Started doing this last year, it works way better than expected.

Two things: a) don't pinch pennies on the filter, get one that's rated MERV 13 or higher, and b) it's loud. You've got to keep the fan on high to move enough air through the filter for the thing to be effective.

You'll find internets where people show you how to get around the noise issue by building a box out of multiple filters and pulling the air through that (more filter surface, less resistance), but I've got limited space so it's not an option for me.

Pro-tip...put the filter on the inlet side of the fan. it will keep crap off the fan blades.

@2

That's not going to do much unless your furnace's air intake is in the room you're trying to clear, is it? Mine's in the basement; I don't spend much time down there.

@4

your furnace should be pulling air(directly or indirectly) from the entire house.

@5

That's just it though-- if it's not pulling directly from a room, the particulate level in that room is going to take a long time to go down appreciably, isn't it? In a large, slightly-leaky volume, that stuff is just going to circulate-- there's no appreciable negative pressure drawing it through cracks or open doors, right?

It's the same reason these box-fan DIY things are only effective in a single room (or a very small home), according to the internets what talk about them (and talk, and talk, and talk)...

Prolly still better than nothing, tho.

If you layer two air filters on top of each other and fill the space between with slices of beef, you'd have a combo air filter and jerky dehydrator.

https://www.youtube.com/watch?v=tZsxFVyqvsY

lol 7

@4 Does it matter where your furnace draws air from when it's heating your house? Nope, and the same thing applies when you're using the furnace fan to filter air. The vents blow out clean air which displaces the dirty air. Depending on how well insulated your house is, most of the dirty air will either get pushed out of the house or it will all eventually circulate through the furnace.

For the last few years I've lived in a region where the smoke has been normal for decades and the furnace filter works great. As a bonus, moving the air around your house tends to keep it cooler.

Nice hack!