Heating Vent Filter Question

[FTL_Ian]

We had a "Heating 101" today as the guy from Cheshire Oil came over to clue us Florida folk in to this radical furnace concept.  He was very knowledgeable and helpful.

There were a couple of intake vents that had filters in them.  They were both clogged up with dust and other crap.  (This was a rental, and the heating system was obviously neglected by the tenants.)  The Cheshire Oil guy said it's best to not have any filters in the system except at the furnace itself, so I have removed them from the intake vents.  I then examined each outflow vent and found filters in them as well.  My roommate Mark commented that one of them looked scorched.  I have since removed them all, but as I was removing the last one I noticed what appeared to be soot on my hands.  Indeed, part of the filter I was removing had a dirty portion about the size of the piping leading to the filter.

Should I have removed the filters from the outflow vents?  Do these trap soot, as it appears?  Do furnaces generate soot?

[slim]

Ian,

Furnances will generate soot (some soot is normal) because of the combustion and impurities in the fuel excessive soot may be due to a improper installtion.    Could you post what brand and model of furnace you have if it is one of the models the company I currently work for I could ask the tech guys here. Also I am not aware of any furnaces that need extra filters on the intake or outflow most of the furnaces I have seen have filters in the furnace.

[FTL_Ian]

Oneida Royal LOR-84A-2B

[FTL_Ian]

This unit definitely has a 20x20x1 filter in the furnace, but it was totally clean.  (Likely due to the pre-filters that were clogged to hell)  I'm guessing that is an intake filter.  You're saying there should be another one in-furnace for outflow?

[Lloyd Danforth]

The part with the soot and the part that handles hot air heat should be separate.  Might not be soot.  If there was much trapped in them, I would clean them and put them back and check them once in a while.

[FTL_Ian]

The part with the soot and the part that handles hot air heat should be separate.

Can you clarify this?  I'm not sure I understand.

[Lloyd Danforth]

The heat chamber where the fire takes place and soot occurs, heats a bunch of tubes that the air coming out of your vents flows thru picking  up heat.  There shouldn't be a connection between the two areas. 
If there is anything wrong, I'm sure the guy would have caught it, but, I would go over to Walmart and buy a Carbon Monoxide detector, just cause its a good thing to have.

[Pat K]

I assume this is a forced hot air furnace.

There should be no soot at all in the duct system.

If this is actual soot and not just acumulated dirt, the system should not be run as this indicates a cracked heat exchanger. This would allow combustion gasses into the house, very dangerous.

This simple for a tech to check and if this is soot it should be checked now.

[FTL_Ian]

The furnace was inspected and cleaned in May when I bought the house as part of the purchase agreement.  I would expect if there was trouble, it would have been caught at that time.  I have no way of knowing the last time the outflow vent filters were cleaned or replaced.  It may have been a long time, as one of the vents had been painted shut around the outside.

Should I put in new outflow vent filters and see if they trap more soot, or leave them off and see if things start accumulating a layer of soot?  I don't want soot on everything, but I also don't want to restrict the airflow with filters.

[Pat K]

I don't know who did the inspection or what they checked.
How old is this furnace ?

I dont want to make a big deal out of this if it's just dirt in the ducts, but I also don't want to minimize the danger if this is soot, people die from CO poisining all the time.

[FTL_Ian]

Cheshire Oil did.

It appears to be circa 1989.

[Pat K]

O-K I would think that the tech there today would have done a draft test to see if the heat exchange was intact.

He is right about the filters, you just need the one in the furnce side air intake. The ones in the supply ducts can screw up the air balence of the system.

[Lloyd Danforth]

If the thing was built for filters then filters are proper.  Clean and/or replace them and check them out after a few weeks and make a judgement about unrestricted airflow verses any detrious found trapped in the filter blowing around the house.

[MaineShark]

Okay, heating tech chiming in here...

Pre-filters are good.  Post-filters are bad.  That is, the return should be filtered, but the outlet should not.  The filter at the furnace (the 20x20x1) should be in the return duct, and I have no reason to doubt that it is, since it would be silly for someone to go to the trouble of changing that (anyone with the skill to do it would know better).  The filter at the return grate in the floor is also a good idea, as it will keep dirt and debris out of the ductwork.

In the case of each, keeping them clean is very important to maintain the equipment in safe and efficient operating condition.  Check the furnace filter monthly, and the filter at the return grate weekly.

As far as soot, if there is any, it is indicative of a cracked heat exchanger, and the furnace should be immediately replaced (heat exchangers can, technically, be replaced, but it is more expensive than replacing the whole furnace).

A draft test alone cannot reliably detect all cracked heat exchangers, and a full heat exchanger test is not typically done during a furnace inspection.  To fully test the integrity, a visual inspection (using mirrors, cameras, etc.) is necessary, and then a smoke bomb is used in the combustion chamber, while the fan is running, to provide a more-visible substance to check for in the ductwork.

If the furnace is circa 1989, then it is getting up there in age (15-20 years is the expected service life for a warm air furnace).

In any case, if you have any fuel-burning appliance in your home, you should have one or more carbon monoxide detectors, and maintain them in good operating condition.  CO isn't anything to joke around with; it can kill silently or, in lowed levels, cause permanent brain damage, or simply lingering illness, similar to a persistent cold.

IIRC, you're in Keene, right?  I expect to be down there in the near future, as my brother is running in the marathon on the 24th, if you'd like anything explained in-person...

Joe

[Minsk]

Might be an idea to snake a cloth into the outflow ductwork to see if there is more crap waiting to be blown out. If so, you'll either want to leave the outflow filters in or spring for a duct cleaning. Depending on the furnace you might be able to force the circulating fan on, which could help blast things out before the heating season starts.

<edit>Yes, there was some actually useful stuff in here at first. But while I was distracted doing other things MaineShark went and posted a better explanation </edit>