I have been trying to get an explanation from the homebuilder on this, but
no explanation makes sense.
New house. Central A/C.
In the master bedroom, when the AC (or just the fan) is on, cool air comes
out of the medicine cabinet in the bathroom (through the slots that hold the
shelves inside of the medicine cabinet).
The flow of air increases when the door of the master bedroom is closed.
But still present when it is open.
There is 1 return in the bedroom, and one outside the bedroom. They are both
the same size filter.
Is there any time where this would be considered normal or part of the
Basically, it is up to me to prove to the builder if there is anything wrong
with the install or not.
They sent a tech from the A/C sub they use and while he could not explain
it, he said it was probably all right.
Can anybody share their comments on this ?
Thanks in advance....
More information is needed. The Bathroom is under a negative pressure
with respect to whatever is feeding the wall cavity. One thing that
could cause this is if they are using the wall cavity as part of a
Another thing is if there is a room in the house with no return path
such as an outside laundry. If the laundry has a supply guct, any air
that blows into the laundry must then go to the outside. That is like
running an exhaust fan all the time the ac is running.
A sun room connected to the house with only a sliding glass door would
have the same effect if the sliding glass door is closed.
If you only have two returns, make sure all interior doors are undercut
so air from all rooms have a return air path. Make sure all
conditioned rooms open into the main house. Make sure all supply ducts
that are outside the pressure envelope of the house are sealed.
Remember, when it comes to duct leaks, supply leaks suck and return
leaks blow. That means a supply leak acts like an exhaust fan, putting
the house under a negative pressure, which will SUCK air in any
available opening. A return leak acts like a makeup air fan,
pressurizing the house. Any house air leaks will then BLOW out of the
house. A room with no return path acts like a supply duct leak.
You definately have a problem. You need a contractor who understands
air diagnostics. Hope this helps.
Your comments imply that you suspect the pressure in the bedroom is higher
than the bathroom and that there is an air leak in the wall which causes air
to flow through the medicine cabnet in an effirt to make up the pressure
If you are on the right track, the closed bathroom door should have air
flowing under it in the same direction as the ned cabnet. It seems like an
open door between the rooms would cancel this flow but you say it only
decreases so the pressure difference within the wall must be from an
Look at the airspace behind the med cabnet, does it connect with any other
cavity open to another room or crawlspace. In some houses I have lived, at
least some of the return air is routed through open wall cavities rather
than closed ducts.
A cooled cabnet may indeed be good for medicine but if fed directly from the
A/C will be heated in the winter. Heating and cooling will also have
contrasting effects on the steam that condenceses on that mirror as well.
Personally, I would just pull the cabnet out, put some duct tape over the
openings and reinstall. As long as the house is comfortably heated/cooled,
what else do you want.
Also take a look at the vent in that bathroom, is air being sucked outside
through it when it is off. Cover the vent and see if this makes a
Our Master bath is on the main floor. Ground level. There is a full
basement below this floor and incorporated in the basement walls are cold
air returns which run between the studs, from floor to floor. These are not
generally "hard duct" but are simply channels between the studs, covered
over with drywall and in some instances and with a foil covered cardboard
type product in others, which channel air up from below and into the cold
air/AC return ducts.
If someone decided to drop a medicine cabinet in the wrong place..between
the "wrong" two wall studs, the occurrence you describe would happen....YMMV
You have already proved something is wrong. Air should not be coming out of
the medicine cabinet.
The tech was clueless. If your house was on fire and he could not explain
it, would he say it is probably all right?
Could be a serious leak in a duct passing in the wall, could be some serious
balance problems, could be a lack of insulation and proper sealing if on
outer walls. I may not know the correct answer, but I don know it is wrong.
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