If you’re seeing ice on your indoor AC unit or the refrigerant line, turn the system off now and let it thaw before running it again. Running a frozen coil can flood liquid refrigerant back into the compressor, and that’s a much more expensive fix than whatever caused the freeze.
How a Coil Freezes
The evaporator coil (the indoor one) absorbs heat from the air moving over it. If something disrupts that heat transfer, the coil surface drops below 32°F and moisture freezes on it. Two things cause this: not enough warm air moving over the coil, or not enough refrigerant absorbing heat properly. The specific culprits fall into a few categories, roughly in order of how often we see them.
The Most Likely Cause: Restricted Airflow
A clogged filter is the single most common reason for a frozen coil. When airflow drops, the coil surface temperature falls and ice builds up fast. Same thing happens with closed or blocked supply registers, a dirty blower wheel, or a collapsed return duct.
Check your filter. If it’s gray and matted, that’s probably your answer. Replace it, let the coil thaw completely (2-4 hours with the system on fan-only mode, not cooling), then restart. If the system runs normally after that, you found it.
Walk through the house and confirm every register is open, including ones in rooms you don’t use. Closing off too many raises static pressure and chokes the system.
Second Most Common: Low Refrigerant
If the filter is clean and airflow is fine, the next likely cause is low refrigerant charge. When refrigerant is low (almost always from a leak, not because it “runs out”), the pressure in the coil drops and the remaining refrigerant gets too cold. Ice forms on the coil surface.
Refrigerant diagnosis and repair requires gauges, proper handling, and an EPA Section 608 certification. Don’t let anyone top it off without finding and fixing the leak first. That’s money wasted on a problem that’ll come back.
Signs pointing toward a refrigerant issue: air from your vents is less cold than usual, the system runs constantly without reaching the set temperature, or you hear a hissing or bubbling sound near the refrigerant lines.
Other Causes Worth Knowing
Dirty evaporator coil. The coil itself can get coated in dust over time. A dirty coil insulates against heat transfer the same way a clogged filter does. This one usually builds up slowly over years.
Blower motor problems. If the fan runs slow or intermittently, you get the same restriction as a dirty filter. A weak capacitor or failing motor often shows up as weaker airflow at the registers.
Running the AC in cold weather. AC systems aren’t designed to run when outdoor temps drop below about 60°F. Refrigerant pressure falls too low and the coil ices over. If you had it running on a cool night, that may be all it was.
What a Tech Actually Checks
When we send a tech out for a frozen coil, the process is straightforward. First, confirm the coil is fully thawed before touching refrigerant. Then check static pressure and airflow, inspect the filter, look at the blower wheel, and inspect the coil for dirt buildup. If airflow is fine, gauges go on the system to read suction pressure and superheat. Those numbers say whether refrigerant charge is the issue. If refrigerant is low, leak detection follows.
Most of the time it’s the filter or a restricted register, and the fix is done in under an hour. Refrigerant leaks take longer depending on where the leak is.
What You Can Safely Do
While you wait, or to rule out the easy stuff first:
- Switch the system to fan-only mode so the coil can thaw (don’t run cooling while it’s iced)
- Replace the air filter if it’s dirty
- Open any closed supply registers and clear anything blocking return vents
- Check that the thermostat is set correctly and the breaker hasn’t tripped
Don’t try to inspect or clean the coil itself, touch the refrigerant lines, or adjust anything inside the air handler. The fins are fragile and refrigerant work requires certification. Poking around in there usually creates more problems than it solves.
When to Call Us
If the system refreezes after you’ve replaced the filter and let it thaw fully, the filter wasn’t the problem. Same if airflow still feels weak with a clean filter in place, or you hear hissing near the refrigerant lines. Those symptoms need real diagnostics, not another filter swap.
We service the Tri-Valley and East Bay and handle frozen coil calls regularly. A technician will tell you exactly what’s causing it and what it’ll cost before any work starts. We’ll get you on the schedule as fast as we can, often same or next day when we have availability. Schedule at adriumservice.com or give us a call.