I think you will be happier with separate vents. If you push air into one branch of a Y duct, it is going to come out of both of the other branches. That would push some air outside and some back toward the other bathroom. Even if there is a damper on the fan so that most of the air couldn't actually get into the bathroom, it will cause turbulence at the Y and make each fan slightly less efficient. The best design for any venting is always the simplest. Just keep the run as short and as straight as possible. The fewer complications you put between the fan and the outside, the better the system will work.

Thanks for your thoughts. The vent levers to the outside wall were opening fully when the fan was on, thus eliminating the possibility of 'stuck levers' or any thing disconnected as being the problem. So, I called Broan and spoke with a man who said it sounded like the motor wasn't doing it's job and in spite of the fan being out of warranty, he was very generous in his offer to send a new motor at no cost. I'm a Broan fan for life as a result. On Monday, our handyman is coming to install the new motor and use spray foam around the opening to the attic which should eliminate the 'attic smell'. I think that sealing up this gap will also improve the fan 'draw'. I can always count on my Gardenweb family to come to my rescue ~ thanks for your time ! gardening annie

As for fan-lights in the shower, it seems ineffective to try to remove water vapor laden air from the shower since most of it will condense on the walls before that can happen anyway. My goal is to remove humid air from the vanity area and allow fresh air drawn from the door undercut to reduce fogging of the mirror and window glass. The entire bathroom will eventually dry out if you use a fan with a built in timer and/or constant low speed feature like Panosonic's WhisperGreen series. I have yet to find a fan-light combination that performs as well as separate units. Here are my criteria: no CFL or LED lamps (LED inappropriate in a bathroom; CFL inappropriate in a house) Max sound rating .8 sone max grille size 13x13 flush with ceiling variable speed fan with timer Panasonic makes a recessed spotlight-fan combination (FV08VRL1) but it uses a CFL. I suppose a halogen PAR lamp could be substituted because the CFL is probably only needed to get an EnergyStar rating but I am not comfortable recommending it. To reduce fan noise it is important to reduce back-pressure by using larger metal ducts.

The main reason is laziness and cost. What they should have done is installed an insulated 4 inch flexible duct to a capped roof vent. It needs to be insulated so the warm air does not condense in the winter and drip into the fan. Your old fan probably got rusty because there was no roof cap and the duct was not insulated.