Frozen or iced-up AC coil
Ice on an air conditioneris always a symptom.
Something forced the coil below freezing, and it was not the coil's idea. Thaw it first — the steps are below and they matter — then find out which of two things caused it, because it will happen again otherwise.
Do this now, before anything else
Running a frozen system is how a moderate repair becomes a compressor replacement. These five steps take two minutes to start and a few hours to finish.
1
Turn cooling OFF at the thermostat
Set the mode to OFF. Do not simply raise the temperature — the system can still call for cooling and keep the compressor running.
Why it matters: ice blocks the coil so refrigerant cannot fully boil off into gas. Liquid refrigerant then returns to a compressor designed to pump gas, and that is mechanically destructive. Every minute it runs frozen is damage you are paying for later.
2
Set the fan to ON
With cooling off, switch the fan setting from AUTO to ON. This runs the indoor blower on its own, with no compressor.
Why this is the fast way: the blower pushes warm household air across the frozen coil and melts it far quicker than leaving everything switched off. Depending on how much ice there is, this is the difference between three hours and most of a day.
3
Put towels down and check the drain pan
Place towels around the indoor unit and check that the condensate drain pan is not already full. A frozen coil holds a surprising amount of water and all of it is about to melt at once.
Why now rather than later: the drain is sized for normal condensate, not for a coil dumping its entire ice load in an hour. Overflowing pans are how ceilings get damaged, and the thaw is exactly when it happens.
4
Replace the filter while you wait
Pull the filter out and hold it to a light. If light does not pass through it, fit a new one now rather than after the thaw.
Why while you wait: a blocked filter is the single most common cause of a frozen coil. Changing it costs a few dollars and it might be the whole answer. If it was filthy, you have probably found your problem.
5
Wait for it to thaw completely, then watch
Give it several hours — up to a full day if the ice was heavy. Once every trace has gone, switch cooling back on and watch it for an hour or two.
What you are watching for: if it cools normally and stays clear, a dirty filter was likely the cause. If it freezes again, the cause is still there and the system needs a proper diagnosis before it runs any further.
Refreezing after a thaw and a new filter? Leave it off and call. That result is genuinely useful information and it tells us where to look.
Why this happens
The coil is supposed to be cold. It is not supposed to be frozen.
Refrigerant boils inside the evaporator coil at a temperature below freezing, and that is what absorbs heat from your home. What stops the coil actually icing over is the constant stream of warm indoor air flowing across it, which keeps the surface just above 32 degrees.
Take away enough of that warm air, or drop the pressure inside the coil too far, and the surface falls below freezing. Condensation that would normally run off into the drain turns to frost instead.
Then it runs away from you, which is why a little frost becomes a solid block so quickly.
STEP 1
A thin layer of frost forms on the coil surface.
STEP 2
That frost blocks some of the airflow through the fins.
STEP 3
Less warm air means the coil gets colder still.
STEP 4
More ice forms, blocking more airflow. Repeat until solid.
Two families of cause, and how to tell them apart
Every frozen coil traces back to one of these. Knowing which one you are dealing with tells you roughly what it will cost before anyone arrives.
Family one
Not enough warm air reaching the coil
Something is restricting airflow, so there is not enough heat crossing the coil to keep it above freezing. This is the more common family and generally the cheaper one.
Airflow at the vents
Noticeably weak, even before the ice formed.
Where the ice starts
Usually on the indoor coil first, spreading outward to the line.
Recent history
Filter overdue, vents closed off, or a blower that has been noisy.
Typical fix
Anything from a new filter to duct or blower work.
Family two
Not enough pressure inside the coil
Low refrigerant from a leak, or a restriction in the metering device, drops the pressure so the refrigerant boils colder than it should. Airflow is fine; the coil is simply too cold.
Airflow at the vents
Normal strength, but the cooling has been weakening for weeks.
Where the ice starts
Often at the outdoor unit on the larger copper line and the service valve.
Recent history
Gradually worse cooling, possibly a previous top-off of refrigerant.
Typical fix
Locating and repairing the leak, or replacing the metering device.
A useful shortcut: if the air from your vents was already weak before anything froze, look at family one. If the airflow felt normal but the house had been cooling worse and worse for a few weeks, look at family two. Neither is conclusive, but it tells you which conversation you are about to have.
The specific causes
Blue markers are airflow. Orange markers are refrigerant side. Costs are typical Fresno ranges, not quotes.
A blocked air filter
Cost of a filterThe most common cause by a wide margin, and the cheapest. Fresno dust and agricultural particulate load filters faster than the packaging suggests, so the interval printed on the box is optimistic here.
A dirty evaporator coil
MaintenanceDust that got past a poor filter settles on the coil fins and insulates them from the air passing over. Cleaned properly during a tune-up. What a tune-up covers.
Closed or blocked vents and returns
Free to correctRegisters closed in unused rooms, furniture over a return, or a rug across a floor vent. Closing vents feels efficient and does the opposite — it raises static pressure across the whole system and starves the coil.
Undersized or damaged ductwork
VariesCrushed flex duct, a collapsed return or ducting that was never adequate for the equipment. Common on systems that have frozen repeatedly for years and had the symptom treated rather than the cause.
A failing blower motor
$600 – $1,300A blower running slow or struggling moves less air than the system needs. Often preceded by a noticeable drop in airflow and sometimes a noise from the air handler.
Low refrigerant from a leak
$400 – $1,600The most common refrigerant-side cause. Pressure drops, the refrigerant boils colder than designed, and the coil ices over. Refrigerant is never consumed, so low always means a leak somewhere. More on leak detection.
A restricted or failed metering device
Quoted after diagnosisThe expansion valve or orifice controls how much refrigerant enters the coil. When it restricts or sticks, the pressure downstream falls too far and the coil freezes even though the charge is correct.
Running cooling on a cold night
Free to correctGenuinely a cause, and it surprises people. Running air conditioning when the outdoor temperature is below roughly 60 degrees drops system pressures far enough to freeze the coil. Rare in a Fresno July, occasionally relevant in spring and fall.
Four things not to do
Each of these is common, and each one turns a manageable problem into an expensive one.
-
Do not chip, scrape or pry the ice off
Evaporator coils are thin aluminum fins over copper tubing, and a screwdriver goes through them easily. Puncture one and you have converted a free filter change into an evaporator coil replacement at $1,200 to $2,800. Let it melt.
-
Do not keep running it because the house is hot
Understandable on a July afternoon, and it does not work. A frozen coil is not cooling anything, and running it sends liquid refrigerant back to the compressor. You get no cooling and a damaged compressor rather than just no cooling.
-
Do not apply direct heat to the coil
No heat guns, no boiling water, no hair dryer held against the fins. Rapid uneven heating stresses brazed joints and warps fins. The blower on ON is faster than you think and it cannot damage anything.
-
Do not accept "it just needs a top-off" as an answer
If low refrigerant froze your coil, there is a leak. Adding more without finding it means the same freeze next season, plus a compressor that has spent months running starved. That is a repair nobody performed, billed as though someone had.
Worth preparing for
The thaw releases a lot of water
A heavily iced coil can hold several gallons, and all of it melts over a few hours into a drain pan and line designed for a steady trickle of condensate. This is the single most common way a frozen coil causes actual property damage, and it happens during the fix rather than during the fault.
- Put towels down around the air handler before you start the thaw, not after.
- Check the drain pan is empty and the line is running before the melt reaches it.
- Look in on it every hour or so, particularly with an attic unit above living space.
- If the system shuts itself down partway through, a float switch has done its job. More on AC water leaks.
Common questions
Why is my AC freezing up?
Because something dropped the evaporator coil surface below freezing. There are two families of cause. Either not enough warm air is reaching the coil, from a blocked filter, dirty coil, closed vents, restricted ducts or a failing blower, or the pressure inside the coil is too low, from a refrigerant leak or a restricted metering device. Airflow problems are more common and usually cheaper to fix.
How long does it take a frozen AC coil to thaw?
Typically three to eight hours with the thermostat set to cooling OFF and the fan set to ON, which pushes warm household air across the coil. A heavily iced system can take a full day. Leaving everything switched off works too but takes considerably longer.
Can I chip the ice off to speed it up?
No. The coil is thin aluminum fins over copper tubing and it punctures easily. A puncture means refrigerant loss and often a coil replacement at $1,200 to $2,800, turning a free filter change into a major repair. Do not apply direct heat either.
Can I run my AC while it is frozen?
No, and this matters more than it sounds. Ice blocks the coil so refrigerant cannot fully boil into gas, and liquid refrigerant returning to a compressor built to pump gas causes mechanical damage. The system is not cooling anything while frozen anyway, so there is nothing to gain and a compressor to lose.
It thawed and then froze again. What does that mean?
That the underlying cause is still present, which is genuinely useful to know. If you also replaced the filter and it still refroze, the problem is likely deeper airflow restriction or a refrigerant issue. Leave the system off and have it measured rather than repeating the thaw cycle.
How much does it cost to fix a frozen AC coil?
It depends entirely on the cause. A filter is a few dollars. A coil clean is part of a maintenance visit. A blower motor runs $600 to $1,300 and a refrigerant leak repair $400 to $1,600. The freeze itself costs nothing to clear — you are paying to fix whatever caused it. See published repair pricing.
Will a frozen coil damage my system permanently?
A single freeze caught early and thawed properly usually causes no lasting harm. Repeated freezing, or running the system while frozen, is what damages compressors. Water overflow during the thaw can also damage ceilings and flooring, which is why the drain pan is worth checking before you start.
Thawed it and want to know why it happened?
We measure airflow and refrigerant side both, so you find out which family it was rather than guessing.
Guidance on this page is general information and not a substitute for a professional diagnosis. Prices shown are typical ranges for the Fresno area and are not quotes. California Heating & AC Repair is operated by Craftline Renovations, Inc. · CSLB License #999179 · Licensed and insured.