This is what happens if you don't change furnace air filters

[Marcus Auerbach]

Yesterday I got a call from a tenant telling me that his furnace would make an unusual noise when he turned on the AC the first time (yes, end of June - that's one of the perks of living in Wisconsin). I asked him to turn off the system and I would check it out after work. When he called me he was surprised that I could not hear the noise over the cell phone, which really got me to worry a little. So I got there turned on the unit and indeed it sounded like when you throw tennis shoes in a dryer and the whole system was shaking. Clearly the fan was completley out of balance, so I checked the filter and to my surprise -  there was no filter! What I found in the fan was this:

It looks like the remains of an old dirty filter! Somehow it must have collapsed and got sucked into the fan. Not quite sure how a filter goes from dirty to shredded?? Makes me wonder if anyone has seen this before or if there was something else going on? I suspect the the heat exchanger is full of dirt and will have the unit cleaned  serviced. My tenant was surprised about what I found - and the fact that filters need to be changed. I learned another lesson here - from now on I will replace filters no matter what my tenants tell me. Looks like this is what most of you guys do as well?