Air coming out of med cabinet when AC is on.

[got 2 go]

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....

[Stretch]

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

[m Ransley]

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.

[Dr. Hardcrab]

Don't most medicines say "store in a cool, dark place"....

;-]

[PipeDown]

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.

[Edwin Pawlowski]

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.

[Rudy]

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

[johnboy777]

I also have this problem only mine comes through the sink base cabinet doors when they are closed but only if our heat pump is on. I can open the cabinet doors and the air is harder to feel but is still coming out somewhat. It may do it with the A/C on but I just noticed it and it is winter. The back of the cabinet is open somewhat for the plumbing to the sink from the basement. I never noticed air coming out of the cabinet until we had a new heat pump installed last summer in 2021.

[johnboy777]

Will air coming out between the bathroom vanity doors affect the heat pump in a negative way such as putting stress on the system? The system still heats our house good. We keep the thermostat on 69 degrees and never have a problem even in the coldest months of teens and 20's.

[Commander Kinsey]

Cold teens? Are there naked young things running around in the snow?

[Ed Pawlowski]

As long as the heat is coming out where needed, no loss. If it is leaking into the garage, attic, outside wall, it is wasted and costly. It should not affect the blower as long as the rest of the heat or cooling is getting to where needed.

[Marilyn Manson]

But is as much air as could be getting to where it is needed?

If air is leaking into the vanity, thus conditioning a space that doesn't need to be conditioned, then some other place isn't getting as much. That could certainly lead to the system needing to run more than necessary to condition the rest of the house. More run time leads to more wear and tear.

And how is it getting into the vanity? If it's leaking up through the walls, then odds are it's not *all* coming out into the vanity. Even it it doesn't really "hurt" the system (other than extended run times) it sure is a waste of energy.

[Marilyn Manson]

I don't see how. Expand on your question.

FWIW, there should never be a return in a bathroom (or kitchen). Odors in those rooms shouldn't be drawn into the system and circulated throughout house.

[Ed Pawlowski]

They should also have a vent to the outside, or at least a window.

My guess is there is a duct joint in a wall that has come apart, thus the air into the vanity.

Sounds like it is working OK but it won't get better, could get worse so should be given a proper inspection.

[trader_4]

Agree. I'd take a look for heat registers on the floor above, ducts going up from the basement below, etc. Only thing that is weird is that the space behind the vanity is typically occupied with water, drain and vent pipes, no room or need to run a duct there, but who knows. Are they sure it's heat coming out, not air being sucked in? If there is negative blower pressure in the basement, it will pull some air from any openings, like those by sinks.

[Marilyn Manson]

True, but that has nothing to do with conditioning the space. Maybe I could see a situation where an exhaust fan pulling air into the vanity. We'd need johnboy777 to explain exactly what is going on.

That's assuming johnboy777 isn't just clickbait for HOH.

If that's the case, then I don't understand your "no loss" statement. As I said earlier, leaky, open ductwork, especially into an area that shouldn't be conditioned, is a loss in terms of overall efficiency.

IMO, conditioned air leaking into a bathroom vanity is not "OK".