Frozen Evaporator Coil: Why Your AC Ices Up and What to Do

What a frozen coil is

Your indoor coil is supposed to be cold, usually somewhere in the 30s to 40s °F inside the tubes, so it can pull heat and water from the air. Moisture condenses on it and drains away.

If the coil surface drops below freezing, that water turns to frost instead. Frost blocks the gaps between fins, airflow falls further, the coil gets colder, and the ice grows.

You may notice weak, warm air from the vents, ice on the copper line at the indoor unit or outside, water around the furnace as it melts, or a system that runs and runs without cooling.

Heat pumps in winter also frost up, but on the outdoor coil, and they defrost themselves. That is normal and different. This page covers ice on the indoor coil in cooling mode.

How a coil freezes

1. Less warm air across the coil, or less refrigerant feeding it → coil temperature drops
2. Coil surface below 32°F → condensed water freezes → frost on fins
3. Frost blocks airflow → coil gets even colder → ice grows
4. Ice spreads to the suction line → little or no cooling

What to do right now

Safe owner steps:

1. Set the thermostat to Off for cooling. Running a frozen coil can send liquid refrigerant toward the compressor.
2. Set the fan to On to speed thawing, or leave everything off. Thawing can take a few hours.
3. Do not chip, scrape or heat the ice; fins and tubes damage easily.
4. Put towels around the indoor unit and check the drain pan as the ice melts.
5. Replace the filter if it is dirty, and open all supply vents and return grilles.
6. Once fully thawed and dry, try cooling again. If ice returns, call a technician.

Causes, and who fixes them

Worked example: how far airflow can fall

Example only. DOE's heat pump guidance says to aim for about 400 cfm per ton and that efficiency and performance deteriorate if airflow is much less than 350 cfm per ton ². Here is a 3-ton system.

The illustrative readings show how small problems stack. A tech measures actual airflow with a flow hood, a static pressure test, or the blower's table. DOE's review found low airflow in 50% to 93% of systems in the studies that measured it ¹, so freezing is far from rare.

Where you will see the term

• On service invoices: "coil frozen, found dirty filter" or "low charge, leak search."
• In smart thermostat alerts for long runs without temperature change.
• In home warranty claims.
• In inspection reports noting water damage near the air handler.

Quick fixes vs real fixes

Why "just add refrigerant" is not a fix

If a frozen coil comes from low refrigerant, there is a leak, because refrigerant is not used up. Adding refrigerant without finding the leak means the problem comes back, and it puts more refrigerant into the air. Section 608 bans knowingly venting refrigerant during service ⁵.

Overcharging to "fix" a freeze-up is also a risk. DOE's review found overcharge in 4% to 50% of systems studied ¹. Too much refrigerant lowers efficiency and can damage the compressor. Charge should be set by the maker's superheat or subcooling target.

Sources: [1] [5]

Cost drivers

HyreHVAC lists no repair prices without enough dated quotes. If the cause is the filter, the cost is a filter. Other causes add up: coil cleaning, blower or capacitor repair, leak search and repair, refrigerant, or a replacement coil. Water damage from melting ice adds more.

If a leaky coil is the cause on an older system, compare repair and replacement with the repair-or-replace calculator.

How a technician finds the cause

1. Confirm the coil is fully thawed before testing.
2. Check filter, blower, returns and static pressure.
3. Measure airflow and temperature drop.
4. Measure refrigerant pressures, superheat and subcooling.
5. If low, search for the leak (bubbles, electronic detector, or nitrogen pressure test) ⁵.
6. Repair, evacuate and recharge to the maker's spec, then retest.

Preventing it

• Check filters monthly ⁴.
• Keep return grilles clear and supply vents open.
• Have the coil inspected and cleaned as needed; DOE notes dirt can bypass a dirty filter and coat the coil ³.
• Have charge checked during spring tune-ups ⁴.
• Avoid very low setpoints on cool nights.

Sources: [4] [3]

When to call a pro

• Ice returns after a new filter and full thaw.
• Hissing or oily stains near the coil or lines.
• Water overflowing the pan or tripping the float switch.
• The outdoor unit hums, clanks or trips the breaker.
• No air from the vents at all: possible blower failure.

Rules that apply

Refrigerant. If a leak is involved, EPA Section 608 rules (40 CFR Part 82, Subpart F) apply: certified technicians, no venting, and proper recovery ⁵. See our EPA 608 research.

Newer refrigerants. Systems with parts made after January 1, 2025 use refrigerants under EPA's 700 GWP limit, such as R-454B ⁶. They freeze for the same reasons. Repairs follow the maker's A2L handling rules.

Sources: [5] [6]

Frozen coil vs related problems

Common misconceptions

Myth Ice means the AC is extra cold and working hard.

Reality It means it is failing to move enough heat.

Myth Running the AC will melt the ice.

Reality Cooling mode makes it worse. Turn cooling off.

Myth A frozen coil always means low refrigerant.

Reality Airflow problems are just as common ¹.

Myth A top-off fixes it.

Reality A leak needs repair, not a refill ⁵.

Climate patterns

When it matters to you

It matters the moment you see ice. Turn cooling off, check the filter and vents, and let it thaw. If it refreezes, book a diagnosis and ask for the airflow and refrigerant readings.

Next step: check urgency with the emergency checker, and read about refrigerant leak repair if a leak is found.

Related terms

Evaporator Coil

An evaporator coil is the indoor coil of a central air conditioner or heat pump where cold refrigerant boils and absorbs heat from the air blown across it, cooling and drying your home's air.

Refrigerant Leak

A refrigerant leak is any escape of refrigerant from the sealed loop of an air conditioner or heat pump, through a crack, a worn joint, a valve core or corrosion, which lowers cooling and heating capacity over time.

Air Filter

An HVAC air filter is the replaceable screen of pleated or fiber media in the return air path of a furnace, heat pump or central AC that traps dust and particles before the air reaches the blower and coil.

Blower Motor

A blower motor is the electric motor inside a furnace or air handler that spins the fan wheel pushing air across the heat exchanger or coil and through the home's ducts.

CFM (Cubic Feet per Minute)

CFM, or cubic feet per minute, is the unit HVAC uses for airflow: the volume of air, in cubic feet, that a blower, duct, register or fan moves each minute.

Superheat

Superheat is the number of degrees by which refrigerant vapor in the suction line is warmer than its boiling (saturation) temperature at the measured suction pressure, found as suction line temperature minus saturated suction temperature.

Heat Pump Defrost Cycle

A heat pump defrost cycle is a short, automatic reversal of the system that sends warm refrigerant to the outdoor coil to melt frost that builds up while the heat pump is heating in cold, damp weather.

Also referenced from: Evaporator Coil, Expansion Valve, Condensate Drain Line, Static Pressure, Superheat, Evaporation Temperature, Short Cycling, Refrigerant Leak, Dirty Condenser Coil, Heat Pump Defrost Cycle, Condensate Overflow Switch, Differential Temperature (Delta T)