What it is

A restriction is a partial blockage in the liquid line that chokes refrigerant flow to the metering device. The usual suspects are a plugged filter-drier, a kinked liquid line, debris or solder in a fitting, or moisture/wax freezing at a low point. The system acts starved — low suction, high superheat — but unlike a true undercharge, the refrigerant is all still in there; it just can't get where it needs to go fast enough.

The classic field tell is a temperature drop across the restriction. When liquid is forced through a small opening, its pressure drops, some of it flashes to vapor, and that flashing makes the line noticeably colder right after the blockage.

How it works

In a healthy liquid line, the refrigerant is subcooled liquid and the line is warm and roughly the same temperature end to end (it gives up a little heat but doesn't suddenly get cold). When you add an unwanted restriction, that spot behaves like a second metering device. Liquid pressure drops across it, a little flashes off, and flashing absorbs heat — so the line downstream of the restriction gets cold, sometimes cold enough to sweat or frost while the line upstream is still warm.

Meanwhile the real metering device downstream is now being fed flash gas and low-pressure liquid instead of a solid liquid column. The coil starves: suction pressure falls, evaporator superheat climbs. Back at the condenser, liquid backs up because it can't move through the restriction, so subcooling reads high. That combination — low suction, high superheat, high subcooling — is the restriction fingerprint.

In the field

  1. Feel and measure the liquid line. Run your hand along it from the condenser to the metering device. A distinct cold spot — especially right after the filter-drier — is the giveaway.
  2. Measure temperature in and out of the suspect component (drier, or across a fitting). Put a clamp probe on the line just before and just after. A drier should have essentially no temperature drop across it. A few degrees of drop, and especially sweat or frost on the outlet, means it's restricting.
  3. Take the full set of readings: suction pressure/temp, liquid pressure/temp at the condenser, and compute superheat and subcooling.
  4. Compare to the low-charge picture. Low charge gives low suction, high superheat, AND low subcooling. A restriction gives low suction, high superheat, but high subcooling. Subcooling is the deciding number.

Normal values & targets

  • Temperature drop across a healthy filter-drier: essentially zero — call it under ~2–3°F. Anything beyond that, or visible sweat/frost on the outlet while the inlet is warm, points to a restricting drier.
  • Restriction signature: suction pressure low, evaporator superheat high (often 25°F+), subcooling high (well above the ~8–12°F you'd expect).
  • Undercharge signature (for contrast): suction low, superheat high, subcooling low.
  • Healthy liquid line: warm, no sudden cold spots, slight subcooling maintained to the metering device.

Common faults & what they mean

  • Cold/frosted drier outlet, warm inlet: plugged filter-drier. Most common restriction. Change it.
  • Cold spot at a brazed joint or fitting: solder slug or debris caught in the line from a prior repair.
  • Restriction that comes and goes, frost ball at the metering device that thaws when the system sits: moisture freezing at the orifice. Change the drier and pull a deep vacuum with a decay test — a new drier alone won't fix wet refrigerant that's already in the system without proper evacuation.
  • High subcooling + high head + normal-to-low suction: liquid stacking behind a restriction. Don't mistake the high subcooling for an overcharge and start recovering refrigerant — you'll chase your tail.
  • Kinked or crushed liquid line: physical restriction, often from rough handling or a tight bend. Visual inspection finds it.

Tech tips & gotchas

  • Subcooling is the tiebreaker. A starved coil with HIGH subcooling is a restriction; a starved coil with LOW subcooling is a real undercharge. Don't add refrigerant to a restriction — it won't fix it and you'll overcharge the condenser.
  • A frosting drier is not "low charge." New techs see frost and reach for the gauges to add gas. Feel the drier first. Frost on a drier means it's restricting, not that the system is hungry.
  • Replace the drier any time you open the system anyway. It's cheap insurance, and a marginal drier is the most likely thing to become your next restriction.
  • Moisture is a sneaky restriction. If the symptom appears only after the system has run a while (ice forming at the metering orifice), think moisture, not metal. The cure is a real evacuation, not just parts.
  • A bidirectional drier on a heat pump can restrict in either direction — check it the same way regardless of mode.

Safety / code notes

Recover refrigerant per EPA 608 (40 CFR Part 82, Subpart F) before cutting in a new drier or clearing a restriction. Always braze with nitrogen flowing to avoid creating the very oxidation/scale that becomes the next restriction. After the repair, evacuate to a deep vacuum and confirm with a standing decay test before recharging — especially if moisture was the cause.