Frozen Evaporator Coil - Diagnostic Tree

Symptom

Customer reports an air conditioner that's running but not cooling - and on inspection the evaporator coil (indoor) is iced over. Often presents as "the unit ices up, I turn it off for a while, then it cools for an hour and ices up again." Root cause is always one of two things: not enough airflow across the coil OR not enough refrigerant in the system. Symptoms look identical; the diagnostic distinguishes them.

Likely Causes

In order of frequency:

• Restricted airflow (dirty filter) - 40 % of frozen-coil calls
• Restricted airflow (blocked return grille, closed dampers, kinked flex duct) - 15 %
• Failed indoor blower motor or capacitor - 10 %
• Undercharged refrigerant (slow leak) - 12 %
• Restricted metering device (TXV or fixed orifice) - 8 %
• Dirty evaporator coil itself - 5 %
• Setpoint too low + low ambient + high humidity (75 °F setpoint at 65 °F outdoor with damp returns) - 5 %
• Other (refrigerant blend issues, wrong evaporator for the condensing unit) - 5 %

Diagnostic Steps

1. Confirm the coil is actually frozen. Pull the access panel. Ice on the coil itself = frozen evap. Ice ONLY on the suction line outside = different issue (overcharge or low ambient).
2. Thaw the coil before proceeding. Turn the system to FAN ONLY (not OFF - fan running thaws faster). Wait 1 - 2 hours OR use a small space heater pointed at the coil with the fan running.
3. Replace the air filter - always, even if it "looked clean." Cellulose filters can be airflow-blocking before they look dirty.
4. Verify return airflow path is clear. Walk every return grille. Closed dampers? Furniture blocking? Kinked flex duct in the attic?
5. Measure indoor temperature delta. With the system running cool and the coil clean + airflow restored:

• Delta of 18 - 22 °F → system OK; problem WAS airflow restriction
• Delta < 15 °F → low refrigerant OR restricted metering device
• Delta > 25 °F → low airflow (filter, blower, ductwork)

1. Gauge the system. With evap coil clean and dry:

• Low suction + low liquid + high superheat → undercharged
• Low suction + normal liquid + high superheat → metering device restriction
• Low pressures + low superheat → low charge with the orifice still bypassing some

1. Measure indoor blower amperage. Compare to FLA. Lower than expected = motor / wheel / capacitor issue restricting airflow.

Resolution

• Dirty filter or restricted return: filter change + airflow restoration. No further work needed once the coil is fully thawed and operating cleanly.
• Failed blower motor or capacitor: replace; verify FLA returns to spec.
• Undercharged refrigerant: leak-search and repair, then recharge. NEVER just "top off" - see EPA warning.
• Restricted metering device: TXV replacement (or fixed-orifice cleaning if the orifice is field-serviceable).
• Dirty evaporator coil: chemical clean per manufacturer label, in-place if accessible.
• Low ambient + low setpoint: customer education. Modern thermostats with lockout below 55°F outdoor handle this automatically.

Prevention

• Annual coil + filter check catches a dirty coil before it freezes
• Customer education on filter change frequency (typically 60 - 90 days, more often for pet households)
• Quote a leak-search test on any system that's had a recharge in the last 2 years

References

• ACCA Quality Installation Standard
• EPA Section 608 leak-rate regulations
• Manufacturer service literature for the specific equipment
• Manuall internal: AC Won't Cool - Diagnostic Tree, Heat Pump Refrigerant Charge Reference