After Phoenix dust storm, renter learns thermostat fan can trap some dust, but not fine particles

When a dust storm rolls through, even a basic thermostat setting can feel like an emergency backup.

After one of those storms in Phoenix, a renter turned to Reddit to ask whether fan-only mode on an HVAC system could recirculate indoor air through the existing filter and help with dust, without requiring any new equipment.

What's happening?

In a post on Reddit's HVAC advice forum, the renter asked, "Will setting the fan to 'on' while the mode is 'off' (not heating or cooling) help with indoor air quality by pulling indoor air through the filter and recirculating it?"

Commenters generally said it can help somewhat. Leaving the fan running will keep air moving through the system's filter, so some larger dust can get caught.

But several replies said the real limitation is the filter itself. Typical HVAC filters often are not built to capture the smallest particles that linger after a dust storm.

As one commenter wrote, "It will filter your air a bit more, but the air handler filter is usually pretty coarse, like MERV 8, so it won't catch the fine dust."

Since the renter also said they could not inspect the unit or make changes because they do not own it, much of the discussion shifted toward air-cleaning options that do not require modifying the HVAC system.

Why does it matter?

Using the fan setting is cheaper at the start than purchasing a separate air purifier, and it could help with some of the dust you can actually see.

Commenters said that may not amount to much, though, if the aim is better overall air quality. They warned that constant fan use can raise electricity use and put more wear on the blower motor, even as the system may still miss the smallest particles.

As one commenter put it, "The filter on your ac is really only designed to stop dirt from getting on the coils."

Others said the finer particles are the bigger concern.

"You need to be concerned with PM 2.5," one commenter wrote, referring to tiny airborne pollution that is tougher to filter and more likely to affect the lungs and overall health.

Humidity came up too. In a lot of places, people said running the fan after cooling can send moisture from the coil back into the home — although one commenter noted, "But it's Phoenix so dehumidification is the least of their concerns."

What can I do?

The advice that came up most often was to get a standalone air purifier if that is an option. Commenters said it costs more upfront, but it does a much better job than depending only on a standard HVAC filter.

For people looking to spend less, commenters also pointed to DIY filtration.

One person recommended a Corsi-Rosenthal box and said to "use qty-4 merv-13 filters," calling it "super simple" and saying it is "less than 80 bucks for materials and can get ~500cfm CADR or higher."

Another commenter made a similar point, writing, "The $85.00 DIY units work as well as a $300.00 store bought unit."

If spending money is not an option, some commenters said the fan setting can still be useful as a short-term measure.

As one person summed it up, "If you're not majorly concerned then yes running your ac fan will filter a bit more air."

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