LESLIE: Dean in North Carolina is dealing with an issue with the furnace. Tell us what’s going on?DEAN: Yes, I have a house that’s about six years old and we have a black dust that accumulates on the windows; inside the windows. In the furnace filter it turns charcoal within probably a month. I put in white filters and they turn a charcoal color within about a month, so – and I’ve got a gas furnace and it seems like it does it even in the summertime with the air conditioner running.TOM: OK, so you have these black streaks that are forming?DEAN: Actually, it’s a coating.TOM: It’s a coating? Hmm. Alright, and are you sure it’s coming from the HVAC system? Because …DEAN: No, I’m not. I’m not sure where it’s coming from.TOM: OK. Because one of the reasons that we get black deposits, especially on walls around windows, is from convection. If you get – if you think about it, warm air rises and, as it does, it pulls the air up; and usually, against a cold, exterior wall or a chilled surface, it will deposit the humidity there. It will condense and it will leave the black dirt, essentially, from the air in those areas. And so that’s a fairly common condition. We see it – sometimes we get streaks because the edge of the ceiling joist, for example, where the wall stud is colder than the wall next to it; will get streaks. We’ll also get deposits above heating registers or air conditioning registers. We’ll get them above windows. That may or may not have anything to do with the heating system itself.However, I will say that getting a good-quality filter in there – which is better than one of those thin fiberglass filters – will help reduce the amount of dirt that’s in your air that’s being circulated. So I would look to install an electronic air cleaner. You can get a good one from Aprilaire. I’ve got one which is their Model 5000 in my house; does a good job. Trane makes a good one called CleanEffects that my sister is putting in. These are good-quality, very efficient units that make a big difference on the quality of air inside your house and I would definitely do that.I would also have the heating system serviced because whenever we talk about black soot, we want to make sure that there’s no combustion problem there. Doesn’t sound like there is but I would definitely have it serviced at the same time.DEAN: Are those room-sized air cleaners you’re talking about?TOM: No, they go on the heating system themselves.DEAN: OK.LESLIE: So they’ll do the whole house.TOM: So it covers the whole house. That’s right.DEAN: OK.