Bath vent fan installation, troubleshooting, repair FAQs:

Questions & Answers on how to design, size, install, wire, or fix bathroom vent fans, ducts, switches & controls.

This article series describes how to install bathroom ventilation systems, fans, ducts, terminations. We include bathroom venting code citations and the text also explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details.

Bathroom Ventilation Code, Fans & Ducting Questions & Answers

Questions & answers about bathroom ventilation codes, ducting, termination, posted originally at BATHROOM VENTILATION CODES SPECS - be sure to review that article.

FAQs on How to install, specify or improve bathroom venting, reduce indoor condensation, avoid bathroom mold. Bathroom vent fans, required bath vent fan capacity, fan noise and sones.

Bathroom vent fan ducts, where to route vent air, duct condensation, ceiling leaks; Photographs of bad or ineffective bath fan installations.

On 2018-08-07 by (mod) - what is the largest bath vent fan(cfm’s) that can accommodate a 3” diameter fkexduct?

I'm not sure I can find an authoritative fan capacity survey skystew, but here are some reference points:

Keep in mind that CFM readings are general and the actual CFM or cubic feet per minute of bath exhaust capacity you will realize depends on additional factors including duct length, elbows, material, termination, and availability of makeup air, fan inlet louver design and cleanliness, and to a small degree, air temperature and humidity or moisture level, and fan location in the bahtroom wall or ceiling.

High capacity 3" bath exaust fans:

Aeropure Slim Fit 120CFM

Broan NuTone 180 CFM Ceiling Vertical Discharge Exhaust Fan (also wall mounted)
Broan AE110L 110 CFM Invent Energy Star Qualified Single-Speed Ventilation Fan with LED

Houzz 120CFM

Panasonic FV-08-11VF5 110 CFM 4" with 3" duct adapter provided

If you go up to 4" fan duct there is the

AXC Bathroom Kit 4", Adjustable Grille, 147 CFM

Still higher cfm bath exhaust fans will pretty much all require a 6-inch exhaust duct, such as the following:

Air King AK200LS Energy Star Deluxe 200 cfm

Broan Nutone Bath Ventilation Fan, Model # L150 157 CFM

Panasonic: WhisperCeiling 380 CFM bath fan

The Broan 210 CFM High-Capacity Ventilation Fan uses an 8" duct.

On 2018-07-29 y skystew

what is the largest bath vent fan(cfm’s) that can accommodate a 3” diameter fkexduct?

On 2016-10-03 by Thelma Lee

Hi,

What is the clearance recommendation for a ceiling fan above a toilet and the top of an overjohn cabinet I want to install?

On 2016-09-03 by (mod) - there are higher-velocity vent fans and even vent boosters (not permitted in all situations).

Gary, indeed there are higher-velocity vent fans and even vent boosters (not permitted in all situations). If you get enough draft and insulate the lines (presuming moving through a sometimes cold space) you may overcome the leak problems.

Normally at the bath fan exit there is a louvered or automatically-closed termination cover to keep pests out of the vent ducting - back drafting can otherwise be a problem.

On 2016-09-02 by Gary

Thanks, Dan. I have measured since the original post. Lengths starting at the fan are: 2' horizontal, elbow, 29", elbow, 14' straight and out. My issue is that I have to go up before going back down to get the "condensation slope".

In this case the 29" "up" part seems long enough to encourage condensation having to go another 16' + elbow. Leak seems to happen at the bottom (first) elbow. I'm tempted to go straight up through the roof with a really good added back draft damper.

If I go this route, my question for you would be should I have a back draft damper at the fan and the roof level?


Thanks, Gary

On 2016-09-02 by (mod) -

Take a look at the distance and number of elbows that the bath vent fan manufacturer says will work. Your run sounds a bit long - depending on the fan model and capacity.

Insulating can help reduce condensation in the vent line, but still there will be a lot of moisture in that bath air when a shower is in use.

On 2016-09-02 by Gary

How high can I go up from my ceiling mounted bath fan in order to achieve downward slope to my attic sidewall vent? The sidewall has an adjoining sloped roof 2 feet below and the available vent area rises toward the ridge. Right now I need to rise 2+ feet to get a 1/4" or less slope.

I'm exiting as low as possible in this layout. Present layout: horizontal from ceiling mounted fan 2 feet, up 2 feet, slight downward slope to sidewall 16 feet. I get condensation and leakage onto the ceiling at the elbows near the fan. I do have insulation around the pipes. Thanks, Gary.

On 2016-08-23 by (mod) - blowing vent air DOWN and then across any long horizontal run is likely to work rather poorly

You can but blowing vent air DOWN and then across any long horizontal run is likely to work rather poorly unless you use solid metal ductwork, properly sloped, insulated, and perhaps with a booster fan.

On 2016-08-22 by Charlie

I have very limited attic space. Can bathroom ventilation be run down through an interior bathroom wall and out through the crawl space?

On 2016-07-25 by Gary Hanson

Thanks for the reply. A bit of a catch tray would keep the condensation off the edge of the drywall up there. I'll start with that. We had discussed an openable skylight and may do it in the future. Thanks again.
Gary

On 2016-07-25 by (mod) -

As my most-admired home inspector in Tampa says - "... it depends" - Mark Cramer. It depends on whether or not you live in a climate where you could install an operable skylight to open and vent out moist air; I'm assuming you want to vent the skylight not the roof around the skylight.

Some skylight models include a small tray at the down-slope end of the glass. The design idea is that condensation runs down into the tray, avoiding damaging the skylight well, and then it evaporates over time between those hot steamy showers.

If you have an effective exhaust fan in the bathroom it will reduce condensation on all surfaces, including the skylight, even if it's not in the highest point of the ceiling; it won't of course entirely eliminate condensation on the skylight glass.

I don't quite understand your active venting plan - adding a powered vent that includes an intake in the skylight well; that is probably do-able but probably isn't necessary.

On 2016-07-25 by Gary Hanson

Sorry- let me complete the question. Right now I have a fan in the skylight sidewall that has 4"steel pipe that is sloped incorrectly and delivering condensation to the wall surface. My plan is to fix the slope, use sealed PVC at the new fan installed in the ceiling and remove the sidewall fan.

I still want to vent the skylight area, but don't have room for a fan up high in the skylight. I still will get condensation on the skylight and likely mold.

Can I add a small vent (only) and backdraft preventer tied via a tee to the downstream airflow from the new fan and get a little Venturi effect venting up there without going to a remote mount multi-port fan? Any help would be appreciated . Thanks,
Gary

What is the best way to vent the skylight area above the tub shower in a small bath? Ri

On 2016-01-03 by (mod) - can I vent 2 bathroom fans (bathrooms are adjacent to each other) through a single roof vent?

Sure but ....

1. you'll need to include a check valve in the duct system so that fan A does not simply blow out through the fan B inlet when B is not running, and vice-versa

2. the duct size will need to be adequate

On 2016-01-02 2 by REA

Can I vent 2 bathroom fans (bathrooms are adjacent to each other) through a single roof vent?

On 2018-04-24 by (mod) - How far away does a bath fan exhaust have to be from the furthest open point on a window.

Jordan

Please see

BATHROOM VENT CLEARANCES https://inspectapedia.com/ventilation/Bathroom_Vent_Termination.php#BVClearance

On 2018-04-19 by Jordan

How far away does a bath fan exhaust have to be from the furthest open point on a window.

On 2018-03-05 by jw

I am developing a bed and breakfast. Those that I have visited often have a whirlpool tub in the bedroom for romantic reasons. When I mentioned this to my contractor, he told me that I would need some whole house (expensive) ventilation if the tub was not located in the bathroom. Is this correct?

On 2017-11-30 by (mod) - use solid metal ducting

If you can use solid metal ducting that will certainly have less Flow Restriction than the common expanded wire and plastic flex duct.

On 2017-11-30 by Anonymous

Okay, that makes sense. It turns out that replacing the 3" pipe isn't going to be an option, so I'm going to just minimize friction/resistance everywhere I can get access. I also discovered that the vinyl flex connecting the fan to the 3" duct

is 3' long and does a complete 360 degree turn because the end of the steel duct is almost directly above the fan housing and only has about 6" of clearance.

Ugh. The only good part about that is there are no other turns - its a straight vertical shot to the attic. Looks like I'll be going with an inline fan mounted in the attic - just have to figure out how to run the wiring.

On 2017-11-30 by (mod) - benefit to having bigger duct

Interesting question Ted.

Based on information that we have posted in our article on increasing water flow rate in a building and borrowing from engineering work reported by Carson Dunlop Associates, it's correct that increasing the diameter of the vent pipe anywhere that you can in the system will improve the overall flow rate for the entire system.

I'm not sure how we would calculate the point of diminishing or diminished return, but most likely it'll be just based on cost and Trouble.

The reasoning we use on water pipes is that you replace the accessible areas for example in a basement or crawlspace and you don't replace piping where it's difficult and expensive to access such as running through a wall cavity.

On 2017-11-30 0 by Ted T

Hi all, I posted this question a minute ago in the building ventilation section but I think this is the correct page.

Question #1: If most of a bathroom exhaust duct run is 3" diameter steel pipe, is there any benefit to having bigger duct (4"-6" round galvanized steel) at either end, and/or a bigger roof cap? I'm guessing that there is a benefit to the average values of the run as a whole, but what would be the point of diminishing returns?

Question #2: How does one calculate air volume, flow, etc., when a run consists of differing diameter duct (i.e. 2' of 4" duct to 18' of 3" duct to 5' of 4" duct? I'm guessing you calculate the values for each section, then average for total length. For EDL, figure for each section then total?

Question #3: Aside from increased noise, what are the major drawbacks to reducing a 4" fan exhaust to a 3" duct? Or a 6" fan exhaust to a 3" duct?

I'm installing a bathroom exhaust fan in my master bathroom and retrofitting two other fans. For the master and hallway bath, both on 2nd flr, I'm installing 6" galvanized round duct and roof jacks. No issues there.

The problem is the guest 3/4 bathroom on the ground floor. I haven't traced out the duct run yet, as access is very limited, but based on house dimensions, room locations and what I can see through the ceiling opening and in the attic,

it looks like it has a short length (maybe 2') of 4" vinyl flex attached to 3" steel pipe (real pipe, not thin-walled duct). I estimate the pipe runs about 8' horizontal, then about 10' vertical into the attic, with at least one, probably two, 90 degree elbows, or one 90 and two 45s. In the attic, 4" vinyl flex is attached to the pipe end and runs vertical around 5' to a 4" roof jack.

My first choice would be to replace the current stuff with 4" galvanized duct directly to the nearest exterior wall (~14' straight run) and exhaust through a wall cap, but I think this may not be feasible without tearing out major stretches of walls and ceiling.

If I'm stuck with the 3" pipe, I'm exploring what I can do to make the best of it.
The bathroom is small (47 sqft) with no exterior wall.

The current fan (Utilitech model #7108-03-L) is 110 cfm and airflow is strong at the roof jack. House framing is steel, and I live in Hawaii so there are no insulation concerns. The fan I'm looking to install is a Panasonic WhisperGreen Select FV-05-11VKSL1, which will run constant at 30-50CFM and kick up to 110CFM with a wall switch.

I would actually prefer the 150CFM model due to the length of duct run but I think that may not work well with 3" pipe. The shower in this bathroom doesn't get much use but my cat's litter box lives in there and the toilet gets used a lot, so fan is mostly for odor-control.

The bathroom opens on to a juncture of guest room, dining room and kitchen, so bathroom smells are very unwelcome.

Besides answers to my questions at top of post, any ideas or recommendations would be very welcome. Thanks much!

On 2017-11-04 by Chris

Can I connect a bathroom fan to an air exchanger exhaust pipe

On 2017-11-03 by (mod) -

Jane,

If you're asking whether there are combined units that provide heat, light, and exhaust venting yes there are and I'm not aware of a code prohibition of their use

On 2017-11-02 by jane

is it code to have a bathroom vent with heating?

On 2017-10-25 by Brett H

I am wondering if there is a way to close off the vent in a 4 unit bathroom fan and do they connect them all together? I am ending up with heavy smoke in my bathroom from some inconsiderate people smoking weed or cigars.

On 2017-10-06 by Ruben Perez

is it safe to install a ceiling exhaust fan in a bathroom & release the removed air into the attic?

On 2017-09-27 by (mod) -

MIranda

Depending on the country, city, province or state where you live, local and model building codes usually require ventilation by fan unless there is an operable window.

On 2017-09-27 by Miranda

We just moved into an apt. We were told that the bathroom exhuast works but it does not.

They have yet to fix this after a whole month and they say they dont know when it will be fixed. Is there a law or code that requires the exhuast to be working? We need our exhaust!

On 2017-05-22 by (mod) -

Don

Where a bathroom vent fan is required it must have a minimum of 50 cfm vent capacity. The bath fan can be wired to be continuously in operation or intermittently used when needed, but the vent fan must be vented directly to the outdoors

- so your installation is in violation of the model building codes for bathroom ventilation. Your local building code inspector/official is the final legal authority for this question.

In the article above you'll see details of this code cited and italicized.

Thanks for asking: many contractors take the easy route. Dumping bath moisture into an attic risks future mold contamination or even roof sheathing damage.

On 2017-05-22 by don

I had a bathroom fan installed. Instead of putting in a separate vent through the roof, the contractor placed the flex pipe 2-3 " away from an attic vent. I am concerned about moisture and whether this meets local building codes.

On 2017-05-03 by (mod) - IF your bath fan does not turn off in response to the switch

IF your bath fan does not turn off in response to the switch the safest thing to do is find and turn off the right circuit breaker in your electrical panel. You'll need to call an electrician to repair either the fan or its switch and circuit. But with it off you at least don't have to face the cost of an emergency nor a fire.

On 2017-05-03 by Ann Greenstreet

When using fan tonight, it started to make a noise, light turned off ok, but site of fan still whirring away .What should I do?
A mere woman!

On 2017-03-25 by (mod) -

Kenji that sounds reasonable to me, provided any needed strapping or reinforcement of the wall are also addressed. But be sure to check with your local building inspector about what she or he will accept.

On 2017-03-25 by Kenji

Hi, I am planning a ceiling bathroom vent ducted to a concrete wall penetration. Because I need to connect down through the 2x4 wall's top plate, I was curious if I could use a periscope vent for the vertical run between the bathroom fan vent and the wall vent. I haven't been able to find much information either way for if this is allowable. thanks,
|Kenji

On 2017-01-24 by kirk

I have two soffit vents for my bathrooms. They exhaust well but allow cold air too flow back into the house. Any ideas on where to get a good vent?

On 2016-11-14 by Randy

when i turn on my bathroom fan-- i get a foul odor. like "poo" i am thinking that since i increased the cfm's sgnificantly that i have caused a backdraft or something similar, and it is pulling air from the stink pipe.?

my old fan did not move enough air/moisture and i had condensation and mildew on the ceiling in bath room. new fan literally creates wind. shower curtain moves.

On 2016-10-29 by (mod) -

Choose a path light fan combination proved for us in that location and make sure that it is properly wired with ground fault circuit interrupter or GFCI protection.

On 2016-10-28 by Jack

Is it ok to replace an existing can light above a shower with an exhaust fan/light?

On 2016-10-22 by (mod) -

I'd make an acceptable looking access cover. It is both bad practice and a code violation to bury an electrical junction box.

On 2016-10-21 by vvicci@aol.com

thanks for the info but i thought about that and having a separate light and fan. I dont like the access cover in the ceiling for looks... Does anyone know if a ceiling fan/light product exists that has a large wiring box?

...

Continue reading at BATHROOM VENTILATION - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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