We are looking to buy a home that has a large bathroom (300 sq.ft.)with a steam shower. The only exhast fan is the small f-fan in the toilet area next to the shower. There is a second floor above, so it's not as easy to go overhead. What is the best alternative to retrofit another vent fan? How do you do this when there is so little space to work with?

Personally I wouldn't do it. I'd have a external vent for each fan. It's really up to the local code in your area. Most of the time it may be permitted if (and only if) there is a integrated damper on each fan to prevent backdraft. And, some municipal codes require that no two fans could operate at the same time. In the cases where code permits multiple exhaust vents into one duct, then they are the type where there is a central fan (sucking) not several fans (pushing). Plus, there are dampers that must block the bathroom vents that aren't being used. Example link below. You also can't or shouldn't terminate in the attic, inside at the soffit vents or inside into a roof vent. In other words it should be hooked to its own dedicated roof jack/vent. Also the duct work from the fan housing to the roof jack should be insulated to prevent condensation from warm vent air and cold duct. This is commonly overlooked and is a big deal in cold climates. Here's a bit of the code: The IRC is pretty clear on this: (underlining and bold are mine) - M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space. That does not leave any room for any air to be pushed back into the living space by any other mechanical exhaust system. Then the IRC adds this: - M1507.1 General. Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section. - M1506.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building. That clearly states that exhaust air from the bathroom/toilet room fan shall be exhausted ... directly ... to the outdoors. Based on the above two code sections, unless the installation instructions specifically state that two exhaust fans may be connected to a single duct, then two exhaust fans may not be connected to a single duct. Unless, of course, permitted by local code. Here is a link that might be useful: Ventilator example This post was edited by audiomixer on Sun, Feb 17, 13 at 22:46

Brickeye thanks. For the record my statement on the neutrals all being tied together was incorrect. The load side of my GFCI feeds a protected outlet in that bath while that outlet feeds two protected GFCI outlets in a downstream master bath. The line side of the first GFCI is multipled to feed a fan and a ceiling light, and those neutrals on the line side (not load) are tied together. Nothing coming off the load side is multipled to non-protected devices. BTW the use of dielectric grease on that Broan QTR140 exhaust fan motor connector seems to have fixed the problem if their was leakage to ground. I still do not understand how moisture on what is now an unprotected fan (multipled on line not load side) causes the GFCI to trip. The load side is an entirely separate circuit. My original thread on this below is confusing because the tripping problem caused by the fan seemed to be fixed several times only to re-occur. Sealing the connector on the fan seems to be a cure for now. I wanted to tie up loose ends for others on these forums if appropriate. Here is a link that might be useful: Original thread on this

there are two separate issues that need to each be addressed entirely on their own. in all of this, remember that your fan duct is a chimney and reacts accordingly. you usually have warm air flow and waste even when not "using" it. if you have condensation and it's all in cold weather, insulate the thing. Not with that trendy gimmicky thin plastic junk but with a whole lot of plain old fiberglass. a lot of it. reseal any seams first, perhaps with brush on goop and fiberglass mesh. the water damage is a separate deal and I would not be any way qualified to suggest anything concrete without eyes-on. both issues do seem to be from poor installs. the s-trap is funny, though.