Honeywell 203373 Replacement 'wire mesh' Pre Filters for model F300, F50F, F50E and F50A Honeywell Electronic Air Cleaners.
This pack contains two (2) 20" x 10" pre filters to fit one Honeywell 20" x 20" air cleaner.

This aluminum pre filter is designed to install in the 1/4 inch slot in your Honeywell electronic air cleaner cabinet.  The pre filter is designed to protect the electronic cells in the air cleaner.  Sold as a two-pack (2 pre filters are required per air cleaner).

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So you've invested in this super advanced and sophisticated Honeywell electronic air cleaner.  You may not be sure of exactly how it works but you feel better just knowing it is providing advanced filtration for your home, right down to the smallest micron!  You take a deep breathe of clean, purified air and rest assured that the air quality in your home is being protected.  But what is protecting those important collecting cells inside  the air cleaner.  Like big, hairy offensive linemen the pre filters are standing guard.  Although they are not very sexy pre filters are an essential part of your air filtration system whose primary function is to keep large debris off the cells.  Honeywell pre filters don't require much attention, however, they should be cleaned periodically and immediately replaced if they become rusted or tattered.   

F300, F50F EAC Operating Guide

20x20 duct or cabinet opening:
Order 203373 2-pack (will fill 20x20 cabinet dimensions).

Cleaning and Maintenance Tips for Honeywell Pre Filters: Clean your pre filters every 4-6 months. Pre filters are very delicate and care should be used while cleaning them. Do not clean pre filters in the dishwasher. Tap water from the sink or an outside hose should work fine. Do not whip or shake the pre filters when drying as this could cause the mesh screen to rip away from the aluminum frame. Always allow the pre filters to dry completely before placing them back in the air cleaner.  Damaged pre filters, especially with the screen hanging loose from the frame, should never be used.  When your heating and cooling system is on air pressure from the fan motor may pull the loose screen into the cells causing arcing.  This can result in constant and loud snapping sounds at best and in some cases severe damage to your electronic air cleaner.