Why is my furnace air filter suddenly collapsing?
I have a Lennox down draft furnace. After adding a room and a couple ducts,my furnace filter collapses during use. It is not overly dirty. I am using a standard 1 inch filter. Not the mohair,but the reinforced paper. The filter has support on three sides.
If you can fit a 2" filter in there, you absolutely should. You'll get better filtration, longer usage and it'll be less likely to collapse.
As far as why the existing filter is collapsing, the obvious answer is too much airflow through
20x25x5 Lennox X6673 Replacement MERV 12 Pleated Furnace Air Filter Box of 1 - Manufactured by Nordic Pure: 
16x25x5 Lennox X6670 Replacement MERV 12 Pleated Furnace Air Filter Box of 4 - Manufactured by Nordic Pure: