What does it mean if the furnace filter collects lots of lint just like what the dryer makes???...Perhaps the dryer is venting inside and the filter is askew and leaks in the return air or filter gap are pulling it into the furnace. Check 'em.
What does it mean if the filter has three-dozen oak leaves pressed onto it?? Today I found an unusually high bill and a handful of foliage fallen from the neighboring trees. In the attic I found a donut of leaves under the vent where the screen had disappeared. The donut hole led to the passageway – an open return duct at the top of a stud run where leaves had blown into the attic only to be sucked into the furnace.
With this info, it was easy to find and explain to the homeowner why the bill was so high, what could be done about the situation, and how it would help. ”You’re heating and cooling the air straight from the attic…see?”
And what does it mean if there's no filter at all?? ...And no cover?? It means the fan is sucking on the zone where the heater is. That fan is strong enough to SUCK the fumes DOWN the flue pipe if it's a gas or oil burner or a woodstove. This depressurization contributes to hundreds of deaths in the US each year as carbon monoxide is colorless, tasteless and odorless besides deadly. Check your filter, read between the lines and then check the filters of those you love. See the whole story in one quick pic.