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Can Bathroom Vent Duct Have an Elbow

Many people are familiar with roof vents, which are a common way to ventilate a bathroom. However, this can lead to confusion if you’ve never installed your own vent before—is it necessary for the ducting to go straight up? If that’s the case, how do you cut each hole, and how would you go around an obstruction?

These are great questions to consider when undertaking a new project. Thankfully there’s no need to worry about any of these things since you can use elbows! However, elbows come with their own set of factors to consider. We’ll be covering some of their drawbacks and what you should avoid when planning to install ductwork with bends.

Bathroom ducts can have elbows. Elbows hamper airflow and create water traps. Fewer and gentler elbows are best. Don’t put elbows by the fan or exit vent. Building codes reduce the allowable total duct length with each elbow added. Local codes can limit the number of elbows. Flexi duct bends count as elbows.

Bathroom Vent Ducts Allowed to Have Elbows

Bathroom vent ducts are allowed to use elbows—in fact, they’re extremely important pieces of the ductwork! If elbows were not permitted, ventilation would only be allowed to be done in a straight line, which is wildly impractical. 

Industrial air duct ventilation equipment and pipe systems installed on industrial building ceiling.

It would cause difficulties with deciding where to put a bathroom vent so that the duct can be strictly vertical. For some houses, this task may not only be difficult but perhaps even impossible—either way, it could lead to many homeowners neglecting to vent their bathroom at all in order to avoid the frustration.

The lack of ventilation itself would be another massive problem as there are many problems associated with improperly ventilated bathrooms.

In addition to more commonly known issues like mildew or misty mirrors, bathrooms with no exit for moisture can cause damage to the integrity of the house itself.

Potential Issues With Elbows

While elbows are useful and often necessary, they do come with a few potential drawbacks that you’ll want to keep in mind.

As you travel along a length of straight pipe, the pressure decreases, and this causes the speed and velocity of airflow to reduce. This decrease normally happens at a slow and gradual pace, but the pressure drop is much more dramatic when you introduce an elbow.

This dramatic decrease in pressure is caused by the bend itself as air is forced to redirect and in so doing, loses velocity.

In addition to causing pressure reduction, elbows also create locations where it is easier for water and other particulates to become trapped.

Elbows Mean Shorter Allowable Duct Length

Since every elbow decreases the rate of airflow by a significant amount, each one that is installed will drastically reduce the total amount of ducting that is allowed to be used.

To be specific, Section M1504.2 of the International Residential Code (IRC) dictates that 15 ft must be subtracted from the total length of allowable ductwork for every elbow that is added.

By decreasing the maximum allowable length of duct, it can be ensured that the rate of air removal remains at an acceptable level throughout the entire length of the ventilation, until it is expelled through the terminal end.

It can be a little complicated to figure out how much ducting you are allowed to use, especially since factors like fan capacity and ducting material come into play.

If you need a guide on duct length and dimensions, I have written one with helpful sizing tables and additional information.

Use as Few Elbows as Possible

While the IRC does provide guidelines on allowable length, some states have their own local codes that further limit the number of elbows you are permitted to use when installing your ductwork, so make sure that you check. For example, Oregon codes say that you are only allowed a maximum of 3 elbows.

Ventilation system ceiling air duct in large shopping mall

No matter what kind of ducting you use, it’s important to look at applicable codes to figure out what is permissible in regards to length and materials. However, this becomes even more important in some cases, where you will want to plan with certain information in mind.

For example, narrow ducting is not able to handle as many elbows as wider ducting is.

Another thing to keep in mind is that flexi duct, while useful, causes slightly more of a pressure reduction than other types of ducting, thanks to the airflow friction created by its ridges.

With all this in mind, it is always best to use as few elbows as possible, regardless of what kind of material is used.

Gentler Bends Are Better

It is important to use gentler bends when possible, in addition to using as few elbows as you can. This is because wider, obtuse angles cause less disruption to air pressure and speed than harsher, more acute angles do. 

Of course, you might be able to use several elbows at harsh right angles and still get the job done within legal parameters. However, the idea is to maximize the level of airflow and minimize the number of locations where water can become trapped.

This, in turn, increases the efficiency of your vent, saving on energy costs and reducing the risk of future water leakage or damage.

Avoid Elbows Near Fan or Terminal Vent

When planning how you are going to install your ducting, you’ll want to avoid putting an elbow too close to either end of your vent system. It’s generally not a good idea to have an elbow right next to the fan itself or to the terminal vent where air exhausts to the outside.

Remember that elbows are great locations for moisture to collect. If you have an elbow too near your fan, you may find yourself dealing with moisture dripping back into the bathroom

Keeping elbows from being placed too near the fan also allows the turbulence from the fan itself to “settle” along the path before it has to travel through a bend.

The same applies to terminal vents, which may collect too much water or suffer from reduced airflow when elbows are placed right next to the exit.

Elbows Must Be the Same Size as the Duct

Whether your ducts are 4″, 6″, or any of the other less popular sizes, you have to have elbows to match. Using an adapter on either side of an elbow will just exacerbate the negative effects on airflow that are associated with the elbow.

Do Bends in Flexi Duct Count as Elbows?

You may be wondering if you can use flexi duct to get away with having an “elbow” that isn’t really an elbow. Unfortunately, a bend in flexi duct does still count as an elbow.

Just like with an elbow made from smooth material, air loses velocity when forced through a corner. In fact, as previously mentioned, the corrugation along the length of flexi duct actually causes air friction that causes more of a reduction in air pressure than smooth elbows.

Note that while this reduction in air pressure does exist, it is marginal, so don’t be afraid to use flexi duct if it’s more convenient. Just remember that an elbow is an elbow!


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