After changing furnace filter, I notice it gets grey/black within 24 hours! This isn't "normal", is it? I've only lived in this house a few months and it is my first home. I'm wondering if the furnace is spewing a fine mist of black dust, as I'm finding furniture covered with a layer of dust just a couple of days after dusting. Furnace is only 4-5 yrs. old. Is this serious?