Shared vent-exhaust stack

Hi.

I would like to tap in to the vertical vent pipe for an exhaust van on 1st floor shower by cutting a section out of the existing galvanized pipe, inserting a pvc "tee" with fernco couplers and hooking up the fan with flex tubing.

1. Would this work from engineering/plumbing perspective
2. Any reason this would be forbidden by code?

Thanks.

Gabe

1) From the engineering standpoint, you are making direct connection for sewer gas to flow into your bathroom.

2) From a code standpoint, see # (1)

I'm not a pro, but this seems like a bad idea.

Exhaust and drain vents are 2 different beasts with different purposes.

Exhaust vents take moisture out of the house. Drain vents let air into the drain line to enable proper drainage, and to protect traps.

When you power on your exhaust fan, you risk blowing some moist air down through the drain and affecting a drain trap somewhere. Even if the risk is minimal, it's not worth it.

When the fan is off, the drain vent has a path into the bathroom, which is definitely a no-no bkz sewer gasses can migrate. Drain vents are almost always required to vent through the roof, or have several feet of clearance from any windows or doors when exiting through a wall.

Apart from that, in my experience/research, flex tubing is bad unless used for very short runs. The ridges and changes of direction drastically reduce the efficiency of the exhaust fan.

My advice is to leave the galv pipe alone. Run a separate pvc vent along the same path to the attic and then tie into the main drain vent stack in the attic. Check that your total width on that exit vent is sufficient, else you'll need to widen.

Thank you all - it is pretty clear this is a bad idea. Is it really necessary to vent the fan to the outside of the house - there is a window in the bathroom - could I simply vent the shower enclosure to the bathroom itself?
Shower stall is very tall (104") so there is a lot of unvented head space over the doorframe.

Thanks again.

Gabe

While some extra moisture may be beneficial in the wintertime because of the heating system drying out the air, it is often not that good to have the aromas of soap, shampoos, etc. wafting around the house, regardless of the moisuture levels. In the summertime, it can quickly lead to mold or extra load on the a/c system. So, it is best to exhaust this to outside; not an attic, or another room. Regarding code compliance...that is a local thing, but many require some means of exhausting the moisture.

Around here if you have a window, you're not required to have an ex-vent.