Frozen AC: How to Thaw It and Why It Froze

Ice on the copper refrigerant lines at the outdoor unit, or on the indoor coil above the furnace or in the air handler, means the coil got colder than freezing. Bryant says an AC evaporator coil can start to freeze once its surface drops below 32°F, usually because of restricted airflow or low refrigerant. Trane lists the signs: the AC runs but blows warm air, ice coats the evaporator coil or its cabinet, the refrigerant lines at the outdoor unit are frozen, and puddles appear as the ice melts. Trane and American Standard say the same goes for the indoor coil of a heat pump.

This page covers summer ice on a central air conditioner or on a heat pump that is cooling. Frost on a heat pump's outdoor unit in cold weather is normal, American Standard says, because the defrost cycle clears it; see heat pump blowing cold air. If the AC runs with no ice anywhere but the house won't cool, see AC not cooling. Water by the indoor unit with no ice in sight points to the drain or the pan, two of the causes Carrier names for a leaking AC; see clogged condensate drain line.

What to Do Right Now

1. Turn cooling off at the thermostat. Trane and Bryant say to switch the mode from COOL to OFF, and Lennox says to shut the thermostat off completely. On a heat pump, Trane warns not to switch to a defrost mode for a frozen indoor coil; defrost only removes ice from the outdoor coil during heating.
2. Set the fan to ON. Trane, Lennox, Carrier, and American Standard say running the indoor fan with cooling off moves house air across the coil and speeds the thaw. Bryant says to do it only if your system and its manufacturer's instructions allow it, so check the owner's manual. Leave a filter in: a Rheem air conditioner manual says never to run the system without one, or the cooling coils will get dirty and may clog.
3. Give it hours. Carrier says a lightly frosted AC may thaw in about 1 to 3 hours, while heavy ice can take 4 to 8 hours or longer. Bryant says not to restart the AC while significant ice remains on the coil.
4. Catch the meltwater (below), and while you wait, replace the filter and open any closed vents, as Bryant suggests.
5. Restart, then watch it. Once airflow is back to normal and everything is dry, the contractor Carrier quotes says you can restart the system; set the fan back the way you usually run it. If the coil ices again, Carrier, Lennox, and Bryant all say to turn it off and call a professional rather than restarting it again and again.

In a heat wave, Trane suggests calling for emergency repair while the coil thaws. If the house is getting dangerously hot, the CDC says to use air conditioning or find a place that has it by dialing 2-1-1 or contacting your local health department, and to use fans only if indoor temperatures are below 90°F.

Expect Meltwater

American Standard says a coil fully encased in ice can release 1 to 2 gallons as it melts, and Bryant warns the water can overwhelm the coil's condensate drain pan. It should leave the way summer condensation does: through the condensate drain line, which Carrier describes as a ¾-inch PVC pipe running from the evaporator coil to a drain outside.

• Towels, a tarp, or a bucket. American Standard suggests old towels around a basement unit, and a plastic tarp, tub, or bucket under a coil in the attic.
• The drain. Check that no water is overflowing from the drain line as the ice melts, a step from the contractor Carrier quotes. Goodman says a steady drip at the outside end on a hot day means the drain is working.
• The float switch. Carrier says many modern systems have a safety float switch that shuts the system down when the drain pan fills. If the AC won't restart after a thaw, a full pan is one possible reason, and our condensate drain guide shows how to clear the line.
• An attic unit. The contractor Carrier quotes says attic systems often have an emergency drain line that exits outside near a sidewalk or doorway; water coming from it means there's a problem.

Dry wet floors and drywall quickly: the EPA says that when wet areas are dried within 24 to 48 hours, in most cases mold won't grow.

Most Likely Causes

Check them in this order. The first five are yours to check; the last three need a technician.

1. A dirty filter. Bryant, Lennox, and Trane all name it a common cause: a clogged filter cuts the warm air reaching the coil, so the coil drops below 32°F and its condensation freezes. Hold the filter up to the light. A filter can also be too restrictive: Rheem's service tips list an air filter with excessive resistance to airflow, and more than one filter installed, among the causes of a frozen indoor coil or suction line. Our filter guide shows the swap and the arrow direction, and the filter buying guide explains MERV ratings.

2. Closed or covered supply registers. Bryant says closing vents in unused rooms can upset the system's airflow balance and contribute to a frozen coil, as can furniture or rugs over them; Carrier's owner's manual adds drapes and toys. Open every register.
3. A blocked return. Trane says blocked return vents cut airflow over the coil, often because a couch or other furniture sits in front of one. Clear every return grille.
4. A thermostat set too low, or one that won't shut off. Carrier says setting the temperature too low can drop the coil below freezing, and Rheem lists low return-air temperature among the causes. Trane says a coil that is frozen only in the morning points to a thermostat that isn't shutting the AC down as the night cools. Raise the setpoint, and if the thermostat misbehaves, see thermostat not working.
5. Cooling on a cool day or night. Lennox says running the AC when it's below 60°F outside can make it freeze up. Carrier's owner's manual for its split-system air conditioners says the outdoor unit isn't designed to run below 55°F without modification, and Bryant's product data for one model requires a low-ambient control, plus an evaporator freeze thermostat, for cooling below 55°F. Your model's limit is in its owner's manual; to cool below it, Carrier says to consult your dealer.
6. A dirty evaporator coil. Trane says dust that collects on the coil, often after a dirty filter, acts as an insulator and restricts the airflow the coil needs. Carrier's owner's manual says the indoor coil sits in a sealed cabinet, and that longer run times and lower efficiency may mean it needs a cleaning by your dealer.
7. A failing blower. Bryant says a blower motor that fails or runs at reduced speed can't move enough warm air over the coil to keep it from freezing; you may hear the system humming but feel little or no air at the vents. Rheem also lists a bad indoor blower motor or motor capacitor.
8. Low refrigerant from a leak. Lennox says low refrigerant, often from a leak, drops the pressure inside the coil until it gets too cold and freezes. Trane says the only ways a system runs low are a leak or an improper charge at installation, and to call right away if you see an oily residue around the indoor coil or hear hissing near either coil. Bryant calls a recharge without fixing the leak a temporary band-aid.

What You Can Fix Yourself

Carrier's owner's manual draws the line: monthly maintenance items and rinsing the outdoor unit are the homeowner's; all other maintenance and all service work belong to a qualified service technician. For a frozen coil:

• Yours: the thermostat's mode, setpoint, and fan; the filter; supply registers and return grilles; towels and buckets; and checking that water drips from the drain's outside end.
• The technician's: refrigerant. EPA rules under Section 608 of the Clean Air Act require anyone handling refrigerant, such as an air-conditioning service technician, to be Section 608-certified, and only certified technicians can buy refrigerant for home air conditioners and heat pumps. The same rules prohibit intentionally venting refrigerant while servicing the equipment.
• The technician's: the coil and the blower. Carrier's maintenance checklist leaves cleaning the blower and the evaporator coil to the technician. Trane recommends hiring a pro over a DIY coil-cleaning tutorial, and Goodman says bent coil fins are a job for an HVAC technician.

Our DIY vs. hire a pro guide puts anything involving refrigerant in the red tier for the same reason.

When to Call a Pro

• It freezes again after a full thaw, a new filter, and open vents (Carrier, Lennox, Bryant).
• It froze with a clean filter while running in its normal temperature range; the contractor Bryant quotes says that's the time to call.
• Signs of a leak: an oily residue near the indoor coil or a hissing sound (Trane).
• The system hums but little or no air comes out (Bryant).
• Water overflows the pan, runs from the emergency drain line, or reaches wiring.
• A heat wave, when Trane suggests emergency service.

Start the thaw before the visit: American Standard says a technician can't evaluate the coil and refrigerant lines until the coil has thawed. Trane says the technician will check the filter, the coil, the thermostat, and the refrigerant level, test for a leak and try to repair it, and recharge only if necessary.

Prevent It

• Filters. ENERGY STAR says check the filter every month, especially in summer, and change it at least every 3 months; Trane and Lennox say every 1 to 3 months.
• Open airflow. Keep supply registers open and returns clear of furniture, rugs, and drapes (Bryant, Trane).
• A spring tune-up. The Department of Energy says to have air conditioners inspected in the spring, and ENERGY STAR's check-up list includes cleaning the coils, checking the refrigerant level, and adjusting blower components for proper airflow.
• A sensible setpoint. Carrier says avoid setting the temperature too low, and that a programmable thermostat reduces the swings that contribute to freezing. The summer guide has ENERGY STAR's schedule, starting at 78°F when you're home.
• The drain. Carrier's manual says water should trickle from the condensate drain after a few minutes of cooling; check it occasionally.

Frequently Asked Questions

Why is there ice on my AC lines outside?

Usually the indoor coil is frozen. Trane lists frozen refrigerant lines at the outdoor unit as a sign of a frozen evaporator coil and says the same airflow and refrigerant problems are behind both. Turn cooling off, run the fan, and work down the list of causes above.

How long does it take a frozen AC to thaw?

Plan on a few hours: Carrier's range runs from about an hour for light frost to 8 hours or more for heavy ice. Running the fan with cooling off shortens it.

Why does my AC freeze up at night?

Trane points to a thermostat that keeps the AC running as the night cools. A cool night can do it too; see the 55°F limit under cause 5 above.

Can I add refrigerant to my AC myself?

No. Under EPA Section 608 rules, only certified technicians can buy refrigerant for home air conditioners, and anyone handling it must be certified. Lennox says low refrigerant often comes from a leak, and Bryant says adding more without fixing the leak is only temporary.

The Bottom Line

Cooling off, fan on if your manual allows it, and a few hours for the ice to melt on its own, with towels under the unit and an eye on the drain. Never chip or heat it. Then fix the filter, registers, returns, and setpoint. If it freezes again, freezes with clean airflow, or shows oil or hissing, call an HVAC technician.

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