My friend woke up this morning to a haze in their house. The worst concentration was in the laundry 'room' portion of the basement where the furnace is located. Air filter was black even though it was replaced about a month ago. Service guy says it's a cracked heat exchanger. It didn't sound like he took anything apart to confirm his diagnosis.
I came by after work to take a look and spotted a hole in the exhaust pipe just above the heater with signs of soot, soot on the bottom of the barometric valve and one of the cleaning ports wasn't closed (don't know if that was the service guy's doing or not). I'm inclined to think this may be a genuine exchanger problem but wouldn't mind hearing some suggestions.
The sticker below the cleaning ports says that improperly fastening can cause flue gas leakage and lead to furnace damage, oil fumes, sooting and severe bodily harm.
The fact that it's so sudden and severe would lead me to believe that the cracked diagnosis may be correct. Unless one of the kids was playing around and undid the cleaning port cover within the last 24 hours.