AC Keeps Freezing Up? 2 Causes, Leak Repair $200 to $1,500

National prices; add a $75 to $200 service call (HomeGuide, HomeAdvisor). HomeGuide figures are from its table.

Do this now: how to unfreeze an AC

1. Switch the thermostat from Cool to Off. Bryant and Lennox both start here. Running a frozen unit puts extreme strain on the compressor, Bryant says.
2. Set the fan to On, if your system allows it, so air moves over the coil and melts the ice (Bryant; Lennox).
3. Do not chip or scrape the ice. Bryant says that can damage the coil or the refrigerant tubing.
4. Put towels around the indoor unit. Bryant says meltwater can overflow the drain pan; if it does, see AC leaking water.
5. Replace the filter and open closed vents while it thaws (Bryant; Lennox).
6. Wait until all the ice is gone before you switch back to Cool. Bryant says it takes several hours, longer with heavy buildup.
7. Leave the unit's panels closed. Carrier warns that AC capacitors store high-voltage electricity that can cause serious injury or death even when the power is disconnected.

What causes an AC unit to freeze up

Restricted airflow and low refrigerant are the two main causes of a frozen AC, Bryant and Lennox both say. Lennox explains the mechanism: when too little warm air crosses the coil, the coil gets too cold, the moisture that condenses on it freezes, and the ice blocks even more air. The airflow problems Bryant lists are a dirty filter, closed or blocked supply and return registers, a dirty evaporator coil, and a failing or slow blower motor.

Low refrigerant almost always means a leak. HomeGuide says an AC never needs more refrigerant unless there is a leak, and Bryant says adding refrigerant without fixing the leak is a temporary fix. Lennox adds a third cause people miss: running the AC when it is below about 60 F outside.

Why does my AC keep freezing after I change the filter

If an AC keeps freezing after a new filter and open vents, the cause is usually a refrigerant leak, a blower problem or a faulty thermostat, Lennox says, and those need a technician. Bryant's check order is the filter and registers first, then the coil for ice or heavy dirt, then a professional look at refrigerant, coil and blower.

• Refrigerant: only an EPA Section 608 certified technician may handle it, and HomeGuide says homeowners cannot legally recharge their own AC. Leak detection runs $100 to $330 in HomeGuide's guide.
• Repeated recharges suggest a leak that was never fixed, PickHVAC notes. Ask for the leak to be found and repaired, not just topped off.
• Blower motor: Bryant says a failed or slow blower can leave the system humming with little airflow from the vents.

How long does it take to unfreeze an AC

Thawing a frozen AC takes several hours, Bryant says, and longer when the ice is heavy. Keep the thermostat on Off with the fan running, and do not restart cooling while any ice remains.

Ice on the outside unit or the copper lines

Ice can show up on the indoor coil, the refrigerant lines or the outdoor unit, Lennox says, along with weak or warm airflow and gurgling, hissing or buzzing sounds. Carrier lists ice buildup on the outdoor unit among the signs of compressor trouble. Turn cooling off and let it thaw the same way. If your system is a heat pump and this happens in winter, see heat pump repair.

Is a frozen AC an emergency?

A frozen AC is urgent when it shuts the system down during extreme heat, Bryant says, and running it frozen strains the compressor, which HomeGuide prices at $1,800 to $2,800 to replace. Treat it as an emergency when:

• Hot house, someone at risk? The CDC says a fan can raise body temperature once indoor air is above 90 F; move people 65 and older, infants and anyone with heart disease or asthma somewhere cool (dial 2-1-1 to find one). Heat stroke signs such as a body temperature above 103 F or confusion mean call 911 (Ready.gov).
• Water from melting ice is reaching a ceiling or floor. See AC leaking water.
• It froze again right after it thawed, which Bryant says is the point to turn it off and call.
• The breaker trips or the outdoor unit makes grinding or banging noises: signs of AC compressor trouble in HomeGuide's list.

Emergency HVAC repair covers after-hours rates: $160 to $250 an hour in HomeAdvisor's and HomeGuide's figures.

How much does it cost to fix a frozen AC

A frozen AC costs nothing to fix when the cause is a dirty filter, and $200 to $1,500 when it is a refrigerant leak, HomeGuide says; HomeAdvisor puts leak repair at $225 to $1,600. Coil cleaning runs $100 to $400 and a blower motor $300 to $900 in HomeGuide's table. A leaking evaporator coil usually means a new coil: $1,000 to $4,500 in HomeGuide's table, about $1,000 on average (typically $600 to $2,000) in HomeAdvisor's.

On an older system, price a replacement before a big repair. HomeGuide and HomeAdvisor both suggest replacing when the unit is over 10 years old, uses R-22, or the repair cost times its age passes $5,000. See AC replacement cost and every repair price on AC repair.

How to keep your AC from freezing up

• Change the filter on schedule. The DOE says every month or two in cooling season; Bryant says every 30 to 90 days, more often with pets or dust.
• Keep vents and returns open, with furniture at least 4 inches from return grilles (Bryant).
• Do not run the AC on cool nights below about 60 F outside (Lennox).
• Get it serviced every spring. Bryant recommends annual service to clean coils, check refrigerant and confirm airflow, and ENERGY STAR lists a spring cooling check. See AC tune-up.

For other cooling problems, start with AC not cooling. The HVAC repair hub covers heating and cooling, and home repair costs compares prices across trades.

Calls go to an independent company that serves your area and takes this kind of job. Ask for its license number and proof of insurance yourself. Some companies take emergency calls; hours vary. How calls work.