwing parts at the liquid line.

The three usual choke points: the liquid-line filter drier (plugs with debris or moisture-swollen desiccant), the metering device (a TXV stuck partly closed, or a fixed orifice/piston fouled), and a physical line restriction (a kink, a crimp from a careless install, a brazing slug, or a flattened section). Each one sits at a different spot in the circuit, and you find it by following where the temperature drops and where the frost line begins.

How it works

A restriction is a pressure-drop point. Refrigerant on the upstream side is high-pressure liquid; as it squeezes through the restriction it loses pressure, and a liquid losing pressure flashes — it boils, and boiling absorbs heat, so the line gets cold right at and just after the restriction. That localized cold spot (often frost or heavy sweat) is your fingerprint. The trick is that the metering device is supposed to be the cold-transition point — that's its job. So localizing a restriction is about finding cold/pressure-drop happening in the wrong place (a drier or a mid-line kink) or finding the metering device behaving wrong (the cold transition there but the system starved as if the valve isn't opening).

Walk the liquid path from condenser to evaporator and the temperature should stay warm (subcooled liquid) all the way to the metering device, then drop. Any cold spot before the metering device is a restriction you can point at.

In the field

  1. First confirm it's a restriction, not a charge problem. High superheat and high subcooling, low suction. (High SH + low SC is undercharge — different fix. See the low-suction differential.)
  1. Walk the liquid line by feel and thermometer, condenser → indoor. It should be uniformly warm/hot (it's high-pressure liquid) until the metering device. Use your hand first, then an IR or clamp thermometer to quantify.
  1. Find the cold/frost transition point — that's the restriction.
  • Cold spot or frost at the liquid-line drier (warmer line going in, distinctly colder coming out) → the drier is plugged. A real temperature drop across a drier — more than a couple degrees — means it's restricting. This is the most common and cheapest fix; replace the drier.
  • Cold transition at a kink, crimp, or flattened spot mid-line → physical line restriction. You can often see the damage (a kinked flare-out, a crushed section behind equipment, a poorly bent line set). Inspect the run, especially anywhere it was handled during install.
  • Cold transition at the metering device but the evaporator is starved (high superheat, low suction, the valve acting like it won't open) → the TXV/orifice is the restriction. On a TXV, this can be a stuck valve, a lost bulb charge (valve closes), a plugged inlet screen, or ice/wax fouling. On a fixed orifice, it's a fouled/undersized piston or debris.
  1. Use the drier as a sanity check on the whole liquid line. A noticeable drop across the drier almost always means the drier (or trapped debris/moisture). No drop across the drier but a starved evaporator points downstream toward the metering device.
  1. For a suspected TXV specifically: check whether it responds. Warm the bulb (valve should open, suction rises) and cool it (valve should close, suction drops). A valve that won't respond is stuck or has lost its charge — that's the restriction. Also check the inlet screen if accessible; a plugged screen mimics a stuck valve.
  1. Confirm after the fix. Once you replace the right part and recharge, superheat and subcooling should both return to target and suction should come up. If subcool drops but superheat is still high, you fixed one restriction and missed another, or the metering device is also involved.

Normal values & targets

Orientation only:

  • Temperature across a healthy liquid-line drier: essentially none — maybe ~1–3°F. A drop of more than a few degrees (and especially anything you can feel as a distinct cold/frost line) flags a restricting drier.
  • Liquid line temperature: should read as warm subcooled liquid (saturation minus ~8–12°F subcool) the whole way to the metering device. It should NOT be cold until the metering point.
  • Restriction signature: high superheat (often 20–40°F+), high subcooling (well above the ~8–12°F target), low suction.
  • Frost at the metering device: normal there if the system is otherwise healthy; abnormal as a starvation sign when paired with high superheat and low suction.

Common faults & what they mean

  • Distinct cold/frost line at the drier, warm just upstream: plugged drier. Most common, cheapest. Recover, replace drier, evacuate, recharge.
  • Cold spot at a visible kink/crimp/flattened section: physical line restriction from install damage or a brazing slug. Repair/replace that section.
  • Starved evaporator, cold transition at the TXV, valve won't respond to bulb temp: stuck/charge-lost TXV or plugged inlet screen. Replace/repair the metering device.
  • Fixed-orifice system, restriction signature, no drier/line cold spot: fouled or wrong piston. Inspect/replace the orifice.
  • Restriction returns after a drier change: you treated the symptom of moisture or debris but didn't address the source (moisture from a bad evacuation, or debris from a prior burnout). Find why the drier plugged.
  • High subcool + high superheat but no findable cold spot: re-verify the readings (instrument error) and recheck for a partially restricting metering device or an internal line issue.

Tech tips & gotchas

  • The metering device is the one legitimate cold-transition point. Don't mistake normal TXV/orifice cold for a restriction. The question is whether the cold is in the wrong place (drier, mid-line) or whether the metering device itself is starving the coil.
  • A plugged drier is the cheapest possible win — check it first. A two-dollar temperature reading across the drier can save you from condemning a TXV.
  • Moisture restrictions wax and wane with temperature. Free moisture can freeze at the metering device and intermittently restrict (a "cold weather only" or "after running a while" starvation). If a restriction comes and goes, suspect moisture and a weak prior evacuation — and replace the drier with a proper vacuum.
  • Don't forget the inlet screen on a TXV. A plugged inlet screen acts exactly like a stuck valve. If accessible, it's a cheap thing to check before condemning the whole valve.
  • Install damage hides behind equipment and in walls. A kink from a rough install or a line set crushed against a joist won't be obvious — inspect the whole run, especially tight bends and anywhere it was handled.
  • Find why it restricted, or it comes back. Debris means a prior failure (burnout, contamination); moisture means a bad evacuation. Fix the cause, not just the drier.

Safety / code notes

  • Recover per EPA 608 before opening the system to replace a drier, TXV, or line section; never vent.
  • Always install a new liquid-line drier when you open the system for a restriction repair, and pull a proper vacuum (decay-tested) — a restriction repair done without addressing moisture just reschedules the failure.
  • Brazing safety: flow nitrogen to prevent oxidation/scale (which itself becomes future restriction debris), and protect against fire and fumes.
  • Pressurized-system and live-panel safety as usual: rated tools, eye protection, PPE.