Bathroom exhausts sharing single duct?!

I just hired my condo management company (a job performed, at my request, for my unit only, at my own expense, which was almost $300 after tax) to extend the two upstairs bathroom exhausts so that they exhaust to the exterior (through the side) of the unit. (These bathroom exhausts were previously vented directly into the attic before which had been damaging the roof's sheathing)
I was not expecting it to be done this way, but the two exhausts are very close in proximity to each other, and the person who did the work decided to use a shared vent for both exhausts. In other words, what he did was connect a flexible aluminum duct to each vent, and then used some type of "T - connector" to join the two ducts together so they are both sharing the same shared flexible aluminum duct that leads to the exterior the unit.
I happened to notice that when one of the two fans is running, it actually causes some air flow to be exhausted out of the other bathroom's idle fan. If the fan in the main bath is running in the main bathroom, I can actually observe the blades of the fan in the master bath start to spin. Clearly air is being pushed from the main bath into the master bath when this happens. If I briefly turn on the master bath's fan and shut it off I can observe it switch direction after it stops turning in the normal direction.
In the opposite situation (master bath's fan running when the main bath is idle), the problem, to me, seems like it could be less severe. The blades do not move in the main bath's fan. There does seem to be a little bit of air flow though the fan opening, but I suspect not as much as in the previously described situation.
Anyway, I just want to get opinions on whether or not this is worth complaining about, or am I just being too picky? It seems to me that if running one fan causes air to be pushed out of the other fan, this will, most likely, cause some mosture to be transerred into the other bath (how much I don't yet know) rather than the exterior. This seems like a less than ideal setup.
Is this type of shared vent setup for bathroom exhausts considered to be normal/common?
Which of the following do you recommend:
1) Immediately complain about this to the management company tomorrow, since clearly there is a problem with the duct setup chosen for this job
OR
2) should I perform a test to see how much steam actually gets into the master bath when the main bath's fan is running (while showering in the main bath) BEFORE complaining about this?
OR
3) Do nothing, since this is a common setup, and likely not worth complaining about.
Let me know what you think.
Thanks,
J.
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<<Clearly air is being pushed from the main bath into the master bath when this happens. If I briefly turn on the master bath's fan and shut it off I can observe it switch direction after it stops turning in the normal direction.>>
In other words, if just the main bath's fan is currently turned on, I can actually see the blades of the master bath's fan spin as the result of air being pushed from the main bath's fan through the master bath's idle fan. If I then briefly turn on the master bath's fan for 5 seconds, the motor of the master bath's fan will wind down after being shut off, but then the blades will start spinning in the opposite direction since the main bath's fan is exhausting air in the direction of the master bath's fan.
J.
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jay wrote:

It *is* done though but the results are unpredictable. Each fan normally has some kind of "flapper" valve inside to prevent backdrafting. You could see if that is present and working.
In the end, I suspect that it won't be a big problem; see what your "steam test" shows.
But you paid for it; it's your call...
Jim
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<<It is not a common setup and may be prohibited. It *is* done though but the results are unpredictable. Each fan normally has some kind of "flapper" valve inside to prevent backdrafting. You could see if that is present and working.>>
Thanks. Will look into this. Most likely there is no flapper. These are Broan fans if that means anything to you. Perhaps, the flapper, if present, doesn't work as well for the master bath's fan because it is mounted sideways in a skylight's opening, rather than on the ceiling. It could be that, when idle, the flow from the 2nd fan, although powerful enough to make the blades spin, still isn't powerful enough to close the flapper, but I'd bet that the flapper is just absent. Will see what I can find.
<<In the end, I suspect that it won't be a big problem; see what your "steam test" shows.
But you paid for it; it's your call...>>
Thanks. I will see what happens with the steam test. By the way, cross flow from the active to the idle fan, although it obviously exists, seems relatively weak when I hold my hand near the idle fan. I will try the steam test and see if this issue really is a problem or not.
By the way, even though I just spent close to $300 extending the exhausts, I'm skeptical that these fans, even when properly installed, really help that much. These Broan fans, even though they SOUND like they work, each can BARELY cause a sheet or two of toilet paper to defy gravity if I place a piece or two across the opening. In other words, I'm skeptical that these fans really suck enough air to get rid of anywhere near ALL the excess moisture/humidity when taking a shower. Perhaps it's only 25% of the excess moisture/humidity tops.
Jeff
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I'm surprised it damaged the sheathing. I found in my previous house that they were just dumping into the attic for many years with no apparent damage. Your attic probably has inadequate ventilation. Fans are available in different sizes but some require bigger duct work which you probably don't want to change now. You might be better off just hooking a timer switch in so the fan stays on 15 minutes after your shower is done and then turns off automatically. They sell fan timers where they sell the fans.

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Art wrote:

It does not always do damage, but it does often enough that I would not allow it in my home. It is very poor work in my opinion. I am glad you have not had any problems.
You are right about the possibility or marginal ventilation. Of course it may have been only be marginal because of the bath vents.
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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<<I'm surprised it damaged the sheathing. I found in my previous house that they were just dumping into the attic for many years with no apparent damage. Your attic probably has inadequate ventilation. >>
I believe the previous owner likely ran the shower at a high volume setting. This is because when I moved into the house, I noticed that my shower faucet has no volume adjustment, but the shower head itself (Speakman 2000) has an adjustment and happened to be set at what, to me, seemed a REALLY high volume setting. Granted the showhead, even at full blast, is supposed to be regulated at 2.5gpm but at the setting the previous own had, I can guarantee this generated a LOT of steam if he took a shower for more than a few minutes at anything above lukewarm.
J.
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There's your answer...............let 'em know it's not satisfactory. Give 'em a chance to fix it, or charge them back for the work they originally did if they won't do anything about it. Be nice to them, and they will reciprocate usually. Just use some tact (of which I am learning I have none apparently..LOL ) and things will work out.

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I would complain about the ducting of two fans into one. If some one stinks up one room it spreads to the other room.
I replaced the fan in my main bath with a Nautilus 130 CFM from a 50CFM day and night difference, there is actually a un-comfortable draft in the room, when drying off after a shower.I don't think you could stand the water temp to be up high enough to steam up the room during a shower.When the fan is on and the door is partially shut, the fan pulls the door open a bit.
Tom

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While it's not a case of out-and-out incompetent installation (unlike the original "vent into attic" setup), it wouldn't be my first choice. Some folks prefer to minimize the number of holes in their roof, 'tho.
Personally, I'd probably just pull down the fan unit that is getting spun by the air current, and put a flapper inline with the outlet (many already have such a beast...sounds like either this one doesn't, or it's not in the best of shape).
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Venting two adjacent bathrooms thru a single combined vent is common and works well if done properly. All that is needed in your current setup is to add a backdraft damper on the each inflow side of the "Y" adapter. A diagram of one can be seen at http://rewci.com/416splobada.html
-al sung Rapid Realm Technology, Inc. Hopkinton, MA
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I wanted to have a central exhaust fan installed to reduce the noise of fans in each bath, but was told by the local code enforcement person that separate exhausts were required. Since that killed the economics of doing a central fan, I just went with standard fans.

Neither. Call your code people and ask them.
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