Honeywell 50000293-004 replacement Post Filters for Honeywell F300E and F50F Electronic Air Cleaners.   
This pack contains two (2) 20" x 12.5" post filters that can be installed in certain Honeywell 20" x 25" air cleaners.

This filter is designed to install in the extra 1/4 inch slot in your Honeywell electronic air cleaner cabinet. Now you can have the benefit of a media air filter and the high efficiency of an electronic air cleaner.  Sold as a two-pack (2 post-filters are required per air cleaner).  Post-filters only fit in the F50F and F300 electronic air cleaners.  They will not fit F50E models.

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Have you ever wondered what that extra slot or track in your Honeywell electronic air cleaner is for? Unless you spend your free time crawling around in the attic with a flash light or endlessly pondering the workings of an electronic air cleaner probably not, right?  But maybe when performing routine maintenance and cleaning those electronic cells and pre filters you noticed that extra little track.  And you thought to yourself, that would be a great spot for an extra little filter to help out with dust, smoke and all those other nasty little particulates floating around in the air.  

Well your in luck, if you've ever wanted to enhance the performance of your air filtration now Honeywell has provided you with a post filter to accomplish just that.  And if you happen to be one of the lucky ones whose Honeywell F300 electronic air cleaner came pre installed with these handy little post filters then no pondering necessary.  You just have to remember to change them every six months or so to keep your air filtration system working at peak capacity.

F300, F50F EAC Operating Guide

Features: 

• New postfilters have been proven to add 8-10% more efficiency at the smallest micron channel (0.3-1 microns), while keeping added pressure drop at a minimum.

• Antimicrobial coating protects against bacteria and fungal growth on the postfilters.

• Electrostatic charge helps attracts airborne particles like a magnet.

• Note:The electrostatic charge will break up if washed.

• Postfilters will elevate the performance of your air cleaner's efficiency in capturing bacteria, smoke, particles, mold spores and other fungi.

• Honeywell recommends that post filters be replaced approximately every six (6) months.