Ice on Your AC Lines or Indoor Coil: Causes and Fixes

You go to the furnace room and find frost on the copper pipe, a sweating coil cabinet or a puddle on the floor. Or you notice the fat insulated line at the outdoor unit is white with ice in the middle of July. The AC is probably still running, but the air from the vents has gone lukewarm. Ice on an air conditioner is a symptom, not the problem itself, and the order you deal with it matters.

What you notice, and what it points to

Why an AC coil freezes

The indoor coil, sitting on top of the furnace or inside the air handler, is designed to run cold but above freezing. Warm house air passing over it gives up heat and moisture, which drips into the drain pan as condensate. Take away some of that warm air, or let the refrigerant pressure drop, and the coil temperature falls below 32°F. The condensate turns to frost, the frost blocks even more air, and the ice grows until it reaches the copper lines and sometimes all the way outside.

Trane lists a dirty filter, a dirty coil, blocked returns, thermostat problems and low refrigerant as the main causes. Lennox adds closed or blocked vents and running the AC when it is below about 60°F outside. Almost every case falls into one of two buckets: airflow, which you can often fix yourself, or refrigerant and mechanical, which you can't.

How to thaw it and check the cause

1. Cooling off, fan on

Set the thermostat to Off and the fan switch to On. Carrier recommends this because the blower keeps moving room air across the coil, which melts the ice faster than leaving everything off. If the blower itself isn't running, skip ahead to the "call a technician" list; the fan can't help, and the coil will thaw slowly on its own.

2. Get ready for water

A frozen coil holds a surprising amount of water. Put towels around the base of the furnace or air handler, and check that the drain line and any condensate pump are working as the ice melts. A pan that overflowed during the freeze may also have tripped a float switch, which will keep the AC off until the water is gone. Our AC and furnace leak guide explains drain lines and pumps.

3. Replace the filter

A wet, frosty filter is a strong hint that ice was dripping onto it from the coil above. Put in a new filter of the correct size. The U.S. Department of Energy suggests cleaning or replacing AC filters every month or two in cooling season; in smoky summers, check monthly. If you're unsure which filter fits, our filter replacement page helps.

4. Open the airways

Walk through the house and open supply registers, especially in rooms you rarely use. Pull furniture, curtains and rugs away from return grilles. Closing a lot of vents to "push cold air upstairs" is a common way to starve a coil of air.

5. Wait until it is completely thawed and dry

Carrier estimates one to four hours depending on how much ice built up. The copper lines at the indoor and outdoor units should be free of frost, and the dripping should have slowed. If you restart while ice is still hidden in the coil, it refreezes quickly.

6. Restart and watch it

Set the fan back to Auto and choose a moderate cooling setpoint, not the lowest number on the dial. After 15 minutes, the air from a nearby register should feel clearly cooler than the room. Over the next hour, glance at the line at the outdoor unit. Sweating is normal; white frost is not.

7. Refroze? Stop and call

If ice comes back with a clean filter and open vents, turn cooling off again and call for AC repair. Running it "just until the technician gets here" risks the compressor.

Problems that need a technician

Refrigerant leaks

An air conditioner doesn't use up refrigerant. A low charge means a leak, or a system that was never charged correctly at installation. Signs include oily residue on fittings, a hissing sound, and icing that comes back within a day or two. A technician should find and repair the leak, pull a vacuum and weigh in the correct charge. Repeatedly "topping off" a leaking system keeps the freeze cycle going and puts refrigerant into the atmosphere. Our refrigerant leak repair service handles this, and our AC not cooling guide explains why recharging without a repair is a poor bet with today's refrigerants.

Blower and control problems

If the outdoor unit runs while the indoor blower stays still, the coil has no air at all and will freeze in minutes. A failed blower motor, a weak motor capacitor or a control board fault is usually behind it. All three live behind panels with high voltage, so leave them to a technician.

A dirty evaporator coil

Years of fine dust that slipped past cheap or loose filters can coat the coil face, acting like a second clogged filter you can't reach. Cleaning it means opening the coil cabinet, which on many systems is sealed with tape or mastic. That is a coil cleaning job, not a homeowner one.

Low airflow by design

Undersized return ducts, a blower set to too low a speed, or an AC that was paired with the wrong furnace can starve the coil even with a new filter. These show up as freezing that keeps coming back in a system with no leak. A technician can measure static pressure and airflow and recommend duct or setup changes.

Colorado specifics

Wildfire smoke and dusty summers

When smoke from Colorado or western wildfires settles over the Front Range, a filter can load up far faster than its label suggests. That is good news for your lungs and bad news for airflow. During smoky weeks, check the filter every couple of weeks and keep a spare on hand.

Cool nights and spring warm spells

Front Range nights cool off quickly, especially in late spring and early fall. Lennox notes that running an AC below about 60°F outside can lead to freezing, because the system has too little heat to absorb. If the house got warm during the afternoon but it's cool outside by evening, open windows or use a whole-house fan instead of pushing the AC to a low setpoint overnight. Denver's average last freeze is early May and the first is early October, per the National Weather Service, so there are plenty of shoulder-season days when this applies.

Coils on top of the furnace

In most Denver-area homes the AC coil sits directly above the gas furnace. Melt water that misses the drain pan can drip into the furnace cabinet, so dry everything before the heating season and have a technician look if water reached the burners or control board. A tune-up each spring is a good time to check the pan, drain and filter fit together.

Altitude

Air at Denver's elevation is thinner, so a blower moves less air mass than the same blower would at sea level. Systems set up without accounting for that can run closer to the freezing line. Our altitude guide explains why setup matters.

When to call

• The coil refreezes after you've thawed it, changed the filter and opened the vents.
• The indoor blower doesn't run while the outdoor unit does.
• You see oily stains, hear hissing, or were told the system was "low" before.
• Melt water reached the furnace or wiring.
• The AC is old and has needed refrigerant more than once; compare options in our Denver HVAC system cost guide.

Frequently asked questions

Why is there ice on my AC's copper line?

Frost on the large insulated line usually means the indoor coil has frozen and the ice is creeping back along the pipe. The cause is most often a dirty filter, blocked vents or a refrigerant leak.

How long until a frozen AC thaws?

Carrier estimates about one to four hours with cooling off and the fan on, depending on how much ice there is. Make sure the coil and lines are fully clear before you restart.

Can I keep running my AC if it's frozen?

No. A frozen coil blocks airflow and can send liquid refrigerant back to the compressor, which can cause expensive damage. Turn cooling off and run the fan until it thaws.

Will a frozen AC fix itself?

The ice will melt, but the cause stays. If a new filter and open vents don't stop it, the system likely has a refrigerant leak or blower problem that needs a technician.

Can I use a hair dryer to melt the ice?

It is better not to. Carrier warns that hair dryers, space heaters and hot water can crack or warp coil parts. Running the fan with cooling off is the safer method.

Why does my AC freeze at night?

Cool nights and a very low setpoint leave the coil with little warm air to absorb, so it drops below freezing. Raise the setpoint or use outside air when it is below about 60°F outdoors.