How Plumbing Vents Work

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How Plumbing Vents Work

When you think about it, plumbing and vents don’t sound like they could go together. A vent in plumbing may sound like another name for a drain, but that isn’t anywhere near the truth. In fact, plumbing vents have a specific purpose that makes them especially important and needed in certain situations, and here’s what they do and how they work.

Drain Vents

Drain vents - plumbing santa cruz, ca

Every building with plumbing essentially has a drain vent. You may be familiar with a pipe that protrudes from your roof, which appears to be doing nothing, but that’s the drain vent for your house or home. Virtually all drain pipes are routed to one or two of these pipes, and in multiple-family homes, there may be several of them in place.

The toilet is directly connected to the main drain vent pipe, while other drains, like sinks and tubs, are connected via branch vents. The main drain vent rises vertically from the main drain, while branch vents are connected on upward-sloping runs until they attach to the main.

Drain vents serve two purposes. The first thing they do is to allow air into the drain pipes while releasing any back pressure from preventing a freely flowing drain. But, the main reason they are installed is to prevent sewer gas from coming into the house. Not only is a sewer gas smell, but it can also be flammable, so venting it out and away is the best way to keep it from coming into contact with your nose or with an open flame.

Air Gap Vents

Air gap vents - santa cruz plumbing, ca

An air gap is a variation on a drain vent, and although, in principle, it works the same way, it has a very specific function.

You will mostly find an air gap vent when a dishwasher has been installed. The cylindrical chrome or plastic piece mounted on the rear of the kitchen sink is the air gap. It is connected to the dishwasher and the garbage disposal unit, if applicable.

In simple terms, an air gap stops contaminated water from getting into drinking water by the use of a gap of air between the drain and the freshwater supply. The air gap is positioned above the drain pipe on a sink so that if any wastewater backs up through the drain pipe, it will discharge from the sink basin. If wastewater continues to flow, like, say, in a flooding situation, because the air gap is above the sink drain, the water will then begin to discharge over the side of the sink and onto the floor. In this way, contaminated water is channeled away from drinking water, even in the most extreme circumstances.

A dishwasher air gap mounted in this position will stop raw sewage from flowing back into the dishwasher or into the potable water system of your house. You’ll never have to worry about contaminated water coming into contact with the dishes you eat or fouling the inside of your dishwasher.


Siphoning santa cruz, ca

Every drain has a trap attached to it. Generally, they are in the shape of an “S,” which is why they are commonly referred to as S-traps. The main function is to hold a bit of water in the trap to, once again, block sewer gasses from entering a room.

If there is no air vent connected to the drain pipes, the water in these traps can literally siphon out and go down the drain, leaving no sewer gas protection from coming up the sink drain pipe. But with a dedicated plumbing vent, no siphoning action will occur, and no sewer gas will enter through any drain.

Virtually every occupied building will have some form of plumbing vent to protect against sewer gas and to aid in draining. If you have any questions about plumbing vents or if you begin to smell foul gas coming from your drains, please contact Duncan Plumbing. You can count on us 24/7 for all of your plumbing emergencies, and we have been serving the Santa Cruz, California, area for over 20 years.

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