Condensation in bathroom vent pipe
terryb
| Posted in Energy, Heating & Insulation on
I’ve had nothing but trouble with bathroom fans. The moisture condenses in the pipe, collects and eventually dumps back into the bathroom. Pipes go from the fans through the attic to a vent in the roof. I’ve tried wrapping the pipes with fiberglass insulation but it doesn’t work. My next plan is to run the pipe under the attic insulation to the eave and install a vent there. Seems like it couldn’t condense under those conditions, ’til it got to the eave. Is there a pre-build vent for this type of setup or do I have to make one? The roof is held up by 2×4 trusses 24″ oc. Seems like this would be a common problem but nobody I’ve talked to has heard of such a thing.
Replies
Bath Fan
The only thing I know about that is if you don't run the fan long enough you can have a similar problem, also if the vent pipe is run with too many angles, they should be run the shortest possible distance. Maybe someone else with post with a better idea.
You're not that far from me
and it's not that cold around these parts, so I'm surprised to read it's a big problem for you. There are definitely things you can do. First, use a big enough fan. Don't know what you've got installed now, but you may want to upsize it. It is fairly common to see 50CFM fans in bathrooms and you might be better off with something like 110CFM. The more air you move thru, the less time the moisture has to condense out of the air onto the pipe, and the more BTUs of heat you're moving thru the pipe to heat it above the dew point. Keep the duct run straight and short, use rigid pipe if possible, and insulate it well.
More info on the setup you have would be good. Fan size, length of duct run, number of elbows, stuff like that.
Occupant habits make a big difference. If you take a steaming shower and then turn the fan off as soon as you leave the room, you're not doing much to evaporate whatever moisture might be inside the pipe. Installing a timer instead of a switch, and letting it run for several minutes after you leave will both help.
There are specially made soffit vents, with counterweighted louvers or some such so that the louvers stay closed when the fan isn't running. Don't know of a source, though.
Condensation occurs because moist air gets into the pipe when the fan isn't running. In cold weather this may be air from inside, in warm (A/C) weather it may be warm air from outside. You want reliable vent closures on both ends to prevent airflow when the fan isn't running. You also should check to be sure that the house isn't pressurized or depressurized such that air is being forced through the vent even with the fan not running.
Couple of things
I'm not sure about there. What's your source of info re moist air entering when the fan isn't running? I suppose in a leaky house there could be stack effect air leakage out thru a vent fan but most of the time that's going to be average interior air, not particularly moist air as when the shower is running. Not saying it couldn't happen but I haven't heard that before.
Also, what would constitute a "reliable vent closure" that would prevent air from moving out? There are usually backdraft dampers on the fan itself and on the vent cap, but those won't do much to prevent air from moving in the direction it's supposed to. I have taken to using Seiho vent caps in a lot of situations, they have spring-loaded flappers (not a stiff spring....) that have to be opened by the fan running, and that's about as good as it's going to get. The damper at the fan itself is usually just a little piece of sheet metal that gravity holds in place until the fan is turned on.
My money is still on an undersized fan, an overly long/complex duct run, and the fan being turned off as soon as the shower is over. We're in a pretty mild climate here, it's going to be hard to get condensation in a pipe in the attic most of the year, unless the exhaust air is really warm and wet..
There isn't enough moisture in the air "trapped" in the duct to create a dripping problem -- you only get a problem if there's a steady supply of moisture.
I'd tend to advise against a soffit placed vent termination. Bad idea. You risk that moist exhaust air being sucked back into the attic if the wind isn't blowing right.
As another poster said. One thing is to run the fan long enough to get rid of the moisture. Are you using flex duct? Is your termination free of restricting air flow? I've seen 80 cfm fans pull no more than 20 cfm ... resulting in condensation on window sills in a brand new construction that would have been deemed very energy efficient. The reason was the ventilation air flow.
If you have flex duct, maybe you don't have the flow you need to get rid of the moisture. Before determining a solution to the problem, you need to assess the problem well. Otherwise you could be spinnin' your wheels doing a lot and getting nowhere. Moisture can be a bit tricky to assess, so think it through and don't do anything without being reasonably sure that the science of what you do makes sense.
How long is the run of duct you have? How long do you run your fan? What size of fan do you have? I'm guessing air flow as your first issue to either check or resolve.
I'd tend to advise against a soffit placed vent termination. Bad idea. You risk that moist exhaust air being sucked back into the attic if the wind isn't blowing right.
I think that's a red herring. So long as you don't put the vent outlet within maybe two feet of a soffit vent there's little danger of substantial amounts of exhaust being sucked back into the attic.
But if you do run the vent duct down through and terminate at, the soffit as I thought I read in the post, it is within 0" of the soffit. Assuming either continuous soffit vent or one every rafter bay ... you are easily w/in 2 ft of an attic vent ... and I wouldn't do it or recommend it.
It's not hard to block the continuous vent for a few feet. And if there's a vent in every other bay that's still 2 feet away.
As long as advice includes your side notes, it would be doable. But a general stamp of approval on soffit located vents is not a good idea IMO.
Well, I suspect the "hazard"
Well, I suspect the "hazard" is vastly overrated.
Generally, you're right, but you run into the occasional situation where the hazard is substantial because of unique site conditions that may combine the right prevailing conditions (e.g. wind/air flow patterns) and the use of the room served by the fan.
Kind of like shear walls are vastly overdesigned ... 'cept for that one instance of seismic activity that pushes it to the limit. Often many 'hazards' are vastly overrated, but codes seek consistency so that when/if the occasional condition does occur, the system has been designed and constructed to reasonably accomodate it. We often get away with conditions that are 'less than ideal' or don't meet code, but it doesn't make it a good idea to do it that way IMO.
One poster (not sure if it was this thread) recently talked about a vent in a soffit and finding years later that the exhausted air routinely got sucked up into the attic vent and was 'working on' the sheathing (black w/ mold).