Air coming out of med cabinet when AC is on.

Hello Group,
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 design ?
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....
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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 duct.
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.
Stretch
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Don't most medicines say "store in a cool, dark place"....
;-]
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You mean you have to prove air is comming out where it shouldnt be! Just tell them to fix it. Seriously some ducts are leaking bad.
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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 difference.
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 additional room.
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 difference.
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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
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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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