Venting Your Plumbing
I was a plumber, among other things, for 18 years as a private renovations contractor. In that time, I have seen and heard all kinds of noises and complaints from and about drainage systems.
Reasons to Vent a System
Most people think plumbing is all about drains and water pipes; and it is. But many people don’t know that those drains have to be vented. The reason for venting is twofold: First of all, noxious gas can build up in a system and if there is no venting it will find an escape through drains and toilets. If you hear a bubbling noise coming from your toilet or sink it is probably because the system is not adequately vented.
The second reason to vent a system is so the pipes remain at neutral pressure. This helps in proper drainage. No venting makes drains slow and can suck out the water from the P trap leaving the house at risk of being flooded with noxious or even poison fumes. This often happens when a do it yourself-er forgets to vent the new bathroom he or she has just finished in the back room addition or basement, or in very old houses that were plumbed before standards were in place.
What Is a Wet Vent?
New houses are all built with adequate venting, of course. The picture on the right shows a typical double drainage system. A typical system contains two kinds of vents: A wet vent and a dry vent. The wet vent is actually the stack. It is typically a 4 inch or larger pipe that runs from the sewer connection under the basement to the roof of the house.
The reason it is called a wet vent is because it obviously carries a lot of water and waste as well as being open to the outside air on the roof level.
All drainage pipes coming from all fixtures in the house like sinks, basins, tubs, and toilets are connected to the stack. Each fixture has its own size pipe. Toilets need 4 inch drainage pipes, sinks are connected with 1.5 inch drainage pipe, basins are typically connected with minimum 1.25 inch pipe, stand alone showers and tubs are usually connected with 2 inch pipe.
All drainage pipes are equipped with a P trap. You can see it if you look under your sink. Check the pictures below. The P trap is a water trap. The water in the trap prevents gases from the sewer going in to your house. But again, this can only happen at atmospheric pressure. If there is a pressure build up due to inadequate venting the gas can get through the water barrier, and as I said before, that water can also be sucked out by negative pressure which can happen if someone upstairs rapidly drains a fixture.
Why Wet Venting Is Inadequate
Therefore, wet venting is inadequate on its own. You also need what is known as a dry vent. A dry vent does not carry water or waste products. It typically connects to the stack below the roof line and runs down the length of the building where it connects to the stack again just above the basement floor.1.5 to 2 inch dry vent stack is typical but codes in your area may vary depending on the material of the pipe. These days it is all PCV pipe but many houses still have cast iron and galvanized pipe in them.
Again, in the photos above, you will see a picture of a typical set up. A basin is connected to the stack well under the bottom of the basin. Another pipe runs off that one upward, well above the basin to the vent pipe. All your fixtures should be connected to it. Sometimes in bathrooms the toilet and tub or shower are wet vented through the basin drain which in turn is connected to the vent stack. Toilets and stand alone showers should be as close to the main stack as possible. But with adequate venting and slop longer runs are possible.
What to Do if Your House Has not Been Vented
So what do you do if your house has not been properly vented and you are getting strange smells and noises coming from your drain pipes? The only rational answer is to add proper venting. If the entire house needs to be done then unless you have done this sort of thing before and don’t need this article, call a good reliable plumber.
You are in for a big expense where walls will need to be cut into, drywall replaced and repainted or refinished.
But if you are just having problems with an addition you can do it yourself if you can find the vent stack. It is a lot of work and still may involve replacement of some drywall. It is a big job but not out of the realm of possibility for someone with a handyman bent.
And yet, for slow drains due to no vent there is an alternative. It is called an air admittance valve. There is a picture of it on the right in a typical application. It is a valve that allows air into the system but does not allow gasses to escape. Though it is not permitted under some codes and is not appropriate for all cases, it is widely used in Europe and is approved for use in some US states and in most Canadian provinces.
Its only drawback is that it does not release pressure. Of course we wouldn’t it to. In most cases that is not a concern, particularly if the rest of the house is properly vented. If your problem is with gurgling noises when no water is being run through the drain, you have bigger problems with venting than this valve can solve.
However, they are relatively easy to install and they are definitely inexpensive at between 20 and 30 dollars, as compared to several hundred to several thousand. All that is required is that you have space to add a T to the system as shown in the photograph.
In conclusion, problems you are having with improper or slow drainage may not have to do with your drains as much as they have to do with your venting. It’s always worth getting to know your drainage and venting system; particularly if you are buying a house or putting in an addition.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
How do you vent an under-counter oval basin that has no opening?
If you mean without a hole for overflow, that's not a venting issue, it just means your basin can overflow if you aren't careful. Obviously, it has a hole for drainage. Therefore if you are just replacing the basin the pipe you are attaching it to should already be vented. If you are doing a complete new install, like building a bathroom in your basement, and you have no previous experience, you will have to study and follow the local code or get a plumber. But in any case, all the basics of venting will be as I've outlined in my paper on the subject.Helpful 2