My Old furnace had a filter mat that you can literally see through. With my families’ allergies, I needed to do something about the primary filter in the furnace but I did not have the cash to get a new furnace just yet- so…My plan was to replace the mat filter with several standard cartridge filters to give me finer filter options and add a little more area since the finer filters I was going to use cause more pressure drop. Here is what I designed- a rack system to accommodate 3 cartridge filters and I ended up using 3M 1000 microparticle filters. It was not too bad using plastic square C-channels, pop rivets, double-sided sticky foam tape, some bailing wire to prevent excessive deflection on one unsupported filter, some acrylic sheet, thick foam weather sealing and a couple pieces of generic sheetmetal pre-holed L-angle. Here is the old filter basket.
Here is the new frame held in with double sticky foam tape and backed-up with pop rivets.
Then with the filters installed.
And with the clear sealing door down and closed.
It has worked now for several years with a noticeable difference in removing pollen and dust in the house and has eased much allergy symptoms. Caution should be exercised not to overload a furnace blower motor by decreasing filter area or increasing pressure drop across the filters beyond a particular motor’s limits (ability to run cool by sufficiently pumping air). Not recommended for the inexperienced. This blower had excess capacity.