Bath fan issue
Just bought a house and this is my first winter in it. Noticed that all the plumbing vents (two bathrooms and kitchen )are tied into one 2" vent. So my first question is:is that ok to have all in one vent out the roof?
The real issue I notice right now is that I have I big backdraft out of my one bathroom fan-the only one connected to the multi vent 2" pipe. This makes the master bath ice cold. The other is a plain 4" flex connected to a dryer type exhaust into the attic which has no draft issue (I know , I'll fix that after all the other major issues get fixed). The drafty fan is piped with 2" PVC starting at the fan and very hard to get to. Is the draft due to the undersized 2" pipe? Any suggestions on how to stop it? Should I add roof vents for both baths?
First, 2" pvc is probably too small for a bathroom vent and when on, would be inadequate to move much air. Second, joining the exhaust from two different fans rarely works with separate motors (i.e., not designed for it). Third, it sounds like the one vent does not have a damper to stop air flow, either back from the roof, or when the other fan runs.
So, without having the specs, you'd want to increase the duct size. Dryer vent isn't the best thing, partly because it isn't insulated, and partly because all of the folds slow the air down, and will trap condensation. Solid ducts work better to minimize friction, and maximize flow.
Ideally, if you wanted to maintain just one roof penetration, you'd switch to something like the FanTec remote motor, and in the bathroom(s), it would just be an inlet grill. This would be quiet, and is designed to work with multiple inlets.
You need to figure out why the damper in the cold bathroom doesn't work - free it up, or add one, if it was removed, as a work around until you can fix it right. You could also add a second penetration, either through the roof, or maybe a gable end, or maybe even move it to a through-the-wall vent.