AC Freezing Up in Foster City

AC Freezing Up in Foster City

An air conditioner freezes up when the evaporator coil runs colder than the moisture in the air can handle, so instead of draining away as condensate it turns to frost right on the coil. Once ice starts forming it snowballs: the frost blocks airflow, less airflow drops the coil temperature further, and before long the whole coil face is iced over and almost no cool air reaches the rooms. Most freeze-ups trace back to one of two things: restricted airflow from a dirty filter or a weak blower, or a refrigerant leak that has dropped the charge low enough to pull coil temperature below freezing. The detail that matters in Foster City is the fill soil settling under the ductwork and the damp lagoon-front air.

Two local facts matter most for a freeze-up call here. Foster City is built almost entirely on filled marshland reclaimed from the bay starting in the 1960s, laced with the lagoons the city is known for, and the lagoon-adjacent air runs damp even on a mild day. The housing is unusually uniform for the Peninsula: 1960s and 70s tract construction on engineered fill, much of it lagoon-front or a short walk from the water. Sixty years of settling in that fill soil has shifted more than a few slabs, which shows up as duct runs that no longer sit level and condensate lines that no longer drain the way they were installed.

In Foster City, we thaw the coil first if it's fully iced, then run the actual diagnostic once the system can cycle normally again, because testing a frozen unit gives false readings. Whatever we find, the part, the charge, or the airflow restriction, goes on a written estimate before anything is replaced. The $99 diagnostic fee is credited toward the repair.

Common causes

Dirty filter or blocked return. Low airflow across the coil is what we find most often on a freeze-up. A loaded filter or a return blocked by furniture starves the coil and lets it drop below freezing. We pull the filter, read the temperature split across the coil, and confirm the return is open and sized for the system.

Low refrigerant from a slow leak. A slow leak drops the charge low enough that the coil runs colder than it should, and once it is cold enough the coil ices over on its own. We find the leak rather than just recharge and leave, because refrigerant is not consumed, only lost.

Weak blower or failing capacitor. A blower that has quietly lost speed moves less air across the coil even with a clean filter. We test the run capacitor and motor amp draw, since a weak capacitor is the most common reason a blower loses speed without anyone noticing.

Corroded or worn outdoor components. Condenser fan motors and contactors wear with age everywhere. The damp, bay-adjacent air here is harder on exposed electrical connections than a drier inland spot would be. When the outdoor unit cannot reject heat properly it throws off the whole refrigeration cycle and ices the indoor coil, so we check the outdoor side on every freeze-up call.

Undersized or restricted ductwork. Sixty years of settling on filled bay soil here has left more than a few duct runs out of level or pinched at a joint. On a retrofit system in particular, that kind of undersizing can restrict airflow enough to cause repeat freeze-ups. We check duct sizing on any retrofit before calling a freeze-up a coincidence.

How we diagnose it

• Pull the filter and measure the temperature split across the coil to confirm airflow is the driver before touching refrigerant.
• Read refrigerant pressures and superheat to catch a slow leak that has dropped the charge low enough to freeze the coil.
• Test the blower run capacitor and motor amp draw against the nameplate to catch a blower that has quietly lost speed.
• Inspect the outdoor contactor, fan motor, and wiring for wear. The damp, bay-adjacent air here is harder on exposed electrical connections than a drier inland spot would be.
• Check duct sizing and returns for airflow restriction on any system added after the home's original construction.

$99 diagnostic, credited toward the repair. You get a written quote before any work begins.

AC Freezing Up in Foster City: common questions

Nearby and related

AC Freezing Up near Foster City: San Mateo · Belmont · Redwood City .

This is usually a ac repair in Foster City job. See our ac repair overview or the Foster City service area.