**Let the filter dry completely, and keep the system off while it dries—or install a suitable dry spare.**

- **Running for three hours without a filter:** A single short period is unlikely to cause immediate damage in an ordinarily clean home, but that is a general assessment, not a guarantee for your unit. Unfiltered air carries dust into the blower and indoor coil; repeated exposure can cause buildup. I would avoid making this your monthly routine. [Trane explains the filter’s protective role.](https://www.trane.com/residential/en/buyers-guide/hvac-basics/filters-help-keep-your-indoor-air-clean/)

- **Putting the filter back wet:** Let it dry fully unless its manufacturer explicitly permits damp installation. Shaking removes loose water but can leave moisture in the filter. Three hours is only an estimate; drying time depends on the material, humidity, and airflow. [Trane’s maintenance instructions require washable filters to be thoroughly dry before reinstallation.](https://www.trane.com/content/dam/Trane/residential/downloads/manuals/dc-uc-web01_packaged_units.pdf)

- **Using a disposable replacement:** Generally, yes, provided your air handler permits it, it fits securely, and its airflow resistance is suitable. Confirm the permitted filter type and MERV rating in your manual or with the technician. A higher filtration rating can be too restrictive for some systems. [Trane’s filter guidance](https://www.trane.com/residential/en/buyers-guide/hvac-basics/filters-help-keep-your-indoor-air-clean/) discusses this limitation.

For your **15 × 21 × 1** filter, check whether those are measured dimensions or the size printed on the frame. Disposable filters have both *nominal* and *actual* dimensions, which differ. Compare actual measurements before ordering; an HVAC supplier may be able to provide a custom size. [Trane explains filter sizing.](https://www.trane.com/residential/en/products/indoor-air-quality/filters/)

A second compatible washable filter would also let you swap in a dry one immediately after cleaning.