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black dirt from vents

Posted by lbelle5 (My Page) on
Thu, Jul 26, 12 at 14:43

I just moved into a 50 year old home. There was a new heating unit put in 7 years ago. An addition was put on in 2004.

Once the old owners moved out, you could easily see where black dirt was sucked into the vents as well as where the vents put out the heat/AC onto the walls. Some areas worse than others.

I was thinking I just needed the ducts cleaned, but after reading up on it, I don't see anyone mentioning being able to see obvious dirt on walls or carpets. It's almost as if someone used a kerosene heater with black oily smoke. There was no mention of a fire at any time in the disclosure.

Any ideas what it is from? Or if duct cleaning is sufficient? Hate to disturb any asbestos like product that may have settled in over the years of remodeling in this house.


Follow-Up Postings:

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RE: black dirt from vents

post some pics
its hard to see from here


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RE: black dirt from vents

Wow, I would call an HVAC guy in to take a look. You don't want to use it without getting it checked out first. It could be something serious and not just dirt.


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RE: black dirt from vents

Can't post pics as carpets got cleaned as did the walls. It wasn't horribly black like a fire, nor is it obvious coming out of the vent. however, I think getting an Hvac guy in might help. I have noticed that the unit has been checked spring and fall religiously according to the sticker on the unit. That's why I'm thinking it's something icky in the ducts...
The carpet cleaner suggested the house needed more venting? Too much negative pressure ore something like that? Odd part is that it's from the intake and output vents


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RE: black dirt from vents

Burning candles for long enough will throw smoke stains, and HVAC filters are not fine enough to catch the particles.

It could also just be from long term coking without
adequate ventilation (or not running the vent fan if it is noisy).

Clean and paint before spending money on duct cleaning.

If the marks start to return THEN take additional action.


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RE: black dirt from vents

if a room is starved for return air it will put
the room under a negative pressure. the room will
then suck air from where ever it can get it..
under sole plates of exterior walls is common if
they were not sealed during construction.

as the air is sucked thru the gap it brings dirt
particles with it, causing dark marks on the carpet.
for example a master bedroom/bath suite with 2 supplies
in bedroom, one in bath, one in each closet with
door to hallway undercut 1" will have a dark mark
under the door to hallway.

air is being supplied with insufficient pathway for it
to get back to return grill.
if homeowners keep the doors open, its not a problem.
but when the bedroom door is closed, the 1" undercut
door only returns for 1 supply. this is why jump ducts
and transfer grills are installed. to provide
a dedicated pathway for air to get back to return.

the dark marks are called ghosting. hvac folk will tell
you it is because of burning candles...but it is really
a symptom of not enough return air.

dirt marks on supply grills are from dirt.
if you remove the supply grill you will see that the
cut in the sheetrock is not sealed where the supply box
is attached. the air is pushed out of the supply box
when unit is running. dirt from the gap around the supply
box is also picked up and pushed out.
if condensation forms, the dirt sticks to the condensation
and grills are dirty.

take a look by removing a supply grill.
post back or show how it looks.
we can help you fix it if we know how it looks.

include the following information.
size of unit, size of return (on filter)
location of return, location of heating system.
type of ducts.

best of luck.


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