Blower motors packed with dust: an AC in Northridge that quit on long runs, and a burned-out motor in Bel Air

A hard case in Northridge

The customer’s AC kept stopping, but whenever we came out, the system was running normally. On those visits we replaced the thermostat and added refrigerant, and the system was working when we left.

We went back several times after that. The customer’s account stayed the same: the system ran while we were there and stopped after we left.

The pattern was time

Eventually it became clear that the system stopped after running for a long time. A service visit is short by comparison. So a system that only fails after a long run can look healthy for as long as a technician is standing next to it.

What we found: a blower motor packed with dust

A closer look at the blower motor settled it. In the photo at the top of this page, the old motor is on the right. Its end and the vents in its body are covered in gray dust, packed with it. On the left is the new motor.

During long runs, that motor overheated. Its thermal protection tripped and shut it off, and with the blower stopped, the system stopped cooling. The label on the old motor reads “thermally protected.”

Why a dusty motor quits only on long runs

A blower motor cools itself with the air moving through and around it. Dust packed into its vents works like a blanket. The heat has nowhere to go, so the motor gets hotter the longer it’s on. When it gets too hot, a protector built into the motor opens and stops it before the windings burn. Once the motor cools down, the protector resets and the motor can start again.

That’s why this kind of fault hides. By the time someone arrives, the motor has usually cooled and reset, so the system starts and runs normally. It takes a long run to bring the fault back.

It took more than one visit

It was hard to pin down, and we didn’t find it on the first visit. An intermittent fault has to show itself while someone is watching, and this one needed a long run to do that.

We replaced the blower motor, and after that the system worked the way it should.

Bel Air: the same dust, and a motor that had burned out

In June 2025 we had a similar case in Bel Air, except that here the motor had died outright. That was clear right away. It had overheated and burned out, and we replaced it.

This photo shows how a blower motor packs with dust. It happens when the filter goes in wrong, when the system runs without a filter, or when the filter stays in too long. In any of those cases, the blower ends up looking like this.

How does dust get past the filter?

The filter sits in the return air, ahead of the blower, and it can only protect the blower when all the air passes through it. Installed wrong, it lets air slip around it. Left out, it stops nothing. Changed too rarely, it loads up until it can’t do its job. Over time, any of the three puts dust on the blower.

If the blower looks like this, check the evaporator coil

When the blower is this dirty, the evaporator coil may look bad too, and it has to be checked. The evaporator is the coil that cools the air. Dust that gets past the filter can settle on its fins and cut the airflow through it.

We wrote up a clinic’s rooftop unit with no filter, where the coil was so packed with dirt that it froze solid.

A filter from another Northridge job

The next photo comes from a different job, also in Northridge: the filter from an air handler, covered in a thick layer of dust. A system can’t work properly with a filter in that state.

Watch your filters, and wash or replace them more often.

What you can check

Find your filter. It’s usually behind a return grille in a wall or ceiling, or in a slot at the furnace or air handler. Turn the system off at the thermostat before you pull it out.

Hold it up to the light. If it’s gray and you can’t see through it, replace it, or wash it if it’s the washable kind. Filters have an arrow on the frame for the direction of airflow, and it should point toward the furnace or air handler. Check that the filter fits its slot without gaps, and don’t run the system without one.

Watch for the signs of an overheating blower: a system that shuts off after running a while and starts again later, weak airflow from the vents, or a hot or burning smell. If you notice any of them, turn the system off at the thermostat and call. Don’t open the blower compartment. It carries line voltage, and the capacitor can hold a charge even after the power is off.

Maintenance saves money later

HVAC maintenance matters, and it saves you money down the road. A filter is a small, inexpensive part. Once a blower motor burns out, like the one in Bel Air, it has to be replaced.

AC repair in Northridge and Bel Air

We repair central AC in Northridge, Bel Air and across the San Fernando Valley and the Westside. For commercial equipment, see what a blower motor replacement involves on a nail salon’s rooftop unit. The $85 service call is credited toward the repair if you go ahead, and repairs carry a 30-day labor and 90-day parts warranty. CA BEAR registered (#A 50636), EPA Section 608 Universal certified, $1M insured. See our AC tune-up and HVAC services, our work in Northridge and Bel Air, or call (213) 205-2055.