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Reuben Saltzman

Plumbing Vents, Why Houses Need Them (forget the water bottle analogy)

Plumbing vents protect plumbing traps. They don’t make fixtures drain faster; in fact, they do the opposite. When it comes to first time home buyers, one of the least understood components of a home seems to be plumbing vents. They’re those pipes sticking up out of the roof that run through the attic and through the rest of the house. All residential plumbing fixtures need to be protected by a plumbing vent.  Vents are frequently connected together inside the attic, which allows for fewer penetrations in the roof.

Plumbing vents

Plumbing vents prevent traps from being siphoned.

Let me repeat that: plumbing vents prevent traps from being siphoned. They also prevent back-pressure on traps, but today the focus is on siphoning. You may have heard that plumbing fixtures will drain faster when they’re vented properly, but it’s not true. The common, improper analogy is to talk about dumping a soda bottle upside down. You watch the water glug out while air replaces it, and this makes it drain slowly.  Once you put a hole in the top, water drains out very quickly because air can replace the water as it drains.

This analogy doesn’t hold water because the top side of every plumbing fixture is wide open. The top of a toilet is open. The top of a sink is open. The top of a bathtub is open. If you wanted to re-create the soda bottle analogy, you would need to block off the top of the plumbing fixture and then try to drain the water out. I can’t think of any instance where this could possibly happen.

As I mentioned in last week’s blog post, every plumbing fixture has a trap, which prevents sewer gas from entering the building. When a lot of water drains through a plumbing fixture, it can be enough water to create a siphon effect, which has the potential to pull water right out of the plumbing trap. In my blog about S-traps, I included a quick video clip of an unvented drain having water siphoned out of it, leaving the trap with far less water than it should have had.

To help demonstrate this, I made a video showing the difference between a vented drain and an unvented drain. I used clear tubing for simplicity, but the physics are the same. You’ll notice that the unvented fixture actually drained about 2 seconds faster than the vented drain. This is because the water that had left the fixture was helping to pull water out. With a vented fixture, there’s no pull. The vent allows water to pull air instead. Check it out:

If you hear a gurgling noise after water has drained out of a fixture, what you’re hearing is air getting siphoned through the trap. This happens when there is no vent present, the vent is obstructed, or the vent is improperly installed. In next week’s blog post, I’ll discuss air admittance valves; devices that are designed to take the place of individual fixture vents without running pipes through the roof.

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Author: Reuben SaltzmanStructure Tech Home Inspections

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No responses to “Plumbing Vents, Why Houses Need Them (forget the water bottle analogy)”

  1. Michael
    October 2, 2018, 4:17 am

    Excellent demonstration and explanation. Thank you.

  2. Mike
    October 2, 2018, 6:27 pm

    Really? Maybe I missed something, but I always thought vents were for gases to escape.

  3. Reuben Saltzman
    October 3, 2018, 2:59 am

    Yes, it’s true that plumbing vents will also allow sewer gas to vent to the outdoors without coming into your home through the traps. It’s a secondary function of plumbing vents.

  4. Spov
    October 3, 2018, 8:50 pm

    Great explanation. I’ve been trying to solve a pesky sink venting issue for a while now. It’s only noticeable when it’s windy outside. My downstairs bathroom sink (added by a previous owner in a remodel) is dirty armed over to the toilet w no studer vent. Toilet vents to roof. Windy days, noxious doors abound. Any advice? I’d say venting swear gas to the outdoors is THE PRIMARY function of plumbing vents. I love the blog and look forward to next one.

  5. Reuben Saltzman
    October 4, 2018, 3:42 am

    Hi Spov, where does the bad odor come from? Your sink or the toilet? Are you sure that there isn’t a hole in your vent somewhere that’s allowing sewer gas into the home? A pressure test on your system would get an answer to that question.

    As for the primary function of vents, it’s to protect plumbing traps. This is done by preventing siphoning and preventing backpressure. Venting sewer gas to the outdoors prevents backpressure. We’re really saying the same thing. Here’s what the MN State Plumbing Code says about traps:
    901.1 Vents Required Each plumbing fixture trap, except as otherwise provided in this code, shall be protected against siphonage and backpressure, and air circulation shall be ensured throughout all parts of the drainage system by means of vent pipes installed in accordance with the requirements of this chapter and as otherwise required by this code.
    901.2 Trap Seal Protection The vent system shall be designed to prevent a trap seal from being exposed to a pressure differential that exceeds 1 inch water column (0.24 kPa) on the outlet side of the trap.

    And here’s what the IRC says:

    P3101.2 Trap seal protection. The plumbing system shall be provided with a system of vent piping that will allow the admission or emission of air so that the liquid seal of any fixture trap shall not be subjected to a pressure differential of more than 1 inch of water column (249 Pa).
    P3101.2.1 Venting required. Every trap and trapped fixture shall be vented in accordance with one of the venting methods specified in this chapter.

    Thank you for reading!

  6. Mich
    October 2, 2018, 11:20 pm

    I was recenrly told these pipes on the roof were for gases to escape??

  7. Reuben Saltzman
    October 3, 2018, 2:59 am

    Yes, it’s true that plumbing vents will also allow sewer gas to vent to the outdoors without coming into a building through the traps. That’s the part that I mentioned about vents preventing back-pressure on traps.

  8. Fluid Mechanics
    October 3, 2018, 5:23 am

    You shouldn’t knock the open bottle explanation and then say that sinks are open at the top. A trap filled with water is just as sealed as a closed water bottle. Water in the trap forms a seal, and thus doesn’t allow air through. Without a vent, air naturally builds up pressure on one higher side of the system, typically the drain side, and then will pull water out of the trap, in order to allow venting from the sink. Naturally water wants to flow, in a
    closed system that loses pressure as it loses water with no air replacement, it will seek to replace lost pressure from your sink drain because the sewer main end tends to be more difficult to access the air, since it’s already filled with so much water. Couple that with the already down flowing water.

    Yes, vents are absolutely necessary, but work on the explanation a little bit, and don’t knock an analogy that is quite spot on. A good understanding of fluid mechanics is key.

  9. Reuben Saltzman
    October 3, 2018, 11:18 am

    Did you watch the video?

  10. Bill e
    October 3, 2018, 9:39 am

    The vent Is after the trap and should come off the top of the drain. The water bottle analogy does work in that it does keep the drain from gulping for air. I think it depends on how you look at it. I would say it does both. I think it depends on how much water is going down the drain also.

  11. Jenna
    October 3, 2018, 11:21 am

    Any suggestions on how a homeowner can clean out a clogged plumbing vent?

  12. Reuben Saltzman
    October 3, 2018, 11:35 am

    Hi Jenna,

    Using a traditional sewer snake from the roof would make the most sense.

  13. Brett
    October 3, 2018, 1:36 pm

    A demonstration using actual drain piping would make more sense visually for people without knowledge of plumbing. 1/2″ tubing does not replicate dfu’s of any fixture in a home. That being said. A oversized dain.. Also a called “wet vent” does not require a vent. Vents came into play because back in the day when S traps were not prohibited. Older homes have more problems with slow drains due to improper trap arm length/sizing and build up blocking drains than they do sewer gas from siphoned traps. A single lav piped in 2″ to the lateral of the trap with a 8″ trap arm… No vent required. Same with a laundry sink, utility sink or a kitchen sink. Unless the kitchen sink has a disposal. “Licenced plumbing inspectors” are more concerned with the length and sizing of a trap arms in regards to venting. With water saving fixtures required as a standard in our part of the civilized world for the last 20yrs and pipe sizing increased from what it was 40yrs ago.. Trap siphoning is less of a concern than creating a slow drain due to the length of a trap arm which will create a slow drain. Although vents are important for proper draining in certain circumstances. They are not required for 50% of fixtures plumbed correctly in a purely basic 2 bath residential home. However every home requires a roof or side wall penetration for a vent. A convenient place for sewer gas to escape as well.

  14. Austin Myers
    October 3, 2018, 6:35 pm

    GREAT demo and extremely informative on how you explained this. Thank you!

  15. Michelle
    October 5, 2018, 1:27 pm

    There is water dripping out of one of our gutters, from the middle of the gutter and a little from the end, but its been dripping off and on, pretty steady for the the past 2 days. It is directly below the plumbing vent on the roof where the gutter is dripping. Could the vent be causing the water? Would you recommend calling a plumber, roofer or gutter cleaner? Thank you so much!!

  16. Reuben Saltzman
    October 5, 2018, 1:34 pm

    Hi Michelle,

    No, that wouldn’t be caused by the plumbing vent. I recommend getting someone out to clean your gutters and patch the leak.

  17. Patrick
    October 7, 2018, 6:49 am

    Your demonstration does not accurately reflect residential plumbing. In your demonstration, the large volume of water and small diameter tubing is making gravity play a much larger role in the equation than is true in residential plumbing. Build the same thing using a lavatory, 1.5 inch DWV and the minimum slope allowed in the code.

  18. Reuben Saltzman
    October 7, 2018, 12:32 pm

    Hi Patrick,

    While water may behave differently in a residential plumbing system, the concepts that I’m demonstrating are the same. But still, just for you, I’ll re-do this demonstration with a clear PVC trap and a laundry sink. Oh, and I’ll add food coloring to the water. When you say the “minimum slope allowed by code”, are you referring to the trap arm or the drain?

  19. Spov
    October 7, 2018, 8:40 am

    The smell is hard to pin point. It’s a really small downstairs bath and I believe it is coming from the wall behind the toilet. I’ve had the toilet reseated. New pedestal sink w new connection and it didn’t help. I suspect a hole the vent pipe that is noticeable on windy days when pressure from the crawl space forces air into the home. 1929 home w half basement half crawl space. I’ve had three plumbers look at it and none seem eager to even try to tackle the problem. I agree pressure test or smoke test is my next step.
    It’s really frustrating because they were supposed to have fixed this when I bought the house but it wasn’t really done.

  20. Calvin T
    October 7, 2018, 3:51 pm

    Have a friend that redid one commode in his bath room. Now when the other commode is flushed air comes up through the redid one. I haven’t seen it yet but I think he said it was on a separate line. He has a septic tank. Could this be coming through the septic if he didn’t vent the new pipe/commode?

  21. Thomas
    October 9, 2018, 10:20 pm

    You are an idiot.
    Master Plumber, 30 years, and live with the strictest plumbing code in the country. Stick to what you know, or don’t know. You are not a plumber and do not know what you are talking about. You have not been trained in any theory, obv. Yes, trained and educated.
    I could go on and on about the fallacies that you are trying to prove.
    Your advice is worthless.

  22. Reuben Saltzman
    October 10, 2018, 3:21 am

    Hi Thomas,

    Thank you for reading, and thank you for your thoughtful and reasoned response. You’ve clearly proven your point.

  23. Robert
    December 9, 2018, 1:34 pm

    Speaking of idiots-the least you could do is prove him wrong rather than blathering on and on….

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