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  • Writer's pictureHandsome Hank

Why We Vent Plumbing Systems

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So why do we vent plumbing systems?

Credit: Mr.Fix-it Inc.

Your plumbing system or DWV (drain, waste, vent) system, if properly constructed, is a combination of properly sloped drains and correctly positioned vents. Drains move waste water from the fixtures to the sewer, and vents provide air to the drains that allow the water to flow smoothly by letting gasses and odours flow out your roof vent and allowing fresh air back into the system. A great analogy is a large tin of juice, when you puncture one hole in the tin, the juice chugs and gurgles when it's pouring out. But, when you add a second hole further up above the first hole, it allows air in and the juice flows nicely into the glass and not splattering all over your kitchen counter. The same physics applies to your DWV system.

In this blog we won't get into all the code requirements on venting, it would take years to accomplish that, but we are going to delve into the basic principles and language of plumbing vents.

Signs of a slow or blocked vent:

-Slow or standing water

-Gurgling sound when draining

-Foul odours (Methane, rotting foods, fecal matter)

Vents control the speed with which your waste water flows.

Photo Credit: 2011 Dream Home Consultants, LLC.

Believe it or not, there is an optimal speed that your waste water should flow through your drains. Approximately 1 to 2 feet per second. In a commercial kitchen with a grease interceptor we call this the drain down time. Often plumbing inspectors will put a ball through your drain and time how long it takes to travel from point A to point B to determine if your plumbing system is working well. If your water travels too fast it will leave solids behind to get stuck on the walls of the pipe. If your water travels too slow it has a similar effect. The top 1/3 of any drain should be air from the vent travelling towards the fixture outlet providing outside air to assist the flow. The bottom 2/3 of pipe is for waste water. Without our vents, our drains would get airlocked and our drain speed would be all over the place.

Vents help to control odour.

If your drains are flowing nicely at a proper rate of speed they should be self cleaning. By this we mean enough flow to wash the sides of the pipe and prevent build up. That build up is pretty nasty and has a foul odour. Properly installed DWV systems should not smell and be self cleaning. Any odour or gases from the sewer that are present should rise through your interconnected vent system and dissipate through the vent stack that runs out through your roof. If you are getting foul odours, you may have a problem somewhere. I would suggest calling a professional because it could be any number of problems, not all obvious, and not always a venting issue.

Vents come in various styles and sizes.

As stated earlier, we are not going to get into interpreting local building codes but we will touch on a few of the important basics.

Plumbing vent running up through roof.

1. All traps require a vent. Most vent connections shall be located 5 ft or 1500 mm from the trap outlet, this is referred to as the fixture vent.

2. You cannot trap a vent or form a system that does not consistently rise out through the roof. All vents must terminate to atmosphere (outside). Without the ability to drain, any condensate that could accumulate and block the vent.

3. Always refer to local building codes when sizing your vents and combinations of vents. Every system is different and often a good mechanic can save time and money by combining vents and changing layouts.

4. Vents shall not be left open inside a dwelling. This would be very smelly and could be dangerous depending on the fixture on which it's located.

5. Vent connections to drains shall be nominally vertical (off the top of the drain) so the air can travel easily out your roof vent.


Drainage is only half of your plumbing system. Improper venting or lack of venting can wreak havoc in all your drains. This is why we have building codes and municipal inspectors. When roughing in new fixtures, it is crucial that the installer has a strong understanding of local codes and good building practices otherwise you are risking a lifetime of aggravation and potential health hazards that are not easily fixed. If you think it's expensive while under construction, wait until you get the bill to do it again properly.

“When it comes to plumbing, there is no such thing as a simple mistake.” — Katherine Applegate

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