Coil cleaning restores heat transfer through the two coils of an air conditioner or heat pump: the evaporator coil indoors and the condenser coil outdoors. In Daly City each one gets dirty in its own way. The outdoor coil sits in ocean air that leaves a salt film, along with wind-blown sand and leaves. The indoor coil runs cold and wet in a damp house, which invites biological growth. Either one makes the system run longer for less cooling, and on a heat pump it costs you in winter as well.
Two coils, two kinds of dirt
| Evaporator coil (indoor) | Condenser coil (outdoor) | |
|---|---|---|
| Where it is | In a cabinet on or beside the furnace, often in the garage; inside each mini-split head | In the outdoor unit |
| What builds up | Dust that slips past the filter, slime and mold on wet fins, sludge in the drain pan | Salt film, sand, leaves, lint, cobwebs |
| What you notice | Musty smell, weak airflow, ice on the coil, water leaks | Long run times, a hot outdoor unit, rising bills |
| On a heat pump in winter | Releases heat into the house | Gathers heat from outdoor air, so dirt cuts heating output |
A heat pump in the fog belt runs most of the year, so both of its coils work through every season.
Salt air and the outdoor coil
Fine salt from ocean fog settles on the aluminum fins. It draws moisture and keeps the metal damp, the fins corrode to a white powder, and the bond between the fins and the copper tubes weakens. Heat transfer drops even when the coil looks clean from a few feet away, and left long enough, corrosion opens pinhole leaks.
Cleaning dissolves the salt and lifts the grime: a fresh-water rinse, a coil-safe cleaner where the dirt is oily, a thorough final rinse so no cleaner residue stays on the fins, then a fin comb to straighten bent rows. Cleaning does not replace lost metal. If fins crumble at a touch or the coil already leaks, we show you and explain the choices, and on replacement equipment a corrosion-resistant coil coating is worth asking about.
Condensers on hillside terraces or in narrow side yards are easy to forget, and they tend to go the longest between cleanings.
Growth on the evaporator coil
A cold, wet coil in a dark cabinet, fed by damp coastal air, suits mold and bacterial slime. The film slows airflow, traps dust, can cause a musty smell when the system starts, and washes into the drain pan, where it clogs the drain. Cleaning means removing the access panel, cleaning the coil faces we can reach with a cleaner made for indoor coils, and flushing the pan and the condensate line. Some cased coils have no service panel, and reaching them takes more work.
Removing mold from the evaporator matters most in homes with allergies or a lingering smell. If growth returns quickly, a UV light aimed at the coil is one option to discuss, and a well-fitting air filter keeps the dust that feeds it off the fins. For smells with other sources, see musty smell.
How a cleaning visit goes
- Power off at the outdoor disconnect and at the furnace or air handler.
- Measure first: the temperature drop across the indoor coil and the system pressures, for a before-and-after comparison.
- Outdoors: clear debris, clean and rinse, comb the fins.
- Indoors: open the access panel, clean the coil, clear the pan and drain, look for rust or damage from icing.
- Restart, measure again and explain what changed.
Most cleanings take one to two hours.
When to schedule it
Coil cleaning is an annual maintenance task and part of AC maintenance. Book it on its own when cooling performance drops, when the system runs much longer than it used to, after a remodel that filled the house with dust, or when a musty smell appears. Ice on the coil can also come from low refrigerant, so if cleaning does not cure it, a leak check is the next step.
What you can do yourself
Turn off power at the disconnect, then rinse the outdoor coil with a garden hose on a gentle setting. Skip pressure washers and household cleaners: the first flattens fins, and the second can leave residue that speeds corrosion. Keep plants trimmed back. Leave the indoor coil to a technician, since it sits next to wiring and the blower.
Price
Coil cleaning runs $150 to $400 per service. A coil that is easy to reach and lightly soiled is at the low end; a cased indoor coil without a service panel, a heavily fouled condenser, or both coils on a heat pump sit toward the high end. You get a written estimate first, and the work carries a warranty on parts and labor.