Basement humidity getting into the A/C system

Old 08-02-10, 09:02 PM
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Basement humidity getting into the A/C system

My 2 A/C units are in my basement which collects enough humidity that there is almost a permanent smell in the basement during the summer. I will need to fix this humidity problem but would like to address first the fact that one of my A/C unit blows air that has the same smell as my basement.

Even though the house was built in the mid 90's, I suspect the duct work on the return was not done properly.

About half the return is made of fixed metal rectangular pieces and the other half consists of several lengths of returns made up from the space between the 12'' floor beams covered only by a metal piece. In those floor beam based returns wires are routed through in different places and even one toilet drain is going through the return space just above the furnace. There is no foil tape or duct tape used to make sure the return components are not leaking even tough the metal covers on the beams are only nailed to the beams and we can see this is not very tight in many places.

Would this qualify as a standard and regular installation? Would sealing with foil tape help resolve the situation?
Old 08-03-10, 08:05 AM
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This is the typical ductwork that was done in the 1980s and early 90s in the northern part of the country. It is ok for the heating system, but not good for A/C. depends on the workmanship, some can be sealed pretty good, some are not. People use that duct to run the cables, wires and pipes after the house was built so they don't have to drill holes through the wall. Anyway, the best thing to do now is seal/tape them as much as you can to prevent the basement humidity from getting into the system.
Old 08-03-10, 08:37 AM
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Hi Jack, clocert is correct, seal everything you can. Those panned ducts are a pain. Sealing the supply ducts also prevents air loss to the basement which forces the return to look for replacement air, which it is apparently finding in the basement.

Dirt floor or concrete in that basement. I assume concrete, but if dirt, measures need to be taken to cover it.

The other issue could be the balance between the supply and return pressure. If there is more supply area than return, then the supply will be pulling harder to make up the difference. Do you have returns in every room that has a supply or just one or two centrally located returns?

One more piece of information, numbers vary, but the natural stack effect of a home, warm air rising and leaking out with replacement air being pulled in where ever it can find an opening, results in ALL of the air within a home being replaced approximately every two hours. A very tight home is still three hours, and the largest portion of that air comes from the basement.

The musty smell you have is a warning of conditions that can support mold. Air sealing where the house rests on the foundation is important along with moisture removal, ie the dehumidifier you mentioned. If you get one, be sure it will perform at lower temperatures.

Old 08-03-10, 04:42 PM
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Clocert ad Bud - Thanks for the replies.

Some answers to Bud's questions:
The basement is concrete.
Regarding returns vs. supply, it is interesting you asked this question as the 2 A/C systems are pretty uneven. The A/C with the problem supplies the entire first floor but provide returns for about 2/3 of the total house (first floor plus about 1/3 - 1/2 of the returns on the second floor). I only bought this house recently but I explain this unbalance to the fact that the second floor is very open and may be no willingness from original owner to compromise the openess.

The A/C- Furnace uses 2 air pipes (in/out) from outside so this might help (although it works in winter but not verified it does during the summer).

If I may ask, what is your measure of a humid basement?


Old 08-08-10, 03:13 AM
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If your basement is musty, you either don't have air movement, or something to get it aired out or dried.

I'd like to keep it under 55%, anything over that mold can start.

So if it's over 55%, get a free standing dehumidifier, or open a vent down there to get some air moving.

I have one vent open down in our home, and it's stays under 50%.
Old 08-09-10, 07:47 PM
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I was recently in a basement that had a bad almost moldy smell. Turned out that termites were chomping away in several spots and their little poop was making the smell, termites....they just love to eat and poop.

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