Monsoon Season and Your AC: Dust, Surges and Checks

The first big dust storm of the year is always a surprise, even to people who have watched thirty of them. One minute the light goes brown, then the wind hits, then the power blinks. The morning after is when the phone starts ringing, and it is almost always one of three things.

When does monsoon season actually run?

Arizona agencies settled on fixed calendar dates in 2008 to make the season easier to track. The National Weather Service uses June 15 through September 30, which is 107 days. Storm activity usually peaks between mid July and mid August, though the distribution moves year to year. In 2025 the first two thirds of the monsoon were abnormally dry and the rain arrived late, with September finishing as one of the wetter Septembers on record.

That overlap with the worst heat is the problem. Phoenix recorded 37 days at or above 110 degrees in 2025. Your system is already working at the edge of its capacity when the dust arrives.

What does the dust actually do?

Three places, in order of how often we find them.

The condenser coil. Fine dust and pollen pack into the aluminum fins on the outdoor unit. The coil rejects heat by moving air across it. Choke that airflow and head pressure climbs, the compressor draws more current, and efficiency drops. Badly fouled coils are the reason a perfectly healthy system stops holding setpoint in August.

The filter. A one inch filter that would last three months in a mild spring can load up in three weeks during a dust event. A blocked filter starves the evaporator coil of warm air, the coil drops below freezing, and you end up with ice on the lines and water on the floor.

The blower wheel. Dust that gets past a poorly seated filter builds up on the blower wheel blades. It comes off balance, moves less air, and the motor runs hotter. This one is invisible until somebody takes the wheel out.

Why do surges kill so many capacitors?

Monsoon storms bring lightning, downed lines and brief outages. When power comes back the voltage does not always come back clean. Air conditioning compressors and fan motors draw a heavy inrush at start up, and a run capacitor that is already weak from heat will give up at exactly that moment.

You can reduce the damage.

• If the power goes out, switch the thermostat off or to fan only. Wait five to ten minutes after power returns before calling for cooling. That avoids a hard start into an unsettled supply.
• Ask about a surge protector fitted at the disconnect on the outdoor unit. It is a small part that protects the expensive ones.
• Replace a weak capacitor before it fails. A technician can measure microfarads against the rating printed on the can. A part reading below tolerance in June will not survive August.

The post storm walk around

Ten minutes the morning after a storm, while the coffee is on.

1. Look at the outdoor unit. Clear palm fronds, bougainvillea, plastic bags and gravel away from all four sides.
2. Rinse the coil from the inside out with a garden hose on gentle flow, with the power off at the disconnect. Never use a pressure washer, because it folds the fins flat.
3. Pull the filter and hold it to a light. If you cannot see the bulb through it, change it.
4. Check for water around the indoor unit or a stain on the ceiling below an attic air handler.
5. Look at the condensate drain outlet outside. If it is dry on a humid day, the line may be blocked.
6. Listen for a change: a new rattle, a hum with no fan movement, a click and silence.
7. On a rooftop unit, look for lifted panels or missing screws after high wind, and for standing water at the curb.

Why do condensate drains fail in monsoon?

Humidity. Most of the year Mesa air is dry enough that an air conditioner produces very little condensate. When dewpoints climb during monsoon, the same system starts pulling real water out of the air, and any algae or dust sludge sitting in the drain line finally gets tested. Older lines clog, the pan fills, and the float switch either shuts the system down or fails to and the ceiling gets wet.

Flushing the drain line before the season starts takes a few minutes and costs nothing compared with drywall.

What about vacant homes?

Plenty of houses around Apache Junction sit empty from May to October. A vacant house is where a small monsoon problem becomes an expensive one, because nobody notices for six weeks. If you are away, arrange a check every two to three weeks: someone who opens the door, looks at the ceiling, confirms the thermostat is holding 85, and listens to the system run.

Should you change filtration for dust season?

Often yes, with one condition. A thicker media filter holds far more dust and lasts longer than a one inch panel, which matters when the air outside is brown. It also adds resistance, so the duct system has to be able to take it. Fitting a dense filter to a system that is already short of return air will make the airflow problem worse. That is a measurement, and it takes five minutes with a manometer. Our indoor air quality page covers the filtration and fresh air options that handle dust season without choking your airflow.

Getting ahead of it

If the system has not been cleaned and checked since spring, get it looked at before the next storm rather than after. We flush the drain, wash the coil, measure the capacitor and check static pressure, and tell you honestly if nothing needs doing.