**Let the filter dry before reinstalling it, and ideally keep a spare so you can run the system while it dries.**

- **Running without a filter for three hours:** A single short period is unlikely to cause immediate damage in an ordinary, relatively clean home, but I wouldn’t make it a monthly practice. Unfiltered dust can accumulate on the blower and indoor coil. Leave the system off while the filter dries, or install a clean spare. Filters protect the equipment as well as improve air quality. [Carrier’s filter guidance](https://www.carrier.com/residential/en/us/products/air-conditioners/air-conditioner-maintenance/how-to-perform-air-filter-replacement/)

- **Reinstalling a wet filter:** Unless your particular filter’s instructions explicitly allow it, let it dry completely. Shaking removes loose water but may leave moisture in the material. **Three hours isn’t a universal requirement**—drying time depends on the filter, temperature and humidity. Trane’s general guidance says to dry reusable filters completely before returning them to service. [Trane’s cleaning instructions](https://www.trane.com/residential/en/buyers-guide/hvac-basics/filters-help-keep-your-indoor-air-clean/)

- **Using a disposable replacement:** Generally yes, provided your air handler permits it, it fits securely without gaps, and its airflow resistance is suitable for the system. Don’t automatically choose the highest MERV rating; check the equipment manual or have your technician confirm compatibility. [Carrier’s filter-selection guidance](https://www.carrier.com/us/en/residential/hvac-resources/what-is-merv-rating/)

For the **15 × 21 × 1** size, distinguish the filter’s measured dimensions from its advertised “nominal” size. Give an HVAC supplier the actual measurements and air-handler model number to identify a compatible replacement. A second matching washable filter is also a straightforward solution: swap them each month and let the washed one dry at leisure.