These filters are exact duplicates of what was supplied with the Aprilaire Model 1750a unit when the unit was brand new. Having these spares filters makes change out a matter of a minute instead of 30 - 60 minutes. Prior to owning them, the procedure was to shutdown, open the filter access panel, remove the filter, clean the filter, (20+ minutes if setup to do so) dry the filter (30+ minutes minimum) reinstall, close it up and restart. I was looking at a minimum of an hour and it had to be done immediately. Now, I shutdown the unit, open the access panel, remove the dirty filter, install the clean filter, close the panel and start the unit up. Less than one minute of down time and I have the option of cleaning the old filter at my leisure and allowing it to completely air dry without forcing the drying time.