THE BASICS OF WET VENTING

It has been a while but Quattro’s Corner is back and we are starting 2023 with a bang. Wet venting is one of the most used types of venting. The purpose of this article is to get your brain going that extra 10.5 per cent.

As always, it’s important to start with definitions. What is a wet vent? As per the Ontario Building Code (OBC) a wet vent is defined as follows: Wet vent means a waste pipe that also serves as a vent pipe.

I am not a big fan of that definition as it doesn’t describe much. I would personally define wet vent as follows: A wet vent is a waste pipe that also serves as a vent pipe. It is a form of group venting which allows one or more fixtures to be flushed in a waste pipe that acts as a vent pipe for a symmetrical fixture. Looking more closely at the requirements for wet venting, 7.5.2.1.(1)(k) truly stands out to me as it sets the tone from the start for wet venting.

(1) A soil or waste pipe may serve as a wet vent provided that, (k) the highest fixture is connected to a vertical portion of the wet vent, upstream of any other fixtures, in the form of a continuous vent. For learning purposes Figure 1 illustrates what this require ment is outlining.

By Andrew Quattrociocchi

the creator of multiple plumbing training courses online through Orderline. He can be reached at aquattrociocchi@king.ca.

lavatory, and one bathtub (with or without shower head) or a shower stall. This explanation is found in the OBC Appendix A 7.4.9.3. Size of Fixture Outlet Pipes. Table 7.4.9.3 Minimum Permitted Size of Fixture Outlet Pipe and Hydraulic Loads for Fixtures is by far one of the most important tables, not only for wet venting, but for pipe sizing as well. I always refer to this table as it sets the tone when utilizing Table 7.5.8.1 Maximum permitted hydraulic loads drained to a wet vent.

As shown in Figure 1, the highest fixture (lavatory) is serving as the principal vent (wet vent) for the entire bathroom group. A bathroom group is considered to consist of one water closet, one

The first question you need to ask yourself when utilizing Table 7.5.8.1 to determine the size of wet vent needed is: is there a toilet or no toilet forming part of the wet vent? This one with water closets. Before we do this it’s important to point

This requirement definitely makes counting fixture units much easier as you get to deduct the hydraulic load from your calculation for the most downstream fixture. When wet venting a toilet, it’s important to know that a toilet must be installed downstream of all fixtures. This requirement is found under 7.5.2.1.(d). Therefore, if there is a toilet being wet vented, the fixture unit rating/hydraulic of the toilet unit is not included. The result would be four fixture units as per Table 7.4.9.3. (See pg98). The last tip I’ll give you is do not look at the location of the fixture itself to determine what is most downstream. With this requirement, most downstream means where its vent connection is located. I have provided a photo below to help you understand this better. an extension of a vertical section of a branch or fixture drain. This photograph matches the description in the definition perfectly. Now we’re on our way to the storey below to utilize our tables.

As you can see in this photo, one would think the lavatory is the most downstream fixture but slow down. The most downstream fixture is not based on the location of the fixture but rather its vent connection. The vent connection for the lavatory is on the storey above. The vent connection for the toilet is in the basement as highlighted in the photo. So, this makes the toilet the most downstream fixture.

Sizing A Wet Vent

The following photos will help you understand how to size a wet vent. As always, let’s look at how we get there. 7.5.2.1 states:

(1) A soil or waste pipe may serve as a wet vent provided that,

(a) the hydraulic load is in accordance with Table 7.5.8.1.

Put a tab for reference in your code book at Table 7.5.8.1. and Table 7.4.9.3. These can be very handy on site or even when writing an OBC examination.

Let’s do this together and size two different types of wet vents.

Here is a second-floor washroom where the lavatory is wet venting the bathtub you see in the back. First thing we need to do is verify that the fixture doing the wet vent is in the form of a continuous waste and vent in accordance with 7.5.2.1.(1)(k). A continuous vent means a vent pipe that is

Here we have the fixture drain on the lavatory (from the previous photo) with a 1.5 fixture unit rating (as per Table 7.4.9.3) wet venting the bathtub, which also has a fixture unit rating of 1.5. Wait, stop and recognize that sentence we talked about under 7.5.8.1(2). The most downstream vent connection is at the Y fitting, which serves the bathtub. Therefore, we do not add the 1.5 F.Us. Using column number two (no water closets), this 1 ½ in. wet vent can only handle a maximum of two fixture units. With the lavatory having a 1.5 rating, this wet vent conforms to the requirements for wet venting.

Let’s take a look at a wet vent with a water closet. Here is a second-floor washroom where the lavatory is again doing the wet venting, but this time we are wet venting a water closet. We need to verify that the fixture doing the wet vent is in the form of a continuous waste and vent in accordance with 7.5.2.1.(1)(k). I know I sound like a broken record or that pet bird who constantly repeats the same words over and over: “continuous vent, continuous vent, continuous vent.”

With wet vent #2 we are focusing on column three. The wet vent must be a minimum of two inches as a 1 ½ in. fixture drain is not permitted. The maximum allowable fixture unit rating through a 2-in. wet vent is three. The most downstream fixture is the water closet. A water closet is four fixture units as per Table 7.4.9.3 and would be assigned to the vent, but maybe this time you have two birds repeating “continuous vent, continuous vent ” and “ most downstream fixture isn’t accounted for.”

The additional fixture that I labelled in the photo is a reminder that the toilet must be the most downstream fixture, therefore that fixture drain must be separately vented and does not form part of the wet vent. Therefore, the toilet fixture units don’t count − toilets can’t wet vent. This is just the basics of wet venting, but a big part of understanding how they work and what to look for. With a bit of practice, repetition and slowing down, the birds will disappear, your brain won’t be going that extra 10.5 per cent and wet venting will become second nature.

Hope you enjoyed reading this. If you have any ideas or topics you want to discuss for the next Quattro’s Corner please reach out. Until the next time.

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