The industry standard recommendation is twice a year: once before cooling season, once before heating season. In Los Angeles, where heating season is mild and cooling season is most of the year, this translates to one service call in spring and one in fall.

That schedule is fine as a baseline. But it misses the things that actually determine how often your system needs attention.

Filter Changes Are Not on a Calendar

Most filters get changed every 90 days. Some manufacturers recommend 30 days. Some homeowners stretch it to six months. None of these schedules account for what is happening in the home.

A MERV 13 filter in a home with two dogs, an active kitchen, and a tight envelope will load faster than the same filter in a home with no pets, minimal cooking, and an older leaky structure. The right interval depends on your specific conditions.

The real check is visual and functional. When the filter is grey and the system is running longer cycles to maintain temperature, it is time. In most LA homes, that means every 60 to 90 days during heavy AC months and every 90 to 120 days during milder months.

More important than the change interval is what happens during the change. Check the filter fit every time. Look at the frame. Look for gaps. Run your hand around the edges while the fan is on and feel for air escaping around the filter. This takes 30 seconds and is the single most impactful maintenance habit.

Coil Cleaning: Annually

The evaporator coil collects whatever the filter misses. In a properly filtered system, buildup is gradual. Annual inspection is sufficient. In a system with filter bypass, the coil loads faster.

A technician should inspect the evaporator coil once a year, typically during the pre-cooling season service. The condenser coil (the outdoor unit) collects pollen, dust, and plant debris. It should be cleaned annually as well, and more often if landscaping is close to the unit.

Duct Inspection: Every Few Years

Ducts do not deteriorate quickly in most cases, but connections loosen over time, especially flex duct in attic spaces where temperature cycling causes expansion and contraction. A visual duct inspection every 3 to 5 years is reasonable.

If your assessment shows unexplained PM2.5 readings or temperature variation between rooms, duct leakage should be investigated sooner.

When to Increase the Schedule

Certain conditions warrant more frequent attention.

Post-wildfire. After a significant smoke event, the filter will have loaded faster than usual. Check it within a week. If the filter was already halfway through its cycle, replace it.

Post-renovation. Construction dust, drywall dust, and VOC loads during renovation are dramatically higher than normal. Change the filter at the start of the project, once during, and once after. Run the fan continuously during construction if possible.

New occupancy patterns. Working from home means more CO2 production during hours the home was previously empty. If occupancy has increased, ventilation needs increase too.

The Fan Schedule Question

Most HVAC systems default to "auto" fan mode, meaning the fan only runs when the system is actively heating or cooling. Between cycles, air sits still. This means no filtration between cycles and CO2 accumulation in occupied rooms.

Switching to "on" during occupied hours keeps air moving through the filter continuously. The energy cost is modest — typically $10 to $20 per month in electricity. The air quality benefit is significant, especially for CO2 and PM2.5.

The trade-off: in humid climates, running the fan continuously can re-evaporate moisture from the coil. In LA's dry climate, this is rarely an issue. Most homes here benefit from continuous fan operation.

Your IAQ assessment will show how your current maintenance schedule is affecting your numbers. $195, about 90 minutes, no upsell. Our free IAQ guide explains the connection between HVAC maintenance and air quality.