Try to explain this one to me??

hello everyone… I did a condo inspection(15yr old) today and found these vents in the back bedrooms with all this black fungi/dust deposits all over them. If you look at the first picture, there is not a spec of anything on the vent to the right?? The living room vent had some deposits, but all the other vents in the house were spotless!! I even popped the register covers on the black one and the duct work was clean. I spoke to the client prior to the inspecion and she mentioned this problem, I explained it could be from a cracked heat exchanger(heater was fine)possibly humidifer, to much moisture in vents, no humidifer. All I can think of now is was there a another source of heat? Kerosene heater in these locations?? Something to make these marks on the registers. I’m reaching for straws and with over 1500 inspections, have never seen anything like this… Any help would be greatly appreciated.



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This board is dead tonight nobody talking just sneaking around and peeking afraid to commit.

That grill does not appear to be dirt or carbon to me it appears from what I can see to be mold that has dried from past condensate on the register. I could not see if there were water stains on the dry wall near the grill or not.

3 things come to mind when looking at those pics and I have to ask:

Is this a 1st floor condo?
Where is the condensor in relation to the vent in question?
Looking at the photo can 1 assume from what appears to be an exterior door that the vent in question is located close to the exterior wall of the condo?

Thanks Charlie for the response… It looks like mold to me as well. It does wipe off easily, not that mold wouldn’t do that, but my delimma is why just these two or three vents??? The ones that are black are the closest to the furnace, but what is causing this???

Pretty good paul…yes 1st floor condo. Back two bedrooms are 10-20 feet from condenser. There is a glass sliding door in the other bedroom. Vent looks about the same as the one in the pic, but the one in the pic only has a window next to it. Back of house faces SW if that helps!

I failed to ask if there is another condo above. So is there another unit above this one?

That vent is somehow getting warm enough for condensation to form on it. Ill bet it is a metal vent cover. If you change it to a plastic cover, that may do the trick.

Yes, There is a condo above.

Thanks william…They are metal covers. Do you think it just took 15yrs of being this close to the Condenser, Coldest part of the house for this to happen.

Now lets see if we can troubleshoot this…

Where is the upper condos bathroom in relation to this vent?

How can the Condensor only be 10-20 ft away from this vent if there is another unit above? Where exactly is it?

What type of siding?

oops…I forgot another very important detail.

What type of property surrounds this complex?

Chris you stated that the grills that have the problem are closest to the furnace. That makes sense to me as the temp of the air is the coldest at the closest grill to the plenum. Duct loss temp could be the reason that you do not have problems at the other grills. If you are located in Ca. did you operate the A/C unit and what were the temperatures. Sorry I went back to your post NJ so to cold to operate the A/C.

The condenser is in the rear of the condo. Back two bedrooms are in back of condo. Bedrooms are located on back walls of condo. Patio is outside and utility room outside back bedroom walls. There is another row of condo’s behind these. Nothing but grass in between? Bathroom is in middle of condo. I would think upstairs is located same area due to drainage. Bathroom vent did have some similar staining. Not nearly as bad…

One last thing for paul… aluminum siding

Had a situation with condensing units bathing the exterior walls with hot air, this caused an increase in negative pressure and drew warm moist air from attic into one room of this large mansion.

The end result was increased levels of heat humidity and microbial growth was everywhere.

HVAC was properly sized and changed out, added/increased insulation over this area, and in-line dehumidification was added. No more problems.


It could be something as simple as the register has been turned off/louver damper closed, causing a sweating/condensation on the metal register.
Lots of people do that to save energy in regularly unused rooms. People sometimes close registers to help force air further down line to areas not getting enough air.
All sorts of reasons may be possible, including a real problem with the Mech. operation of the system.
Typical design of HVAC systems in Multi-Family properties, have been marginal to poor at best, typically due to cost cutting, space constraints for ductwork design, etc. Usually no design for balancing the system airflow, except by the registers.
By the way, I have seen mildew, mold, etc on Plastic registers as well.

Thanks Barry… I was just sitting here thinking a similar senario. Register vents in ceiling, warm air rises and condenses on cold metal air register vents, back of home faces SW. Fifteen years later this is what they look like. Any help on wording this in my report???

Thanks Greg!!