There is soot on my daughters hot water tank(from base halfway up) as well as on the top of the tank. She changed her furnace filter and it was totally pitch black with soot. Both of these appliances are outside in a furnace room. We had a furnace man come and look at it. He was stumped. He said there is no blockage in the pipes and the vent(about 6" hole) seems adequate for air intake to make furnace and hot water tank burn properly. The condo is 9 years old. He said this has been happening for at least a few years. (daughter didn't notice it, duh) There is soot everywhere in the house. She changed the filter about a month ago and it no longer shows any sign of soot. Furnace man said hot water tank needs to be replaced and furnace can just be cleaned. He said to hold off until he figures out why it happened. My question is why did it happen and if we haven't done anything to correct the problem isn't it still happening? Should I call for a 2nd opinion? I am worried that there may be carbon monoxide although she has 2 detectors & they aren't going off. Thanks so much, Kris

One more thing. I don't know if this has anything to do with it...She was throwing out an old computer. She opened it up to remove the hard drives & found both had caught on fire & melted. The most soot ,was on the cold air return in that room. Also soot was streaking out of electrical outlet. Although I doubt if that would cause soot on hot water tank, would it?