What to know
Early spring, before you need cooling
- Fit a new filter. Whatever is in there has done a winter.
- Run the system in cooling for half an hour, once outdoor temperatures are reliably above about 60°F, so any problem surfaces while you still have time. Below that, check your unit's instructions first — most air conditioners are not built to run in cold outdoor conditions.
- Clear leaves and winter debris from around the outdoor unit.
- Open any registers that got closed during the winter, including in rooms nobody uses.
Doing this in spring rather than mid-summer is the single most useful habit on this list, because appointment times get scarce once the heat arrives.
Mid-summer
- Check the filter again. Cooling moves a lot of air, and a filter that would last all winter often will not last a summer.
- Notice which rooms are struggling and write them down. It is much easier to describe in September than to remember.
- Keep the outdoor unit clear of cottonwood, which arrives all at once and mats the fins.
Autumn, before heating season
- New filter again.
- Walk the house and pull two or three register covers. Wash them, and look inside with a torch while they are out.
- If you can get safely into the attic or crawl space, look along the accessible runs for anything sagging, kinked or disconnected.
- Check that furniture moved over summer has not ended up against a return grille.
Autumn is the right time for the closer look, because you are about to ask the system to work steadily for months and any weakness will show.
Mid-winter
- Watch for a room that is not keeping up, which is the clearest signal a duct system gives.
- Check the filter once more if the house has pets or the system runs hard.
- Listen at start-up for anything new.
Once a year, whenever suits
Look at the main return grille properly. It shows accumulation before anywhere else, and it is the single best indicator of how the system is doing between professional visits.
Take a photograph each year from the same spot. Comparing this year against last tells you far more than looking at it once ever could.
What is not on this list
Duct cleaning at a fixed interval, because it does not belong on a calendar. It belongs on a trigger: remodelling, a house you have just bought, pets, or airflow that has changed for reasons the filter does not explain.
If your seasonal check turns up any of those, that is the point to have somebody look. Otherwise this list is genuinely the whole job.