What is an S-Trap and why should I care?

An S-Trap is an arrangement which traps water in a return bend, much like a P-Trap, but has a second return bend into a vertical drain line instead of a J-Bend into a horizontal drain line. This could also include a standard P-Trap which connects to a horizontal drain line which turns down into a vertical drain line without proper venting. In either case the configuration can cause loss of trap seal through siphonage.

Pipes are filled with static air. When a flow of water or waste enters the pipe it displaces the air – pushing it forward down the pipe. As the water travels down the pipe a vacuum is created behind the flow drawing in new air, preferably through a vent in the piping system. In the case of an S-Trap the suction of the water moving down the vertical pipe pulls the water out of the trap until air can be drawn in through the drain.

Water closets are designed to take advantage of this action and are a perfect example of an S-Trap.

Water passing through the trap and down the pipe pulls all of the water and waste out of the bowl until the vacuum is broken by air. Then the tank valve slowly refills the trap and the bowl so that the seal is maintained. An S-Trap can work – as in the case of a water closet – if there is a mechanism to replace the water in the trap, otherwise the trap flushes and the seal is broken.

Today we rarely encounter S-Traps except in flushing fixtures. They are forbidden under the plumbing code. The most common mistake in a plumbing rough is making the drain arm – from trap to waste stack and vent – too long. If the top of the waste pipe at the point where it meets its vent is below the water level (Trap Weir) in the trap, that is technically an S-Trap – and acts as one causing the trap to siphon.

Since the waste line requires a minimum grade, the longer the horizontal pipe is the greater the difference in elevation. Eventually the top of the pipe at one end is the same elevation as the bottom of the pipe at the other. This is known as being out of the vent. That is because the top half of the pipe is helping air pressure balance between the vent and the trap. When the down side of the pipe gets too low you have created an S-Trap.

There are instances where S-Traps are virtually unavoidable. Most often this occurs on floor drains in a large area such as a commercial kitchen or warehouse. In these instances long combined waste and vent mains are used with the top half of the horizontal main acting as a vent. But as the line gets longer the distance from trap to drain becomes longer due to grade on the main piping.

Long vertical drops between drains and traps can cause another problem. The falling water gains momentum and at a certain point it will have enough momentum to pass straight through the trap leaving it dry. For that reason the code restricts the vertical drop to 24”. But what happens when your main is more than 24” below your drains?

This is one of those gray areas adjudicated by the local official. If you keep your trap within the trap weir then the riser to the drain is too long. If you raise your trap above the main waste/vent then you have created an S-Trap. Most inspectors prefer the S-Trap. In most instances the floor drain will be protected with a primer or other means which will insure the trap seal, so possible siphonage is not an issue, but a long drop in the vertical pipe will risk even the water from the trap primer overrunning the trap and leaving it dry.

We often find situations where piping cannot be installed by the strict letter of the code. That is why the code allows for the discretion of the inspector. This is also why it is important, not simply to memorize the code, but understand the intent of the code.

In our example the intent is to maintain the trap seal. Using an S-Trap, though forbidden by code, and a trap primer to insure sealing the trap is better than a long standpipe, also forbidden by code. By understanding the reasoning behind the code we can find efficient and effective installation techniques that solve the problems, or perhaps – as in the case of the flushable water closet – use the very problems that caused the S-Trap to be forbidden to design a fixture that revolutionized sanitary plumbing design.

The plumbing code is written as a guide to minimum acceptable standards. We should be working toward best possible practices and standards. To do that we need to understand why the code is what it is. We must ask ourselves why something is required or forbidden and then we will know not only what is required as a minimum, but how we can provide a better system. If you are chasing best practices instead of minimum standards you'll never worry about a failed inpection.

One thing to remember, however, if you are studying for the exam – the exam is based on the letter of the code and minimum standards. If a vent can be 1 1/2” by code and you size it 2” you are counted wrong. In practice there would of course be no problem oversizing a vent, and the inspector wouldn't say a word, but the test is a test of your knowledge of the code.  

Vents and Venting 101

When I talk to people in the industry – plumbers, designers, and engineers – I find a lot of confusion surrounding proper venting techniques. Venting in a piping system is based on one simple principle. If you place a drinking straw in a glass of water, then place your finger over the end of the straw and remove it, the straw stays filled with water. The water in the straw cannot flow out until air is allowed in. The straw is air-locked.

In a piping system, as fluid flows through the pipe, air is pushed ahead of the fluid and air must enter the system behind the fluid. This creates pressure changes in the system with pressure ahead of the flow increasing and pressure behind the flow decreasing. In a closed system the flow would stop, as it does in the straw. In an improperly vented plumbing system the flow is restricted, pressure on downstream traps can cause sewer gasses to push through, and negative pressure on upstream traps can siphon them dry. The restricted flow can also cause stoppages.

The principles behind proper venting allow these pressure changes to be eliminated. The most basic and effective vent system would be to individually vent every fixture in the system, and some engineers use this configuration for its simplicity. But this is lazy design and adds unnecessary piping which in turn adds cost to the customer. It is not necessary to individually vent every fixture in order to create a fully functional vent system. With a little thought, vents and their related cost can be eliminated without changing the effectiveness of the system.

Two of the chief confusions I find comes from terminology used and proper application of principles. I will focus on three main concepts that represent the same basic principle and seems to create the most confusion – Wet Vents, Circuit Vents, and Loop Vents. Keep in mind our guiding principle – fluid flowing in a pipe creates pressure fluctuations. We want to limit those fluctuations and allow air to freely enter and exit the system to keep the fluids flowing.

Circuit Vents and Loop Vents are in essence the same thing. In both cases a vent is extended up from the horizontal piping between the last two fixtures on a branch in order to vent a group of fixtures. The only difference is where the vent terminates. In a Loop Vent the vent loops back to the Stack Vent. This is only possible on a stack with one branch interval or the topmost interval of a multi-interval stack. Circuit Vents terminate individually or tie into a vent stack or vent header.

Loop Venting has advantages over Circuit Venting as it creates a relief vented loop with a vent before the first fixture on the branch (the Stack Vent) and a vent before the last fixture on the branch which are then tied together providing circular air flow within the branch. We will discuss relief venting later. In the Circuit vent we are limited to a single vent to provide air to our system. Yet often that is all we need.

In a Circuit Vent, as in a Loop Vent, the horizontal branch line is vented, providing a common vent for all fixtures connected to the branch. This eliminates the need for individual vents. We are limited to eight total fixtures on the branch, and only four can be water closets unless we add another vent before the first fixture – a Relief Vent. With the relief vent, or with a Loop Vent (which provides its own relief vent) we can vent up to eight water closets. That eliminates up to seven individual vents!

We can also connect multiple circuit vented branches in series, so in essence we only need one vertical vent for every eight fixtures on the horizontal branch (Remember: only four can be water closets unless we relief vent). In applications such as gang toilets or gang showers the cost savings for the customer really add up. And we are still providing ample air flow to our system. The principle behind this technique is based on the physics of fluid flowing in the pipes. In a properly sized and graded horizontal drain line (the grade on a circuit or loop vented drain should be no more than 1:12) the fluid stays in the lower half of the pipe, leaving air in the top half. This allows the drain itself to partially act as a vent.

That brings us to the Wet Vent. A wet vent is simply a vent that is also used as a waste for another fixture – generally a lavatory or sink drain. In a bathroom group, or two adjacent bathroom groups, all of the fixtures can be vented through the lavatories as a wet vent. This configuration is very much like the Circuit or Loop Vent – the fixtures on the horizontal branch are commonly vented through the waste riser serving the lavatories – but the wet vent can be the last fixture on the branch. It is also permissible in Circuit Vents and Loop Vents to utilize a Wet Vent for any or all of the required vents or relief vents mentioned previously.

A Wet Vent can also be used to vent another fixture as a Common Vent, such as venting a floor drain with a lavatory drain. Also, one fixture above another on a stack, such as a sink connected above a washing machine or a lavatory above a water cooler, can act as a wet vent for the lower fixture. We can even stack fixtures floor-to-floor, such as hand sinks or mop basins on multiple floors connected to a single vented waste stack. There are of course restrictions to what fixture drains can serve as wet vents, and pipe sizes required, but often one vent can serve multiple fixtures and thereby lower costs and complexity of the plumbing system.

Proper venting is not that complicated when we keep the basic principle of pressure changes in mind and follow the simple sizing and maximum fixture loads spelled out (with charts!) in the NC Plumbing Code. And when in doubt, an extra vent here or there will not hurt anything. You cannot over vent, but you can waste the customer's money by adding unneeded vents. The balance we seek is adequate air flow with minimum vent piping for the most efficient design and installation.

A few points to keep in mind:

• Eight fixtures on a single branch (no more than four water closets) can be vented with a single vent located between the last two fixtures on the branch – Circuit Vent
• More than four water closets on a branch requires a relief vent before the first fixture on the branch – Circuit Vent or Loop Vent
• A Loop Vent is, in essence, a Circuit Vent tied back into the branch's relief vent (Stack Vent)
• A Wet Vent is a vent that receives discharge from another fixture
• A Wet Vent can be used to vent two complete bathroom groups
• A Wet Vent can be used as the vent and/or the relief vent on a Circuit or Loop Vented branch
• Multiple fixtures can drain into a single, properly vented stack

A few “Gotchas” in Mecklenburg County:

• Urinals on a Circuit or Loop Vented line should tie-in to the branch on the horizontal and cannot be used as a Wet Vent
• Fixtures downstream of a water closet must be individually vented (some inspectors allow floor drains on a Circuit Vent with water closets – but better to individual/common vent and be safe)
• Pumped lines, such as a washing machine drain, cannot be used as a wet vent. Some inspectors consider a dishwasher to be a pump discharge – or a pump from an icemaker or condensate line.

In closing; I would suggest a quick reading of the NC Plumbing Code, Chapter 9 – Vents. It is very clear on when and where vents should be placed. I will also suggest a quick Google Image Search of Circuit Vent. As I said at the beginning, much of the confusion seems to stem from the terminology. There is very little difference between Circuit, Loop, and Wet Vents. They are all based on the same principles.