bathroom fan ducting?

[Henry W]

We have two issues with our bathroom exhaust fan - one is that it is noisy and very slow to start working once switched on (sometimes several minutes before the fan starts moving). I presume that will be taken care of with a new fan.

The second issue is the heart of my question: What is the proper ducting for a BR fan?

Our current one is ducted from the BR ceiling, into the attic and up through the roof (white flex duct). In cold weather, we get enough condensation from humid shower air that water drips from the ceiling fixture. Not great, or safe! So we don't use it during cold weather.

How does one avoid this?
My brother in law in Ontario Canada says that there in new houses they run insulated duct horizontally with a soffit exit - final air movement is down. Is that the right approach here in NC? Anyone know what the codes are? Would an insulated duct into attic and roof exhaust be enough to prevent serious condensation?

Other ideas?

Thanks,
Henry

 

[redknife]

I'll get the conversation started. I researched the code and best practice recommendations for several projects, most recently our master bath remodel.
Duct insulation for bathroom fan ducts is not required by code in unconditioned space but can be beneficial in reducing condensation. If the run is entirely vertical, the condensation run back will likely still occur, just precipitate closer to the cold/warm interface. Horizontal runs can reduce the run of condensate back to the fan. You can orient the run so there is a short horizontal section realizing that the more turns and length, the higher the static pressure.
You might consider if there are other ways to vent to the outside but this is often impractical. In order to keep static pressure as low as possible, the run should be as straight and short as possible. Flex duct has a higher static pressure than smooth duct. Per code, the duct must vent to the outside and must not be vented into a soffit, walkway, or crawl space.

Regarding fan noise, the fans available on the shelf at the big boxes may still be fairly loud so don't assume that a replacement will be better or ultimately acceptable. There are fans available online that have a better sone specification (quieter). For some reason, at the stores near me they only seem to sell noise ones.
Do you have access to the fan from above? If so it is easier to replace the fan with your choice of fans keeping in mind that smaller size may require drywall patching. If you don't have access from above, it can be more difficult to switch models and brands. You can often find a replacement fan motor for your existing fan and that is the easiest and least expensive option. Although the replacement fans sometimes are labeled as quieter, they are often still quite noisy. We have a bathroom in our basement that we did a fan motor switch out- it was quieter but still quite loud. We then pulled drywall and revised the fan to a lower sone new unit and then repaired the drywall.
Another option to consider, assuming you have access above would be an inline fan where the fan motor is placed in the attic. For instance, Fantech makes kits. We switched our upper floor to this model of exhaust fan and they are essentially inaudible. We also added timer switches because it was really easy to accidentally leave them on indefinitely.
Finally, a note on sizing. The sizing calculators you will find online often won't account for the static pressure in your ducting or the makeup air availability. Consider your duct configuration when you size your fan. I generally go bigger. The upside of bigger is function. The downside is that the more air you evacuate from your conditioned space, the more you will suck in unconditioned air from elsewhere.
HTH

 

[Roy G]

We thought we would shorten the path for the exhaust air in the attic and made the duct almost vertical to the outlet vent. Big mistake. A lot of condensation. We went back to having some horizontal pipe before it went up to the vent and it was much better. Now we have the pipe insulated and we don't have a problem.

Roy G

 

[ehpoole]

Another option if you are seeking a quieter fan is to install an inline fan rather than a ceiling mounted fan. The inline fan is installed inline with the ductwork (within the attic space) so there can be several feet of duct between the actual fan and your bathroom ceiling, which can greatly reduce the sound level as well.

 

[Lowlander]

Running the duct horizontal is and then terminate it through the soffit is best. I always rip out the cheap plastic duct and replace it with a quality 3 or 4 inch air duct depending on the outlet of the fan. A nice feature on a bath fan is a auto on/off depending on the moisture level in the room.

 

[CrealBilly]

Personally speaking... opening up the bathroom window while showering helps a lot. My house has both bathroom fan and window. We rarely use the fan since we open the window while showering and yes even in the dead of winter.

Remember if you ain't numb - your not having fun. :lol

Never do anything during the day that will keep you awake at night

 

[redknife]

Since you asked about code and it has come up in discussion:
Operational windows may be substituted for a mechanical fan (minimum 50cfm)

regarding soffits:
M1501.1 Air shall not be exhausted into an attic, soffit, ridge vent or crawl space

I believe the exclusion of soffit discharge was new in 2012.

 

[gator]

Can't tell you anything about duct runs but for the noise and slow start - have you pulled the cover and check the fan unit? Mine started making noise and running slow (not stopping but just running slow) and when I pulled the cover, the fan blades and shaft was covered in lint. Pulled all the lint out and vacuumed the area and performance went back to normal.

George

 

[Rwe2156]

In an addition we had built the vent is in the wall just below soffit.

 

[AlexSwansboro]

I just did a master bathroom addition and had my final inspection last week. I bought a light, exhaust fan, heater combination from Broan. It isn't too loud and it is nice to have the extra heat in the winter. I ducted it to the soffit and it and it passed inspection just fine. Doing so allowed me to have most of my ducting installed horizontally. The big thing to me is that you install the duct work the way the installtion instructions say to do so. Then, keep the box and the installation instructions for the inspector. If the manufacturer says you can do it, then in my experience, the inspector has no leg to stand on. .

 

[junquecol]

When you vent thru soffits, warm air rises thru vents, travels up the bottom of roof before exiting attic area. Every roofing nail will be a drip point. Soon, plywood is rotten and needs to be replaced. Much better to go thru roof.

 

[JimD]

Anybody try using a plumbing vent stack? Could pull water out of traps, I guess, but I don't think the airflow would be great enough to do that. Advantages are it isn't a new opening, really goes to the outside, and any condensation just goes into the plumbing. I would attach horizontally well above drain flow. If you hate that idea (not sure I like it) what about a fake vent pipe? Still pretty easy to install but some work on the roof is required.

 

[redknife]

Anybody try using a plumbing vent stack? Could pull water out of traps, I guess, but I don't think the airflow would be great enough to do that. Advantages are it isn't a new opening, really goes to the outside, and any condensation just goes into the plumbing. I would attach horizontally well above drain flow. If you hate that idea (not sure I like it) what about a fake vent pipe? Still pretty easy to install but some work on the roof is required.

I think the main issue besides code would be backdraft of sewer gas.

 

[JimD]

Maybe if you use the sewer vent. If you use a separate vent that wouldn't be an issue.

 

[ehpoole]

Maybe if you use the sewer vent. If you use a separate vent that wouldn't be an issue.

There is nothing like setting off a small explosion at the start of your day to get the blood flowing (which is a very real possibility if any sparks are present within the vicinity/room). Sewer gasses are both explosive and hazardous (as well as unpleasant) and the job of the vent stack is two fold, to vent those sewer gasses safely outdoors and to relieve the positive and negative pressurization that can occur as a large volume of water is suddenly flushed through the drain pipes so that the water drains quickly and consistently without allowing solids an opportunity to settle out.

The vent stack exists for one critical reason, it should never be repurposed for other purposes. You are by no means the first person to suggest repurposing the vent stack as many seem to jump to that as their proposed solution for piercing their roof rather than doing so properly, but it is good that you took the time to ask first as it is not something that should ever be done (and such can come back to bite you if you later decide to sell the property).

While you could certainly do a fake vent pipe there would be the obvious problem of what one does with the rainwater that enters as it would all run through the ducting, past the fan (an electrical appliance) and into a puddle on your bathroom floor. Fortunately, to do so is unnecessary as there are already roof mount bathroom exhaust vents readily available that are designed specifically for the task of venting moist exhaust air and protect against rainwater entering them (and puddling on your bathroom floor) by design.