AC freezing up: how to find the cause in 20 minutes

The 8 real causes of a frozen evaporator coil

A residential AC removes heat by boiling refrigerant inside the indoor coil. The boiling point of R-410A at normal operating pressure runs 35 to 50 degrees, set by the suction-side pressure of about 102 to 145 psig. When something drops that pressure or that boiling point below 32 degrees, water vapor in the air condensing on the coil freezes instead of draining. The eight causes below all reduce coil temperature the same way, but the underlying fault is different for each and so is the fix.

• Dirty air filter. Most common cause. A loaded MERV 11 or MERV 13 filter on a one-inch return slot can push static pressure over 0.4 inches of water column on its own, dropping CFM below the ACCA design minimum of 350 to 400 per ton. Below roughly 280 CFM per ton the coil ices. Cost: $8 to $25 for a new filter.
• Closed registers or blocked returns. Homeowners shut bedroom registers to redirect air, or place furniture against a return grille. Same outcome as a dirty filter: not enough warm air across the coil, coil over-cools the smaller air mass, condensate freezes. Cost: $0.
• Dirty evaporator coil. Dust film insulates the coil and reduces airflow. Field studies show even thin fouling cuts CFM 20 to 30 percent. Cost: $100 to $400 in-place, $400 to $700 for pull-and-clean.
• Low blower CFM. A PSC motor with worn bearings, a slipping belt on older equipment, or an ECM motor with a failed module. Running 30 percent under spec on a humid day will freeze the coil. Cost: $300 to $600 for PSC replacement, $600 to $1,500 for ECM.
• Low refrigerant from a leak. Undercharge drops mass flow, suction pressure crashes, coil temperature falls below 32 degrees. Visual tell: ice on the suction line back to the compressor. Cost: $200 to $1,500 depending on where the leak is and what the refrigerant costs to replace.
• Restricted refrigerant flow. A failed thermal expansion valve, a plugged liquid-line filter-drier, or a kinked liquid line. Same low evaporator pressure as undercharge, but charge is fine. Cost: $250 to $850 for a TXV replacement, less for a drier.
• Outdoor too cold for cooling mode. Below about 60 to 65 degrees outdoor, head pressure on the high side drops, suction pressure drops with it, and coil temperature falls under 32. Standard residential AC has no low-ambient lockout. Cost: $0, just stop running cooling under 60 degrees. Or add a head-pressure control if you need cooling year round.
• Zoning fault. On zoned systems, a closed damper or a failed bypass means the compressor runs against minimum airflow. Same freeze mechanism as closed registers. Cost: $250 to $600 for damper actuator replacement.

Step 1: Thaw the coil before touching anything else (1 to 4 hours, $0)

Switch the thermostat from Cool to Off. Switch the fan from Auto to On. The blower will keep pushing room-temperature air across the iced coil with no refrigerant flowing. A lightly iced coil thaws in about 1 hour with the fan running. A fully iced coil that has run all night takes 3 to 4 hours with fan-on, or up to 24 hours system-completely-off. Carrier and Trane both publish 3 to 4 hours as the homeowner baseline; in practice the fan-on method is faster because moving air thaws ice faster than still air at the same temperature.

Put towels around the air handler. As the ice melts it will drain through the condensate pan, but a fully iced coil holds more water than the pan can handle. Air handlers in attics or closets are the worst for water damage. The condensate drain calculator will tell you whether your drain line is correctly sized; an undersized or sloped-wrong drain is one reason an iced thaw turns into a ceiling stain.

Do not chip the ice. Do not use a hair dryer. The aluminum fins are thinner than a credit card. A bent fin section reduces airflow permanently, which makes the coil freeze again the next time conditions get marginal. Patience here costs nothing. Forcing the thaw costs a coil.

Step 2: Read the air filter and the returns (5 minutes, $0 to $25)

Pull the filter from the return grille or the air handler slot. Hold it to a light bulb. A clean filter is grey-white and you can see the bulb through it. A clogged filter is black or dark grey and rigid. The single most likely reason your coil iced is sitting in your hand. Replace it with the same nominal size printed on the cardboard frame.

MERV rating matters more than people realize. A standard residential air handler is rated for about 0.5 inches of water column total external static pressure. A loaded MERV 13 filter on a one-inch return slot can use 0.4 of that 0.5 by itself, leaving almost nothing for the ducts and the coil. The result: low CFM, frozen coil, repeat. If you have been upgrading filters chasing allergy relief, you may have downgraded your AC. The MERV filter calculator will tell you the highest MERV your system can actually handle.

Walk through the house and confirm every return grille is open and unblocked. Move the couch off the floor return in the hallway. Open the bedroom door registers. Check the basement and crawlspace returns for cardboard boxes against them. A return grille blocked by furniture behaves exactly like a clogged filter.

Step 3: Check static pressure before refrigerant gauges (5 minutes, tech only)

Static pressure is the airflow problem's blood pressure. A digital manometer in the supply plenum and a second probe in the return reads total external static pressure across the blower. Residential equipment is rated for 0.5 inches of water column TESP. Readings of 0.6 to 1.0 indicate severe duct or filter restriction; readings over 1.0 indicate the air handler is fighting a duct system it cannot move air through. Either way, the coil ices and refrigerant gauges will lie to you about the actual fault.

A tech who skips static pressure and goes straight to refrigerant gauges sees low suction pressure and may add refrigerant to compensate. That covers the symptom for a week. The underlying airflow restriction is still there, and now the system is overcharged when the filter gets changed. Static pressure first is not optional. The static pressure calculator will tell you what your TESP target should be given equipment ratings; a service tech with a manometer will tell you what it actually is.

Coil pressure drop alone should stay under 0.2 inches of water column on a 0.5-rated blower. A coil reading over 0.3 means the coil itself is dirty enough to be a flow obstruction. Pulling the coil and cleaning it is a $400 to $700 job; an in-place spray-and-rinse is $100 to $400 but works only on the first inch of fin depth.

Step 4: Find the ice and read where it is (2 minutes, $0)

Once the coil is thawed and the system has run for 15 minutes, watch where ice forms first. Ice location is the single most useful diagnostic before opening gauges.

• Ice on the suction line only (the thicker insulated copper line running back to the compressor): airflow or undercharge. Static pressure tells you which.
• Ice past the filter-drier with a measurable temperature drop across the drier: refrigerant restriction. The drier is plugged, or the TXV is stuck partly closed downstream.
• Frost only at the TXV outlet, dry inlet: TXV stuck closed. Refrigerant cannot expand normally.
• Ice forming only on cool nights and clear during the day: outdoor temperature too low for cooling mode. The fix is to stop running cooling under 60 degrees.
• Ice forming repeatedly within hours of every recharge: there is a leak. Adding refrigerant covers it briefly. The next step is electronic leak detection or a nitrogen pressure test.

Step 5: Refrigerant gauge readings (tech only, 15 to 30 minutes)

With static pressure normal and the system fully thawed, the tech connects gauges. The diagnostic depends on whether you have a thermal expansion valve or a fixed-orifice metering device. TXV systems use subcooling as the primary indicator. Fixed-orifice systems use superheat. The superheat calculator and subcooling calculator give the target values for your indoor and outdoor conditions; here are the patterns a tech looks for.

• Low subcool plus high superheat: undercharge. The system is low on refrigerant; there is a leak somewhere.
• High subcool plus high superheat: restriction. Charge is fine; refrigerant cannot get through the liquid line, the drier, or the TXV.
• High subcool plus low superheat: overcharge. Someone added refrigerant chasing a different problem.
• Normal subcool and superheat with low suction pressure: airflow problem. Go back to static pressure.

Target subcool on R-410A is 8 to 12 degrees. Target superheat varies with conditions; a chart in the superheat calculator gives the number for your indoor wet-bulb and outdoor dry-bulb. Readings outside those ranges tell the tech which branch of the diagnostic tree they are on.

Step 6: Find the leak if charge is low (30 to 90 minutes, $100 to $330)

If the gauges show undercharge, refrigerant has to be going somewhere. Adding more without finding the leak is a violation of EPA Section 608 venting rules and a guarantee the coil freezes again. Three methods, in increasing thoroughness:

• Electronic leak detector (heated diode wand). Tech walks every brazed joint and the coil itself. Finds most leaks at $100 to $200 in service fees. Misses very small leaks under a few ounces per year.
• Soap bubble check at suspected joints after the electronic detector flags an area. Confirms exact location. Adds $0 to the bill; same visit.
• Nitrogen pressure test to 150 to 250 psig, holds for an hour. Catches leaks the electronic detector missed. Required when the leak is intermittent or the system was recovered. $200 to $330 typical.

Common leak locations: the suction-line Schrader valves (cheap fix, replace the cores for under $50), brazed joints at the evaporator coil itself (expensive, sometimes requires a coil replacement), the indoor copper U-bends (very expensive on cased coils), or a corroded condenser coil on systems near salt air or pool chemicals. Coil replacement runs $1,200 to $3,500 depending on tonnage and access.

Refrigerant recharge cost by type

Once the leak is repaired, the tech recovers any remaining refrigerant, pulls a 500-micron vacuum, and recharges to the manufacturer's spec or the calculated subcool target. Per-pound cost depends on what refrigerant your system uses.

• R-410A (systems installed roughly 2010 through 2024): $40 to $75 per pound installed at current prices. Was $25 to $50 before the AIM Act phase-down began constraining supply. A typical 3-ton residential system holds 6 to 8 pounds, so a full recharge after major repair runs $240 to $600 in refrigerant alone, plus diagnostic and labor.
• R-22 (systems installed before 2010): $90 to $250 per pound, often $400 per pound when supply houses are dry. R-22 production ended in 2020 under the Montreal Protocol, so all R-22 sold today is recovered and reclaimed. A 3-ton recharge can cost $700 to $2,000 in refrigerant. For most R-22 systems with a major leak, the replace-vs-repair calculator will return a strong replace verdict.
• R-454B and R-32 (systems installed 2025 and later): $50 to $90 per pound at install. Per-pound pricing is still settling as these refrigerants replace R-410A under the AIM Act phase-down. Both are mildly flammable (A2L classification), which adds installation requirements but does not change recharge mechanics for a certified tech.

Handling refrigerant without EPA Section 608 certification is a federal violation under the Clean Air Act. 40 CFR Part 82 Subpart F applies to every residential AC, and the per-violation civil penalty for venting currently reaches over $44,000 per day. This is why the leak-repair and recharge steps are not DIY, even on small systems. The refrigerant charge calculator will tell you the target charge in pounds for your line set length and equipment tonnage; only a certified tech can actually add it.

When the freeze keeps coming back

A coil that freezes once and clears after a filter change is a homeowner-level fix. A coil that freezes twice in a week, or freezes within 24 hours of a recharge, or freezes only on cool nights, is telling you something the basic diagnostic missed. Three patterns to watch for:

• Refreeze after recharge. The leak was not found. Adding refrigerant covers the symptom for days to weeks. The next freeze comes back at the same time of day or under the same weather, because that is when the leak rate exceeds the charge margin. Demand the leak search before the recharge.
• Refreeze after coil cleaning. The static pressure is still over spec. Coil cleaning addresses 0.05 to 0.15 inches of water column at best; if the system is reading 0.8 to 1.0, the duct system itself is undersized. The duct sizing calculator will tell you whether your trunk and branch sizes can move the CFM your tonnage requires.
• Refreeze on cool nights only. The system has no low-ambient lockout. Either stop running cooling under 60 degrees, or have a head-pressure control added to the outdoor unit. Common on systems homeowners use as dehumidification in shoulder season; the fix is usually a standalone dehumidifier sized with the dehumidifier sizing calculator.

What it costs to fix, in order from cheapest to most expensive

Order matters because the cheap fixes resolve 70 percent of frozen-coil calls before anyone opens the refrigerant gauges. Working through them first saves the diagnostic-and-recharge fee when the answer was a $15 filter.

• Filter replacement: $8 to $25. Resolves about 40 percent of homeowner-reported freeze events.
• Register and return cleanup: $0. Open everything, move furniture.
• Coil cleaning, in-place: $100 to $400. Works when the coil is mildly dirty.
• Coil cleaning, pull-and-clean: $400 to $700. For coils dirty enough to need full submersion.
• Blower motor replacement: $300 to $600 (PSC) or $600 to $1,500 (ECM).
• Filter-drier replacement: $200 to $400 including recovery and recharge.
• TXV replacement: $250 to $850 typical, $1,000 to $2,000 on R-22 systems or difficult access.
• Leak detection only: $100 to $330.
• Leak find, repair, and recharge: $300 to $1,500 most cases. Up to $3,500 if the leak is in the evaporator coil itself.
• Coil replacement: $1,200 to $3,500 depending on tonnage and access.
• Full system replacement: $7,000 to $14,000 installed for a 3-ton heat pump or AC. Use the replacement cost calculator for your zip code and system size.

The repair-vs-replace breakpoint for a refrigerant leak on a system over 10 years old is usually replacement. R-22 systems with major leaks are almost always replacement, because the refrigerant alone can hit $2,000. The replace-vs-repair calculator weighs system age, repair quote, refrigerant type, and remaining equipment life against the cost of a new install.

Common questions about a frozen AC

Will a frozen AC unit fix itself?

No. The coil will thaw when the system stops calling for cooling, but the cause is still there. Filter is still dirty, refrigerant is still low, registers are still closed. The next time the AC runs, the coil freezes again. Running the system while it ices is what destroys the compressor. Stop the compressor, let it thaw, find the cause, then turn it back on.

Can I just keep running the AC and let it work itself out?

No. Liquid refrigerant returning to the compressor is the most expensive way to fail. The reed valves in the compressor head are designed for gas. Liquid bends them on the first hit and breaks them on the fifth. A compressor that should last 15 years can die in a single afternoon of slugging. The repair cost on a residential compressor is $1,500 to $3,000 in parts and labor, which usually makes the system a replacement candidate.

Why does my AC freeze at night but work during the day?

Outdoor temperature. Below about 60 to 65 degrees, head pressure on the outdoor unit drops, suction pressure follows, and coil temperature falls below freezing. Daytime highs of 80 degrees keep the system in its operating band; overnight lows of 55 push it out. Standard residential AC has no low-ambient lockout. The fix is either to stop running cooling overnight when the temperature is going to drop, or to add a head-pressure control to the outdoor unit if year-round cooling is needed.

Why does my coil freeze right after a recharge?

The leak was never found and fixed. Adding refrigerant covers the symptom for a few days to a few weeks; then the system loses enough charge to drop suction pressure below freezing again. The freeze rate is a direct readout of the leak rate. Any tech who recharges without an electronic leak search or a nitrogen pressure test is selling you the same diagnostic visit twice. EPA Section 608 actually requires leak repair on systems with verified leaks; the rule on residential systems is the 5 to 15 percent annual leak rate threshold depending on system charge size.

What about a heat pump that freezes in cooling mode?

Same causes as a straight AC. The reversing valve is in the cooling position, so refrigerant flow direction matches an AC, and the diagnostic tree is identical. The cold-climate heat pump guide covers the separate freeze pattern that heat pumps see in heating mode (frost on the outdoor coil during defrost), which is different and not a fault condition.

For a working AC, the delta-T calculator verifies the coil delivers 16 to 22 degrees of cooling across it, which is the final check after any freeze repair. A coil that ices because of airflow restriction often produces a delta T over 22 degrees once it thaws, because the small amount of air that does get through gets over-cooled. A delta T under 16 degrees after a recharge means the leak was probably not completely fixed.