Bathroom vent tied into vent stack?

[JeffeVerde]

I've got a downstairs bathroom (toilet/sink only) with the typical cieling ventilation fan. Looking at the roof in that part of the house, the only roof vent is the vent stack. Is it common/permissible to tie the exhaust of a bathroom fan into a (dry) vent stack? Or is the fan just exhausting into the ceiling space?

[Chewy]

If there's no sign of a roof our exterior exhaust outlet, it's likely it's just pumped into the attic. Not the greatest way to do it, but permissable (anyone want to back me up on that?).

NOT permissable to tie a exhaust fan into waste vent. Nope, nuh-uh, no way.

[JeffeVerde]

If there's no sign of a roof our exterior exhaust outlet, it's likely it's just pumped into the attic.

That's the thing - there is no attic space. This is the first floor of a two-story home. So if it's not vented to the outside, then it's just dumping into a 5' long joist bay with another room above it. With no place to exhaust to, the fan wouldn't be able to "exhaust" the bathroom - just make noise - which may be the case.

I'm curious why connecting a bathroom exhaust fan into a dry vent stack wouldn't be permissible. The vent stack is intended to provide a vacuum break to the wet stack, so I don't see what negative effect the slight positive pressure of an exhaust fan could cause. And the exhaust fan has a flapper "check valve" in the exhast duct to prevent reverse air flow.

The vent stack from this bathroom is 2", where the rest of the house is 1-1/2", and I wonder if the larger line was used to support the exhaust fan.

[suemarkp]

Can you follow the joist bay to an exterior perimeter wall? If so, cut a 4" hole there, put some sort of bug and rainproof cover over it, and tie that cover into your vent fan via a duct.

[Chewy]

Simply put, you can't because you can't. IPC 901.4 says you can't vent anything other than the plumbing system with the plumbing vent system.

Functionally, if you were to attach to the vent stack, it'd have to be at an upward angle. When the rain comes, it'll drip down the inside of the vent stack and right into your aluminum bathroom exhaust duct and, probably the fan itself.

As far as the existing routing, I'd just pull the fan and see if you can at least get the direction it's heading. I'd check the roof, the exterior walls, as well as the soffits (if any) for the exit point.

And regarding the vent sizes, 1-1/2" is typical for just about all fixtures. A toilet requires a minimum of 2".

[joed]

Venting a fan into the vent stack would allow sewer vapours to enter the house through the fan when it is not running.
If you can't find the exit by pulling the fan try a smoke test.

[mfarver]

A lot of first floor bathroom fans, especially in half baths do not exhaust outside, They recirculate the room air and have a activated carbon filter that is supposed to be replaced (and never is). They are usually a little smaller than you'd expect a fan to be, and rectangle in shape. The ones I find most often have a grill on each end, and a small slot in the center where the filter can be removed. Follow the advice above and take the cover off. Usually by looking at the insides you can figure out how it works, and the rough direction any outlet goes.

Mark

[scuba_dave]

also possible if house was resided someone covered over/removed outside vent cover

[JeffeVerde]

A lot of first floor bathroom fans, especially in half baths do not exhaust outside. . .

Nope - it's a standard fan. Pulling the cover, I can see the exhaust port, and that it's attached to something. The wife has decided that she doesn't like the existing kd texture on the walls and ceiling and wants it smooth. Just to satisfy my own curiosity, I'll open the ceiling to see what's up with this fan before I replaster.

[mrikking]

No shower you should be fine. It just removes Poo air and allows freash air to come in. If there were a shower/Bath you would need to vent, moisture creates mold and thats not good.

[JeffeVerde]

No shower you should be fine. It just removes Poo air and allows freash air to come in. If there were a shower/Bath you would need to vent, moisture creates mold and thats not good.

That's what started the whole question. Directly above this toilet-only bath is a full bath. The full bath is vented by a small window, that does a poor job of clearing the steamy air from a hot shower. I want to add a fan to this upstairs bath and figured I'd tie into the vent for the downstair room - only there doesn't appear to be a vent from the downstairs bath. So, thinking that the original builder wouldn't have been so lame as to install the fan with the exhast dumping into a fully enclosed 5' long joist bay, I figured that maybe it was tied into the vent stack.

[mrikking]

Gotcha, I was confused reading the other posts. I have heard of people using indoor dryer vents attached to bathroom vents but i do not suggest this. but possibly could be what was done, just guessing.