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Bathroom Exhaust Fan | Can It Be Tied Into Plumbing Vent?

Home construction can be confusing and overwhelming. You may find yourself wanting to cut corners here or there for convenience and money’s sake, but a bathroom ventilation system is not the place to do so.

Many consider venting their bathroom fans into the plumbing vent rather than an exhaust duct, but not only is this non-compliant with the International Residential Code, but it can also create many problems for homeowners.

Bathroom exhaust fans cannot be tied into the plumbing vent. Such a setup violates the International Residential Code. The bathroom can also fill with sewer gases, which are odorous and potentially harmful and dangerous. The systems will also not function effectively.

Bathroom Fans Must Terminate Outside

According to Section M1505.2 of the International Residential Code (IRC):

Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or circulated to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms, toilet rooms and kitchens shall not discharge into an attic, crawl space or other areas inside the building.

Tying the bathroom fan into the plumbing vent, which is not an outdoor space, is a direct violation of this regulation.

Illustration of a bathroom fan exhaust connected to a vent outside

Plumbing Vents are Only for the Plumbing System

The IRC also states in Section P3101.3:

The plumbing vent system shall not be used for purposes other than the venting of the plumbing system.

This means that venting the bathroom fan into the plumbing vent violates this regulation as well because you would be using it to vent the bathroom fan ventilation system.

In addition to code violations, there are several practical and health reasons to avoid this.

Practical Issues

House Sales and Insurance

When it comes time to sell your house, the last thing you want to be dealing with is code violations that make it harder for you to do so.

Potential buyers are going to look at an incorrectly vented bathroom as a big (and expensive) issue for them to fix if and when they decide to buy your home from you.

Thus, it is not unlikely that the offers for your home will be lower than your asking price, as potential buyers will be looking to save money that they can then use to redo the bathroom ventilation system.

As for house insurance, homes must be up to code and compliant for damages to be covered, meaning that if you have an incorrectly vented bathroom fan, insurance will not cover any claims that were caused by the incorrect venting.

This obviously affects you, the owner, but it can also affect the sale. The idea of having to pay out of pocket for any damages may be enough to turn a potential buyer off.

Sewer Smell From Bathroom Fan

Not only is venting an exhaust fan into a plumbing vent against code, but it can also turn out to be quite the smelly issue.

Plumbing vents emit sewer gas that can leave a lasting smell of ammonia in the bathroom when the fan is not running.

The gases are supposed to be contained in the plumbing vent system and released outside through the main stack vent. When the bathroom fan is tied into the system, these gases can travel from the plumbing vent into the bathroom fan ducts and escape through the fan into the bathroom.

Plumbing Vents and Exhaust Ducts Are Different

In addition, to carry noxious gases outside, a plumbing vent functions to regulate the pressure of air in the plumbing system, which allows water to go down your drain system properly.

Illustration of a plumbing vent and its pipe connections inside the bathroom

The volume of air and gas passing through the vents is not large. So, to be up to code, plumbing vents only have to be 1.25″ in diameter or more to be code-compliant.

Bathroom exhaust fan ducts, on the other hand, are to be about 4-6″ in diameter. This size is important. Anything less than 3-4″ would reduce the exhaust duct efficiency too much.

Health and Safety Issues

The sewer gases escaping through the fan and into the bathroom are not only unpleasant to smell, but they can also pose a health risk.

Sewer gas exposure can cause hydrogen sulfide poisoning, which is characterized mainly by eye irritation.

Asphyxiation may occur due to extremely high levels of methane in the air. Methane is expelled by sewer gas and is extremely dangerous in small and enclosed spaces.

As methane levels increase, oxygen levels decrease, which can quickly lead to dizziness, unconsciousness, and, in extreme cases, suffocation and death. In less extreme cases, one may develop a headache, nausea, vomiting, slurred speech, and/or impaired vision.

Both hydrogen sulfide and methane are extremely flammable, so venting the bathroom fan into the plumbing vent increases the fire risk.

Moreover, the bathroom fan is going to be working overtime to exhaust the old air outwards due to the small diameter of plumbing vents. This leads to overheating, which is a potential fire hazard in itself. Add the flammable gases and you could have a serious problem on your hands.

Save Space With a Through-Wall Bathroom Fan

If your duct space is limited and you’re looking for an alternative way to vent your bathroom, look into venting it through an exterior wall instead. This allows fresh air to be vented through from the outside while the stale air from the bathroom is pushed outdoors.


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