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eries · Jan 29, 2006, 08:47 AM

Checked my gas furnace filter and found it to be totally black. This condition occurred a few months earlier this year. The filter is changed monthly,so there were two filter changes that appeared normal. Is my flue loading? The furnace is 8yrs. Old.

RickJ · Jan 30, 2006, 04:55 AM

For a start, disconnect the furnace vent from where it goes into the chimney.
Clean it out well in there.

Let us know if that takes care of it.

labman · Jan 30, 2006, 05:09 AM

There should be absolutely no connection between the air filters and the combustion products of the furnace. Most furnaces have the filters in the intake side of the furnace. If the furnace was producing soot, it would have to leak through the heat exchanger, and then circulate through the house and back through the return. Any leak in the heat exchanger is a very serious problem, and unlikely to come and go.

I would look for other problems in the house. Could cooking misadventures have produced soot a couple of times? Check the return vents and see if any of them are extra dirty. That could give a clue were the soot is coming from.

RickJ · Jan 30, 2006, 05:31 AM

Thanks, labman!

[Rick knuckles self on the forehead!!] :(

Of course! I was thinking of the soot that can get backdrafted due to an obstructed vent/chimney - which, of course, cannot get on the filter. Silly me - having a brain cramp.

Sorry 'bout that! :o

labman · Jan 30, 2006, 07:57 AM

This could be extremely dangerous. If your furnace is producing soot for whatever reason, it is producing carbon monoxide too which would get into the supply air too. Carbon monoxide kills. I would open up the furnace and take a good look at the area from the filters to the blower to the heat exchanger. If there is soot in that area, I would take a careful look at the heat exchanger. It separates the air from the blower from the combustion chamber. If you see cracks, gaps, missing cover, streaks of soot on the air side, or any other way for carbon monoxide and soot to get from one side to the other, do something at once. If you do find soot in the blower area, and do not see where it came from, or how to fix a problem you do see, call a professional today.

This site is about people sharing knowledge to help people fix their problems themselves. You may or may not be able to find and fix the problem if the heat exchanger is leaking. If not, call a professional. There may a draft problem. If so, it needs fixed too, but even with a good draft, you will still be getting poisonous carbon monoxide in the house if the heat exchanger leaks.

Do you have a working carbon monoxide detector? If so, and it hasn't gone off, likely the heat exchanger is OK, and the soot hasn't come from the furnace.

fredg · Jan 30, 2006, 08:09 AM

Hi, eries,
I have read the answers you have received so far... very good.
I am 64 yrs old, and believe me, if I thought something serious was wrong, I would call a reputable repair person; not fooling with it myself!
My suggestion is to call one.
I do wish you the very best.

labman · Jan 31, 2006, 10:42 AM

Since you have successfully changed the filter yourself, I think you should be able to open the housing and look and see if there is soot downstream of the filter. If there is, you might look further and see if something looks wrong.

There are some people that are better off not trying to do much. On the other hand, I have worked with many people here guiding them to successfully fixing the problem with their furnace. It is a shame you have let somebody talk you out of even looking at the problem.

n6chr · Jun 19, 2010, 08:48 PM

Another thing that causes a black furnace filter is burning candles in the home. That should be a consideration. A working carbon monoxide detector is a must these days.