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I recently discovered that my furnaces (Rheems) have reusable "permanent" filters, but they also have a disposable filter tray. Are they intended to both be used simultaneously, or are homeowners expected to choose only one?

The disposable filters are in a slide-out tray that's easy to access and are first to take in air. The washable filter (see photo) comes next after the disposable and requires unscrewing and taking the cover off the furnace. It's not an electronic air cleaner unit, just part of the furnace. I didn't actually discover/notice that there was a reusable filter until a year or two after having bought our house. Incidentally, there was a crazy amount of lint caked onto the reusable filter.

So, does using both filters cause unnecessary strain on the motors, or is this an intended design?

Rheem permanent filter

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Given that more restriction actually lessens the load on a typical furnace blower (no, really, you can prove it with an ammeter - less mass of air moved means less work, and homeowners "opening things up to reduce the load" can actually increase the load to the point they burn out the motor) that's not a concern.

Essentially you have a prefilter (disposable) and what appears to be a washable electrostatic for the fine stuff that makes it past the prefilter. The prefilter keeps the dust bunnies out of the washable. Not sure why they would make the washable harder to access (annoying design philosophy) but yes, use both.

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  • Your claim that the blower motor does less work when the filter is dirty sure sounds plausible and matches my recollection of learning in Physics 101 that pushing on a wall as hard as you can performs no work. Given that conventional wisdom says the opposite, do you have any references? More importantly, a reference that an increase in airflow is inversely correlated to motor life would be helpful.
    – glenviewjeff
    May 26, 2020 at 14:10
  • EE Portal says that one cause of motor failure is from windings heating up due to efficiency losses. I wonder if a motor spinning a fan with less airflow available lowers the motor efficiency and heats up the motor, thus decreasing motor life.
    – glenviewjeff
    May 26, 2020 at 14:11

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