How Many Drains Can Be On One Vent?

How many drains can be on one vent? It basically depends on the size and attachments, and so you must keep some better idea of this, of course.

Drains are the veins of a plumbing network,facilitating the transportation of atmospheric pressure. The plumbing system of our homes won’t work without an adequate drainage network,

And vents are the mechanisms that enable the drains to function properly. That brings forth the question – is venting a must for drains? The answer is a resounding ‘yes’.

Each and every drain to and from your laundry, toilet, and sink of the kitchen, etc, needs vents. But what is the number of drains that we can attach with one vent?

Stay with us to find out.

How Many Drains Can Be On One Vent?

Let’s have a look at this table first:

Diameter of Vent piping in Inches	1 ¼	1 ½	2	3
No. of Maximum Units Allowed  to be Attached Vertically	1*	2**	16***	48****
No. of Maximum Units To be Attached Horizontally	1	1	8	35

This table shows the Plumbing Code vent requirements. Plumbers nationwide follow these guidelines known as the Uniform Plumbing Code (UPC), or otherwise known as the American National Standards.

Though different states may seem to have their own plumbing venting rules, all of the codes are in harmony with codes laid out by the UPC.

Let’s get the asterisks explained:

* In a venting pipe where the diameter is 1 ¼, there should be no Trap Arm

**In a venting pipe where the diameter is 1 ½, sink, urinal, and dishwater should not be shared for one vent. In other words, a 1 ½ diameter venting pipe should also have one drain per fixture.

***There should be no six-unit Traps or waterclosets shared.

**** There should be no more than three water closets or six-unit Traps shared in one vent.

The Main Vent

Each fixture in a plumbing network has to have its own vents. And these vents should end up out in the open – usually at the rooftop. But in reality, our households have a singular vent pipe that stretches through the roof. The vents of different fixtures all end up in this pipe. Plumbers call it the main vent stack.

The vent stack ensures the proper air pressure regulation in the plumbing network. As the drain pipes take waste and water away from our home, the venting system of a plumbing network gets rid of gas and bad smells. Fresh air gets inside the plumbing system through vent piping and creates atmospheric pressure. This helps the flow of water through drains.  

Connecting Drains Into The Vent Stack

The vent of a fixture must be half in diameter as its drain pipe. As we have seen in the table, the minimum requirement for a vent pipe is 1 ¼ inches. Generally, plumbers use a 1 ½ inch pipe for venting for a drain that is 2 inch in diameter. This makes sure that the drain gets enough air to prevent clogging.

As we have mentioned, there should be a vent for each drain for every plumbing fixture. But it may not be always practical or possible to have separate vent piping for each individual drain. That’s why drains are shared for venting. We have mentioned how sharing a vent should be done as per the Plumbing Codes.

Each vent piping must arise on top of the overflow line of that plumbing fixture. This precautionary step prevents a vent from turning into a drain. It can happen when the drain of a fixture becomes clogged.

When the vent inclines up to the main vent, a Y-fitting is used for tying the vent with the main vent. A vent tee should be used when the vent rises to the connection level and moves horizontally from that point on. In this case, plumbers choose a vent tee that doesn’t have a sweep. 

This Video Will Help You Too!

Frequently Asked Questions (FAQs)

Final Words

Venting is what makes a plumbing operation efficient. We have discussed how many drains can be on one vent with reference from UPC. Hopefully, it clears out any confusion you might previously have.

Every now and then, harmful gases can be inside plumbing networks. These gases are harmful to us, and we need to remove them. Then there is the purpose of keeping the flow of water through the pipes, which requires air pressure. Venting ensures that both of those functions are performed efficiently.

So, it’s important that you are careful about maintaining proper venting. And get professional help whenever you feel necessary.

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