What it is

A frozen evaporator coil is ice building up on the indoor coil until it blocks airflow and the system stops cooling. It's a symptom, not a diagnosis. The coil freezes because its surface temperature drops below 32°F and stays there long enough for condensate to turn to ice instead of draining away. Two root causes drive almost every frozen coil: not enough airflow across the coil, or not enough refrigerant in the system. Your job is to figure out which, because thawing the ice without fixing the cause just buys a day before it freezes again.

How it works

The coil is cold because refrigerant is boiling inside it at a low temperature. Normally the air moving across the coil keeps the surface above freezing — there's enough heat in the airstream to keep the boiling refrigerant warm enough that the fins stay above 32°F and condensate drips off as water.

Break that balance two ways:

  • Low airflow: not enough warm air reaches the coil, so there's not enough heat to keep the surface above freezing. The refrigerant pressure (and saturation temperature) drops, the coil goes below 32°F, and ice forms. Once ice starts, it blocks even more airflow — a snowball effect that buries the coil.
  • Low charge: with too little refrigerant, the pressure in the coil drops, the saturation temperature falls below freezing, and the coil ices up even with good airflow. Low charge also raises superheat, so you'll see that signature on the gauges.

Either way the mechanism is the same: coil surface below 32°F. The difference is why it got there, and the gauges tell you.

In the field

First, thaw the coil completely (fan-only, or heat, or just time) so you can take real readings — you can't diagnose a block of ice. Then run it and look at the signature:

Airflow problem signature:

  • Suction pressure low, but evaporator superheat is normal or low.
  • Evidence of restriction: dirty filter, caked blower wheel, closed registers/dampers, undersized or blocked return, blower speed set too low, collapsed flex duct.
  • Air handler clearly moving weak air.

Low-charge signature:

  • Suction pressure low AND evaporator superheat is high.
  • Subcooling low (true undercharge) — confirm there's no restriction faking it.
  • Often a leak somewhere; the system has lost refrigerant.

So the deciding number is superheat: a freezing coil with normal/low superheat is an airflow problem; a freezing coil with high superheat is a charge (or restriction) problem.

Normal values & targets

  • Airflow target: roughly 350–400 CFM per ton in cooling. Below ~300 CFM/ton and the freeze risk climbs fast.
  • Evaporator split: normal ~16–22°F at proper airflow; a frozen-prone coil from low airflow often shows a very high split before it ices.
  • Coil surface: must stay above 32°F. Saturation temperature in the coil normally sits in the high 30s to low 40s°F for AC; dropping into the low 30s and below is freeze territory.
  • Superheat: normal-to-low with an airflow problem; high (25°F+) with a low charge.

Common faults & what they mean

  • Dirty filter / caked blower wheel / closed vents: classic low-airflow freeze. Normal-to-low superheat confirms it. Restore airflow.
  • Blower speed set too low for the tonnage: under ~350 CFM/ton, the coil can freeze even when everything else is clean. Reset the speed tap / ECM setting.
  • Low charge from a leak: high superheat, low subcooling, low suction. Find and fix the leak, then weigh in a correct charge — don't just "top off."
  • Running AC when it's too cold outside: low outdoor ambient drops head pressure and coil temperature; running cooling below the equipment's minimum outdoor temp can freeze the coil. Not a fault, an operating-condition issue.
  • Restriction (drier/metering device): starves the coil and freezes it with high superheat AND high subcooling — looks like low charge but the subcooling tells the difference (see the restriction article).
  • Oversized system / very low load: short, low-load operation in very dry/cool indoor conditions can drop coil temp; less common but real.

Tech tips & gotchas

  • Superheat splits the diagnosis. Frozen coil + normal/low superheat = airflow. Frozen coil + high superheat = charge or restriction. Memorize that fork.
  • Don't charge a frozen coil. Ice on the coil throws off every reading. Thaw it fully first, then measure, then decide. Adding gas to a coil that froze from low airflow makes it worse.
  • Check the blower wheel, not just the filter. A clean filter in front of a dust-packed blower wheel still starves the coil. The wheel is the airflow killer techs forget.
  • A leak fix is the real repair for low charge. Topping off a leaking system that froze guarantees a repeat call. Find the leak.
  • Look for water damage clues. A coil that's been freezing and thawing overflows the pan when it melts — check for water staining around the air handler, which confirms a recurring freeze.
  • Thaw gently. Never chip ice off aluminum fins; you'll bend or puncture them. Fan-only or heat melts it safely.

Safety / code notes

When the coil thaws it dumps a lot of water — make sure the condensate drain and pan handle it per local plumbing/mechanical code, or you'll cause water damage. Recover/handle refrigerant per EPA 608 (40 CFR Part 82, Subpart F) when connecting gauges or repairing a leak. De-energize before servicing the blower or coil.