Troubleshooting guide

Why Is My AC Freezing Up?

Low airflow, a dirty coil, low refrigerant and operating conditions can all ice an AC coil. Ice alone does not name the cause. How to tell them apart.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

The evaporator coil works below the dew point of the room air so it can remove moisture. When its surface drops below 0 °C (32 °F), the condensate freezes onto it, the ice blocks the airflow, the coil gets colder, and more ice forms. Ice is the result of that loop, not a diagnosis.

Refrigerant work is regulated and needs certification in many places, and the equipment has high voltage and moving fans. Charge checks and electrical diagnosis belong with qualified technicians. If the coil is iced, turn the cooling off and run only the fan to thaw it before anything else.

Symptom	Likely cause	What to check
Ice on the coil and weak airflow at the registers	Dirty filter, closed or blocked registers, blocked return, slow blower speed, or a dirty coil	Filter, return grille and registers first. Blower speed setting and coil condition after thawing
Ice, with normal airflow, and the air is only slightly cool	Low refrigerant charge from a leak, which lowers the coil temperature	A technician measures pressures and finds the leak. Adding refrigerant without finding the leak does not repair it
Ice forms only when it is cool outside	Running cooling at low outdoor temperatures, where many systems are not designed to operate	Check the minimum outdoor temperature in the equipment instructions
Ice forms after a few hours in humid weather	Long runtimes with marginal airflow, or an oversized coil load of moist air	Airflow per ton, filter and blower setting, and return-air leaks
Ice on the refrigerant line outside	Refrigerant-side problem such as low charge or a restricted metering device	Technician check of pressures, superheat and subcooling
Ice returns soon after thawing	The cause has not been fixed. The filter is only part of the picture	Go through airflow, charge and controls in order. Check the blower motor and capacitor

Why low airflow makes a colder coil

The coil absorbs a fixed amount of heat, and less air means that each pound of air must give up more heat. The air temperature drop across the coil rises as airflow falls. For a coil that removes 27,000 Btu/h of sensible heat from the air, the temperature drop is ΔT = Q ÷ (1.08 × CFM):

Airflow (3-ton unit)	CFM per ton	Air temperature drop
1,200 CFM	400	20.8 °F
900 CFM	300	27.8 °F
600 CFM (blocked filter)	200	41.7 °F

A bigger air temperature drop means a colder coil surface. Once the surface falls below 32 °F, ice starts to form. That is why the first check is always airflow.

Why ice alone does not identify the cause

Low airflow and low charge both ice the coil, and they need opposite fixes. A dirty filter is cheap to fix, while a leak needs a repair. The distinguishing clues are in the airflow you can feel, the air temperature at the registers, whether the outdoor unit and the refrigerant lines behave normally, and whether the ice comes back after the airflow has been restored.

Write a useful problem statement

Equipment type, size and age. Thermostat setting and outdoor temperature.
Where the ice is (indoor coil, indoor refrigerant line, outdoor line) and when it appears.
Airflow at the registers (normal, weak) and the air temperature drop if measured.
Last filter change, any recent duct or filter changes, and whether the problem has happened before.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Is it safe to chip the ice off?

No. Chipping can puncture the coil. Turn the cooling off, run the fan, and let the ice melt. Expect water, so check the drain.

Can I just add refrigerant?

A sealed system does not use up refrigerant, so a low charge means a leak. Adding refrigerant without repair is only a temporary fix, and handling it requires certification in many places.

How long should I let it thaw?

Often several hours. Running the fan with the cooling off speeds it up. Restart the cooling only after the airflow problem has been corrected.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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