Installing a bathroom exhaust fan and the proper way to vent it...

[trader4]

"I seem to recall, either in Fine Homebuilding or a CMHC publication that under certain circumstances in cold climates venting up through the roof can lead to thermosyphoning hot air continuously when the fan is off. "

Veeery interesting! I had never thought about that. I suspected a similar and worse effect occurs when people run a bath vent to the underside of a roof mounted fan. When the roof fan runs in summer, it will suck cool airconditioned air from the bathroom, even if the bathroom fan is off. I would think as you pointed out, that just having a straight up run from the bathroom to a regular roof vent would provide and ideal path for convection.

[BP]

Only you are talking about venting into the attic. No one else. Try to keep up.

[BP]

You keep talking about "the code". Please tell us where you live (not the street address) so we can look up your states building code on line. If there is a restriction from venting a bath fan into the outside air through a dedicated hole in the soffit I would really like to see it.

[BP]

I would want to see "the code" that says this. I think a lot of people are confusing the hack job of pointing the end of the ducting into a vented soffit, with the proper installation which is to run the duct out *through* the soffit terminating with a grill or louvered vent cap.

[trader4]

" I would want to see "the code" that says this. I think a lot of people are confusing the hack job of pointing the end of the ducting into a vented soffit, with the proper installation which is to run the duct out *through* the soffit terminating with a grill or louvered vent cap. "

I agree!

[jdk]

i have a 1962 built ranch in cols ohio. the above is what i did when i replaced a bath fan. put the metal pipe about 1/2" or more past the original wood soffit. it has vinyl soffit over that. i then used the vent soffit over the pipe. next vent opening 2 to 3 feet away. my only question is when i did this the old fan had no vented piping at all. just emptied into the attic. there was no damage over the last 30 plus years. so if the attic has good ventilation you may not have to worry which way you go depending on you climate?

[BocesLib]

I used 6" flex duct myself for a fan that I recently put it. (I used the insulated stuff too)

I vented it to the soffit and put on a auto opening grill to vent the bathroom air to the outside.

Nothing like having a 200 cfm van venting the bathroom. No more smells, no fogging the mirrors nothing. Just sucks the hell out of the heat in the house :)

Didnt want to go with a through room install. My feelings are the less holes in my roof, the better off I am.

Tom

[Chris Lewis]

According to :

As I mention, that's merely wasteful... It gets vastly worse if you have condensation dripping into the room.

I don't know whether putting a "trap" (S curve) into the vent (as others have mentioned) will be sufficient to break the thermosyphon. I would suspect not in most cases, but, at least it should prevent condensation draining _through_ the fan... But, the "trap" might get rather yucky after a few years.

Yeah, running the bathroom vent to the underside of a roof mounted fan would be bad even in the summer too if you had AC.

[yellowbirddog]

This may be a little OT, but if a person was dead set for some reason on venting to the attic OR they came across such a set-up, you can simply change your habits to minimize risk of attic condensation.

For instance, don't run your fan when it gets chilly enough outside to see your breath (or thereabouts). In cold weather, heated interiors experience a drop in humidity and can absorb lots of moisture without the humidity getting high. It may be a good idea to run your forced-air furnace and keep the bathroom door cracked when you take a shower to keep an influx of low-humidity air into the bathroom. In the winter, I can take a shower without running the vent fan and the mirrors don't even begin to "steam up".

[TP]

Yes, I did the same thing. Insulated flex duct out to the soffit. Very nice setup.

TP

[jayhawkjtp]

MOLD! Homes that have poor venting that just vent into the attic eventually grow MOLD. MOLD is unhealthy! People don't buy moldy houses!

I've had my best friend walk away from 3 houses because the inspections found mold in the attic due to poor ventilation. 1 house was bulldozed after inspections found it was uninhabitable.

ALWAYS vent to the outside air. It may cost a bit, but well worth it if your house is your nest egg. Nothing worse than seeing your nest egg bulldozed because of toxic mold.

[jayhawkjtp]

MOLD! PEOPLE PLEASE DO YOUR RESEARCH! New Jersey has some of the worse mold cases/stories because old contractors and do-it-yourselfers vented into the attic and not to the outdoors! Home inspections now often include mold inspections when the house is older; my friend has walked away from 3 homes in New Jersey because all had toxic levels of mold. 1 house was bulldozed recently because it was unihabitable due to mold. Nice home, wasn't trashy BUT THE OWNER VENTED THE BATH INTO THE ATTIC FOR 15 YEARS!

[gary]

Good the picture came through.

[Dennis]

Gary, I just can't agree. There should be no water or snow infiltration in a properly designed termination. (It has a damper in the exterior cap to prevent just that.)

By adding unnecessary twists and turns, you are adding some back pressure for the blower to overcome, which would degrade its performance (to whatever extent would depend on the specific system.) Also adding a trap could possibly cause some moisture in the air stream to condense prior to being evacuated (this would especially true if your roof cap has no damper and allows rail to enter). This would be undesirable as standing water could deteriorate the vinyl ducting over time. (I prefer metal ducting as the high heat in an attic can cause plastics to deteriorate over time).

In any case, a duct that runs as straight as possible is the most desirable way to vent, and millions of homes exist with roof terminations where this presents no problems at all.

[Dennis]

Unlikely. You would actually have to chance the laws of thermodynamics to make a difference. One area you might not be considering is that it's not only against code, it's also more expensive to heat the home. Bath (and kitchen) fans carry a tremendous amount of moisture. Dump this into the attic in cool/cold weather will cause this moisture to settle into the insulation; degrading the performance and moving the dew point closer to the heated side. It's not out-of-the question to consider a R-value loss of 50% or more (one test showed much more) on cold days.

[CBHVAC]

The OP isnt showing, so....

The proper way to run a bathroom vent is simple, and to do it by code is simpler yet..

If the run is over 14 Feet, it needs to be metal snap lock pipe. Period. If water is getting into a vent pipe that feeds to the roof, the wrong cover is on it. If condensation is an issue, then you have the pipe obviously not ran in the best location and it should be relocated if possible, and again, if over 14 feet, metal ducting used, and if needed, insulate with proper duct wrap. Condensation, can also occur when you are using a fan with too low of a CFM for the space, or, trying, as others have said to make it harder than it needs to be, and installing a bunch of so called traps (that are not to code here anyway) and raising the static pressure of a tiny POS fan..unless you are using Panasonics.

[yellowbirddog]

You entirely missed what I wrote. The whole idea is NOT to dump moisture into the attic by NOT running the fan during cool outside conditions.

[gary]

Hi Dennis, did you see the picture I posted. That is how all the new buildings have their bathroom fans installed now.

[Goedjn]

It occurs to me that this isn't necessarily true of half-baths, though.

[Dennis]

Your right, I didn't get that from what you originally wrote. But that defeats the whole purpose of ventilation, doesn't it? The purpose it to ensure use year round. But I have to admit, you are correct. If you don't use the fan, not much moisture will get trapped in the attic.