By Hood Service Company · Corsicana, TX · Updated
The short version
- Ice on the indoor coil or refrigerant lines means something is wrong, even in July.
- Restricted airflow and low refrigerant charge are the two most common causes.
- Thaw the coil with cooling off and the fan on, and never chip ice off it.
- If the coil freezes again after a new filter, the cause needs a technician.
What is an evaporator coil, and why would it freeze?
The evaporator coil is the indoor coil where refrigerant absorbs heat from your air. It normally runs cold but above freezing; when too little heat reaches it, moisture on its surface turns to ice.
The evaporator coil sits above or beside your furnace or air handler. Warm air moving across it keeps the refrigerant inside from getting too cold. Cut that airflow, or lose refrigerant, and the coil drops below freezing. Ice then blocks air, airflow falls further, and the ice spreads.
How can you tell the coil is frozen?
Look for frost or ice on the larger refrigerant line at the outdoor unit, weaker airflow from the vents, air that isn't cool, and water around the indoor unit as ice melts.
- Frost or ice on the insulated refrigerant line at the outdoor unit or where it enters the indoor unit.
- Airflow from the vents that's weaker than usual.
- A system that runs constantly while the house gets warmer.
- Water in the drain pan, on the floor or dripping from the indoor cabinet.
What causes an AC coil to freeze?
Most freeze-ups trace back to restricted airflow or a low refrigerant charge. Less often, a weak blower, a dirty coil or a duct problem is to blame.
Running cooling on cool nights can also push a marginal system into icing.
- Dirty filter: the most common cause, and the easiest to fix.
- Closed or blocked vents and returns: furniture, rugs or closed registers starve the coil of air.
- Dirty coil: dust on the coil itself blocks air even with a clean filter.
- Blower problems: a weak or failing blower motor moves too little air.
- Low refrigerant charge: a leak lowers the pressure in the coil, and its temperature falls below freezing.
- Duct problems: a collapsed or disconnected return duct; see weak airflow and ductwork problems.
How do you thaw a frozen AC coil safely?
Switch the thermostat from Cool to Off, set the fan to On, and let the ice melt on its own. This can take several hours. Replace the filter while you wait.
- Set the thermostat's system switch to Off and the fan to On, so air keeps moving across the coil.
- Replace the filter if it's dirty.
- Put towels around the indoor unit in case the drain pan overflows, and check that the condensate line is draining.
- Don't chip, scrape or heat the ice. Coil fins and tubing are thin and easy to puncture.
- When the lines are dry and ice-free, switch back to Cool and watch for ice over the next day.
What if the coil freezes again?
A refreeze after a fresh filter means the cause is still there, most often a refrigerant leak, a dirty coil or a blower problem. Those need diagnosis, not another thaw.
Repeated freezing strains the compressor and can let liquid refrigerant reach it, which is how a small leak becomes a major repair. A technician will check airflow, clean the coil if needed and test the refrigerant charge, finding and repairing any leak rather than simply adding refrigerant. Schedule AC repair service if the ice comes back.
How do you keep the coil from freezing?
Keep airflow steady and the charge correct. Change filters on schedule, keep vents and returns clear, and have a spring tune-up that checks the coil, blower and refrigerant.
If you're not sure a symptom fits, the HVAC troubleshooter can help sort it out. Regular AC maintenance is where a dirty coil or a slow leak usually gets caught.
When should you call a technician?
- Ice returns within a day or two after thawing and a filter change.
- Oily spots appear on refrigerant lines or fittings.
- The blower doesn't run, or runs weakly, with a clean filter.
- Water overflows the drain pan or leaks through a ceiling.
- You hear hissing or bubbling near the indoor or outdoor coil.
Frequently asked questions
Can I keep running the AC while the coil is frozen?
No. Running cooling over an iced coil moves very little heat and can damage the compressor. Keep cooling off until the ice is completely gone.
Why would the coil freeze on a hot day?
Outdoor heat doesn't stop a coil from freezing, because the coil's temperature depends on airflow and refrigerant pressure. A clogged filter or a low charge can freeze a coil in the middle of a Texas August.
Will a frozen coil fix itself?
The ice melts once cooling is off, but the cause stays. Unless the cause was a dirty filter you've replaced, expect the coil to freeze again.