View Full Version : Bath fan exhaust venting

11-16-2004, 07:41 AM
I would like verification that the following can be done -
I have a master bathroom with separate toilet / shower / open areas. The toilet area has a bath fan that vents to the side of my house (stucco). The run is @15'-20'. The tiled shower area only has a light - no exhaust venting, therefore we get frequent mildew buildup in the shower. (we do have a ceiling fan in the master bath, but that does not help much).

I'd like to install a bath light/fan kit in the shower, replacing the light - but I'd prefer not to cut another vent into my siding. I was hoping that I could use some kind of 'T' into the existing toilet fan ducting that is currently vented to the side of my house. This way, each could operate independently and I would not have to cut an additional vent into my siding.

Can this be done with no concerns? Do I need to be concerned about duct sizes (the shower fan would likely need to be sized much bigger than the toilet fan). What about backdrafting? Any special parts I may need?


11-16-2004, 07:51 AM
Use a wye instead of a tee to reduce backdraft. Also, most fans have an integral backdraft damper. If you have a 4" duct on the toilet fan, that is adquate for even a very large CFM bath fan.

Fans and lights directly over a tub or shower are required to be UL listed for that specific application. This may limit your choices.

In any event, such a fan must be connected to a GFCI protected circuit.

11-16-2004, 11:25 AM
If you have mildew then it sounds like you need a larger fan to move more CFM or let it run longer. I have not seen a ceiling fan in a bathroom. You must have a very large room.

11-16-2004, 11:42 AM
It is a large master bathroom, the only venting to outside is in the 'enclosed' toilet area (bath fan and window). This is the reason I need to vent the shower area.

I just found that the existing ducting (from the toilet room to outside) is 3". The intended bath fan I'd like to install in the shower requires 4" ducting. Does this mean that I am out of luck in trying to tie the 2 bath fans into the same outside vent? I assume the 4" ducting above the shower would lose efficiency if I tap into the existing 3" ducting.
If so, my choices appear to be:
1) Run a separate 4" duct for the shower and cut a new vent in my siding for it
2) Upgrade my toilet fan to a 4" duct, enlarge the existing vent, and tie the 2 together.
3) Let the 4" duct share the 3" duct/vent and hope efficiency is OK


11-16-2004, 12:01 PM
I'd consider a remote fan that connects ducts to both places. That way it is quiet, and efficient. You can probably replace the existing 3" vent cap with a 4" or larger without messing up the siding, but I've not done it (I'm not a pro).

Does this have an attic above, or does it have living space above? I put in a Solatube ceiling fan/light/circular skylight in my bathroom. Very quiet, and free light. Check out their website as an option. My guess is that you could use this in the shower as well, and when the sun is up, you wouldn't need the light.

11-16-2004, 12:18 PM
There is an attic above the master bath.

Thanks for the suggestion, but I'm looking for a quick and easy fix rather than the Solatube solution.

11-16-2004, 12:53 PM
I suspect you could up the ductwork to 4" and possibly keep the same outlet fitting at the exterior wall.

11-16-2004, 12:58 PM
Thanks to all for your responses.

11-16-2004, 01:11 PM
I have coupled two together with no problem and I have coupled two together with a problem.

Same fan and duct size with minimal duct runs and it seems to work fine.

Different fan size and longer / different ducts you will have a problem. When both are in use at the same time, the stronger fan can reduce the output of the weaker fan to almost no discharge.

Try and keep it as balanced as you can.

11-18-2004, 03:29 PM
since we are on this subject of bathroom fans, i am replacing an exhust fan and not sure how to take out the old? it's in a finished ceiling, so i don't know if it is nailed in or what. i took out all visible screws and something is holding it in. any tips?

11-18-2004, 04:38 PM
Some of them use hangers - sliding bars that go between the studs. Depending on how concentious the installer was, it could be a pain getting it out. Most of the time, they are just tacked in. If it is up against a joist, look for a nail or a screw through the side into the joist. WIthout access, how do you plan to get the new one in and stable? There often isn't enough play in the exhaust vent to do the wiring and make an airtight connection, then put it in place.

11-18-2004, 08:57 PM
Not sure what you mean. so, if it is on a hanger then what is the best way to get it out? can i just pull it down by force? also, i can see the wires and it appears that i can get my hands it there and wire it together. is a possible change?

11-19-2004, 04:47 AM
IF you pull it by force you will pull down a portion of the ceiling also. Most fans are larger than the opening in the ceiling so you will probably have to cut a section out to remove the old and install the new one.

11-19-2004, 05:00 AM
As that piece of foolhardy advice suggests, yup, your a plumber

11-19-2004, 06:40 AM
You indicated in an earlier post that there was an attic above. Time to crawl up there. Once there, it should be obvious how it is attached. You will be able to then make a much better connection to the vent pipe and do the electical connections as well. Probably need to be up there to get it attached properly, too. You don't want your connections on the vent pipe to be leaking. You're exhausting moist air - it will condense and potentially make a mess up there. Depending on the length of the run, it will probably get some condensation. It is much better to use the metal ductwork for this rather than the corregated stuff, unless run vertically through the roof. That way, you can pitch the run to the outside so any condensation will run outside rather than drip back into the room. Seal the joints, too. Some of the cheaper fans do not have a damper in them to block cold air from coming back into the room. This may be a useful thing to add, if necessary. Much easier in a solid vent. Most vent caps have one, but not all (they don't seal that well usually, and I personally like to have one closer to the unit. If it gets really cold there, insulate the pipe, too. Helps keep the moisture as a gas rather than condensing.

11-21-2004, 01:32 PM
While we are on the subject, might I also ask a bathroom fan/exhaust question. I don't have a fan in the bathroom of my small old (70s) nothing fancy home. The problem is mildew or mold, whatever that black stuff is the grows on the ceiling. A couple of years ago, a very kind painter painted with KILZ underneath, which stopped it for quite a while.
As other people above, my fear about putting in a fan is that, although this is just a very inexpensive house (basically a tract home), it is very very tight against the very very cold NH winters. And I don't want to interfere with that. I guess a fan could be put in to exhaust either directly out (through the wall), or up into the unfinished attic, or maybe even down into the basement/garage. Any advice or thoughts would be very useful in terms of not having this turn into a fiasco. Thanks! R

11-21-2004, 05:09 PM
It's not recommended to vent into the attic, although it is done alot. You are exhausting moist air - anytime except the summer, it will condense in the attic, wet the insulation, and just move the problem.

There is usually enough leakage in a house so you won't run into a problem running an exhaust fan. IF the house is truely very tight, it could be a problem (but I doubt it). If you wanted to do it high-tech, you'd put in a heat recovery system - this would extract heat from the outgoing air, and use it to warm the incoming air. Neat, but not usually necessary unless you have people showering all day.

At least one company makes a fan control with a humidistat. It runs the fan only until the humidity drops to the specified level - this way it doesn't run forever and waste your heat and/or a/c. BTW, the heat recovery system also moderates the incoming air in the summer - it cools it as the exhaust goes out.

11-22-2004, 08:14 AM
Ok, thanks so much for this information. I don't like high tech stuff, just more things to give problems later. Is it necessary to put in an exhaust fan? If so, if I understand correctly I might be able to run it out the wall of the house? What about the idea of exhausting it to the basement below, as this wouldn't require that I have a hole cut in my tight NE house. Also, if you have any recommendations of equipment that will not give problems for ages, perhaps you could let me know. Thanks again< R

11-22-2004, 05:40 PM
This is an opinion, not through experience in the trade. It just doesn't make sense to me to exhaust it in a different part of the house. Now, in the winter, it may not be a horrible thing, but there is more than water there - the perfumes from shampoo, shaving cream, soaps, etc. now are wafting around more of the house than before. Dumping it into the basement thatmay not be as warm as the rest of the house could cause the moisture to condense on the cold walls of a typical basement. YOurs may be finished and heated, so thatmay not happen. Many times, the vent is passed out through the nearest wall, so that is usually not a problem. I needed to vent a 2nd story bathroom - couldn't go out the sidewall, as it is a Mansard, and didn't want it on that part of the roof, so went up to the upper roof. I used the Solatube 10" circular skylight with light kit and fan kit. The motor is at the roof, so it is very quiet. Provides light even at night if the moon is up.


11-23-2004, 11:08 AM
Ok, appreciate your opinion and ideas. I guess my 2 best choices would be either to go out the side wall, or to go up through the attic out the roof. Either way I have to put a hole in my house, but it seems that out the side wall might be the simplest. Thanks again, R

Johnny K
11-27-2004, 11:29 AM
Monkeyboy.. please keep us posted on what you end up doing.. I'm in the same boat..

We have large master bath with a high vauletd ceiling.. in the bath room there's a seperate toilet room that has a window and an exhaust fan mainly for light and for the riddance of any foul odors :o

Well the vaulted ceiling collects the steam and moisture from the shower (wife takes really long ones.. not sure what's going in there, but that's another story..) We've started to develop small black spots (mold) on the textured ceiling..

So I bought a fan for the main bath area (130 cfm), tapped into electric for the light above the shower. I also ran the flexable ducting down to tie into the venting used for the one in the toilet room... I couldn'y find any Y's made for such an application, so I got a 3" (i.d.) pvc connector that had a straight thru piece and a curved connector coming in.. used large screw clamps to make sure it all good and tight..

Problem now is with the new bathroom fan on, and even though the toilet room fan has the flap in there to stop air from coming in, there's enough gaps in the flap for air to be pushed into the smaller fan (path of least resistance).. the run for the new fan is about 15 feet of ducting before it hits the pvc connector and about a 2 foot run from the pvc connector to the toilet room fan and 2 foot to the outside vent..

If I can't solve it now, I'm tempted to vent the toilet room fan to the attic and use the exterior vent for the new bathroom high volume one that will be used during and after showers.. when the toilet room fan is on, the door is usually shut and little or no moisure is present (again it for odors) and if a shower is taking place, the main fan will be on.

Anyone have any feedback or suggestions... if I vent the one for the toilet room to the attic, it makes sense to have the venting pour into an open area and higher up in the roof space (no directly into any insulation).. the attic space is huge (could dry wall it and live up there if someone wanted to).. a roof vent isn't a good option as it's 2 stories up.. if we hever get any roof repairs done, I'll install a vent then and be done with it..

Thanks for any suggestions!!

Johnny K
Cleveland, OH

11-30-2004, 03:27 PM
A lot of building codes require either a bathroom fan or a window that opens, since the window that opens is cheaper a lot of builders don't bother w/ a fan. But unless you can get the person showering to open a window quite a bit in the middle of the winter it's not going to get the moist air out and then you get mold and or mildew.

Some places also allow or did allow a vent to the attic. But as has been noted you're only moving the problem to a different part of the house if you do that or vent to the basement. Go up in an attic in cold climate in the middle of winter in a house with a fan vented to the attic and you'll see icicles hanging from the nails and other places. Then you'll have mold and mildew in the attic (or basement). You've got to get the moist air out of the house. Even venting just the toilet room to the attic is probably not advisable, all the air in the living space is moist and it will condense in a cold attic, except maybe in a place that's dry and warm all year. Consider the condensation on a single pane window in a bedroom in the winter, same thing will happen with any air from the living space into the attic.

Joining two vent pipes is a tricky deal that involves balancing flows etc and that might involve installing a couple of different dampers to prevent back flow as has been noted here. As with most things doing it right is usually not the easiest way. The easiest for you might be closing off the line to the toilet and installing just a high volume exhaust in the shower area. It will draw air from the toilet room for some odor relief, assuming that they're adjoining. Or probably better just bite the bullet and put a new fan in and vent it out the wall or the roof it's really pretty easy. You could probably do it in an afternoon if the wiring is available. Working around the insulation is a hassle and you have to be able to get to the outside of where ever the holes is. But check "This Old House" (http://www.thisoldhouse.com/toh/knowhow/handbook/article/0,16417,689843-4,00.html) or this
Black and Decker (http://www.blackanddecker.com/ProjectCenter/DocumentView.aspx?DOC_ID=p_2_53_16405_16444.html)

You could install an inline fan in your existing joined ducts like this (http://www.fantech.net/bathroom.htm)
to suck the air through instead of pushing it and that would eliminate some of the balance and back draft problems. They're a little more expensive and probably no easier to install than the regular ones, but they're usually higher quality and quiet and might eliminate the need for new holes for Johnny K that has existing toilet fan.

As for weather tight you will get some backdraft but it's better than mold in my opinion and no house is air tight. The air comes in for the furnace, water heater, and out for the stove, dryer, and lots of other tiny holes.

Any way that's the opinion of a lowly mechanical engineer.

12-01-2004, 06:33 AM
A roof vent is not that hard to put in, and done correctly leaking is not a problem. I have done many of them, in my oppinion they are easier than a side wall installation. The only down side is that snow and ice can block it for a while during the winter, if you are in a cold area.

12-03-2004, 03:49 PM
To address your concerns about heat loss due to venting your fan to the outside. It is well known that heating moist air is much much harder than heating dry air. So, by venting the moist air out of the house you are making the building easier to heat.

Further, mold is nothing to take lightly. It can cause many serious health problems. By venting into an attic or basement you would be promoting wood rot, insulation damage, higher heating costs and mold growth. In short, it is a very bad idea to vent moist air into another part of the building.

12-12-2004, 07:46 AM
The exhaust fan in my upstairs bathroom makes a popping sound every 5 seconds or so when it is windy outside. I'm assuming that this is caused by changes in air pressure lifting and then dropping the backflow preventer flap. Is there an easy fix for this problem? The exhaust tube goes up, then does a 90 and exits on the side of the house. Thanks for the help!

12-14-2004, 09:35 AM
It doesn't sound like that big of a deal but a couple of possilities:
1. Put some sort of foam rubber weather strip at the seal of the damper to cushion to cushion the closing.
2. Change the outlet of the vent on the wall with one that has a spring loaded damper that might help keep it closed during the windy days
3. Put one of the outlets on that is more of a louvered damper those don't seem to slam.
4. It might be as simple as making sure the door to the bathroom is closed during windy periods to prevent the quick pressure swings between outside and the bathroom. Of course if you live in Wyoming or someplace like that everyday is windy.

The problem might be that the damper that is slamming is actually the one that is a component of the fan. It's at the outlet of the fan into the duct. That might not have an easy fix but keeping the bathroom door closed would help that.

12-14-2004, 07:12 PM
Thanks for all of the suggestions. I think the popping sounds might be coming from the damper that is built into the fan because it is a fairly loud and solid noise...the outside vent is just made up of 4 or 5 light-weight plastic flaps. I tried taking the fan apart but the ceiling is sheetrock and it's difficult to get anything done...I'd probably have to go up in the attic and see if I could get to it from there. Do you think a rubber seal on that damper would help? I'll try keeping the door shut and see if that helps. I live in ND so finding another windy day shouldn't be a problem. =)