Our house seems very dusty. The tops of bookshelves, computer screens, etc seem to accumulate a thin film of dust just a few days after cleaning. Certain other co-inhabitants of my house have allergies, and want me to do something about the dust.
So I figured, I'll start with the HVAC system and see what's up. I removed the filter from my Weather King Air Handler (14AHJ11S01C01), and discovered that it uses a "permanent washable filter". It had a sprinkling of dust, but not the thick cakes I've removed from other types of permanent filtration systems. What alarmed me was the extremely loose weave of the thing:
I mean, look at this thing:
This looks like it would maybe intercept any stray birds and meteorites that find their way into my air ducts, but not much else. Is this doing anything at all? Am I missing a piece of the filter?
From what I've read most furnaces don't have enough force to push through something like a HEPA filter, but surely they could use something with a weave tighter than a pasta strainer, right?
I was thinking of just attaching a piece of thin foam to the filter, like this: 
At least that might catch a bit more dust. Will I burn my house down if I do this?
Best Answer
the air filter is designed to clean the air flow prior to passing through the fan or the heat exchanger. its a common misconception that the air filter cleans the air for us humans. its there to keep the air clean for the machinery. its a secondary benefit that the air is cleaner for us. if you get a buildup of dust inside the fan, you get overheating of the motor, inefficient air flow across the vanes, even fires from localized buildups. if you get dust buildup in the heat exhanger, it can massively shorten the life of the furnace (particularly in electric furnaces) or cause an outright fire.
if you cant get a cartridge filter to fit your furnace, get an hvac company to come in and install one between the cold air plenum and the furnace intake. then you can keep the furnace cleaner longer, which will mean a longer life for your furnace.