When most plumbers think about air in a drain system, they think about venting traps. However, this is not an article about installing an individual vent to serve a trap. It’s not even about different venting systems that could vent several traps. This article is about the necessary air in a drain system and what it’s doing that you may not even know about. 

The first trap was introduced into plumbing in 1865 in America, and since then it has been modified to prevent the loss of its trap seal. Some modifications have been great, and others have had a negative impact. Most plumbers understand the importance of the trap seal and how to maintain the trap seal with proper ventilation by following plumbing codes. The importance of air in the drainage system has been widely overlooked, and most plumbers are not even aware of that importance. 

THREE REASONS PROPER VENTILATION MATTERS

Proper ventilation throughout the drainage system prevents three major difficulties. I’m not going to say that one is more important than the other as they all serve a necessary task to help with efficiency and therefore allow more drainage fixture units to be installed on smaller diameter piping without overtaxing the pipe. 

The most common would be protecting the trap seal. We’ve been hammered with the knowledge that every trap needs a vent and where to install that vent for the trap seal to be adequately protected, but proper ventilation doesn’t stop there. 

Restricting airflow is one that is commonly overlooked, or even not mentioned at all. This is a huge part of why we have reduced our pipe sizes over the last 75-plus years. There was once a time that a 4-inch line could only handle six water closets, six washbasins and six bathtubs, now a 4-inch line can handle seven times that amount. Maybe it’s not just a different piping material, maybe it’s actually the increased airflow.  

Material deterioration or slime and fungi growth is another side effect of little or no ventilation in a drainage system. This has become somewhat of a lesser issue since we went from a metallic material to a plastic. 

HORIZONTAL FLOW, VENT STACKS AND PRESSURE CONTROL

Do you think the average plumber would know these three components to ventilate a drainage system?

Let’s start out with our horizontal piping. We install our horizontal piping to flow about half full. Not only is this to keep the solids suspended with the water, but to also have enough air in the drain to prevent siphonage or increased pressures. If we were to have more than half-full drains, we would restrict airflow, potentially creating sluggish drain systems. 

Most plumbing codes require a “main” vent stack. The main vent stack is required for each building having a gravity building sewer and is required for many reasons. One reason is that the aeration of the drain system would lessen the noxious gas production. Some other reasons are to keep the municipal sewer well ventilated or to create an established vent that your individual or branch vents may connect to. Whatever reason you like, they are all needed to maintain a safe drainage system.   

Increased pressures or unbalanced pressures will have a negative impact on both horizontal and vertical drains. With increased flow velocities and pressure in a drain stack, the water will restrict the capacity from the horizontal branches discharging into the drain stack. The discharge of the horizontal branches into a drain stack should not cause excessive turbulence with the sheet of water flowing down the stack. If this were to happen backpressure in the horizontal branch could result.  

DRAIN STACK DISCHARGE, HYDRAULIC JUMP AND BRANCH CONNECTIONS

We need to protect the horizontal branch connections by limiting the amount of discharge into one branch interval. The amount of fixture units in a drain stack may be limited by the amount of fixture units entering the drain stack at each branch interval. If you have a drain stack with more than 7/24 full of waste, the horizontal branch connection may have a hard time penetrating the sheet of water in the drain stack causing a backup in the horizontal branch not only restricting drainage, but also could restrict airflow as well, potentially creating stoppages. The discharge of the horizontal branch can enter the drain stack by mixing with the vertical flow or by deflecting it. The deflection can only be accomplished by developing a head pressure in the horizontal branch. This head pressure is created by the flow in the drain stack restricting water flow in the horizontal branch creating a backup until the head pressure is great enough to change the flow pattern of the sheet of water in the drain stack to allow the water from the horizontal branch to enter the drain stack.  

This has been studied, and with empirical data it is permitted to create a backup as long as this backup does not allow water backing into a shower stall or similar fixture as well as if it does not create a sluggish flow in the drainage system. 

Once the water flow from the drain stack enters the bottom most horizontal drain, commonly referred to as the building drain or house drain, the increased flow velocity due to elevation, which may have reached terminal velocity will slow down rapidly and that water will fill the cross sectional area of the horizontal piping creating “hydraulic jump” that will increase the pressure in that part of the piping just upstream of the hydraulic jump. 

Terminal velocity is achieved by different size drain stacks at different heights. Hydraulic jump usually occurs within the first 10 pipe diameters or less, which means any branch connection downstream of the 10 pipe diameters has no negative impact on that branch connection. However, if you do have a branch connection within 10 pipe diameters of the base fitting of a drain stack, the results of the hydraulic jump are not just the increased pressures, it will also affect the drainage discharging into the horizontal drain potentially causing a backup.  

If you do need to have a branch connection within 10 pipe diameters it will have to enter the horizontal drain from the top half of the drain, essentially “falling” into the drain so that a stoppage will not occur.  

There has been so much change to the amount of air required in the drainage system since plumbing first started. Imagine how much more could change with ventilation in the next 50 years.