How Do Your Plumbing Drains Work?

We are going to look into how waste water disappears; it’s called your drain-waste-vent (DWV) system. It’s often called your “black water system”. While getting supply water to a sink or shower is usually reasonably easy, getting it to drain away, especially in an existing home, can be much more difficult. Your DWV system maintains a neutral air pressure in the drains, allowing the flow of waste water down the drain using gravity. Every home has 3-4” diameter pipes, called the main stack, that act as the primary drain for all your fixtures, including your sinks, toilet, tub, and shower. In some parts of North America, it’s also called the “True Vent” or “Soil Stack.” This vent is that round black pipe you see sticking out of your roof. From there, it continues down through your home and becomes the drain that flows to the municipal sewage system or to your septic system, if you live in the country. Some homes may have another, called a secondary stack. This is usually the case when you have a laundry sink, kitchen, or bathroom at the opposite end of the home from where the main drain leaves the home.

To better understand why you need this vent, think of pouring water out of a full plastic bottle. The water flows out unevenly, often stopping and starting. If, however, you punch a hole in the top of the bottle, the water will flow out smoothly. This hole you have added is operating in the same manner your house vent does, letting air into the bottle to allow the water to free flow.

Now that you understand how and why, we will add in more terms. A revent is added to the system to assist your sink or shower to properly drain. These vent lines are added to the drain lines coming from your bathrooms and kitchen. They are joined into the main stack above the fixture at the point where the main stack becomes the path for your waste water to flow down this stack or drain. Both the drains and main stack are usually made of ABS plastic or PVC, if code required. Older homes, built after WW2 and up until the late 60’s, have drains and a main stack made of copper. If your home was built before WW2, they are likely made of cast iron. These older drains are on the “hit list” of most home insurance companies as they are known to decay over time, crack or split and emit sewer gas or, in some cases, actual sewage.

Your DWV system also contains several “clean-outs” that are strategically located to allow the plumber to unclog a blockage created by a buildup of waste in the line. In most municipalities you are not allowed to cover your main cleanout. This allows the city workers to clear your drain if they have a large blockage in their sewer lines, which does happen at times. Birds, leaves and even ice buildup in the winter can block the top of your vent. If you see bubbles in your toilet, your drains are slow or you get a sewage smell coming from your sinks, check your vent first. It is normal to hear a “sucking” sound when your sink empties.

Some homes have the main drain empty above the floor and, if you want a laundry sink or bathroom in the basement, you will have to install a pump-up unit. In the case of a bathroom, you have a couple of choices. One company makes a toilet and pump unit called “Sani-Flo” which will operate a combination of fixtures from a basic toilet to a three-piece bathroom. If necessary, you can break up the concrete basement pad and install a pump-up basin that will allow you to add more than one bathroom if needed. You will also have to saw cut open the concrete pad to install the necessary drains from whatever location you decide you need to drain towards the pump-up unit.

If you live in the city, your waste water, a combination of grey and black water, will leave your home to the municipal sewage treatment plant. If you have a sump pump, it is recommended that you do not discharge this into the city system. This adds clear water to the grey/black water that goes through a costly treatment process, a waste of use for the municipal system. If you live in the country, you will have a septic system and, while these systems are generally trouble free, age and improper materials flushed down the system can seriously affect the septic tank and drain field. We recommend a septic tank be pumped every five years to clear the solid waste and inspect the condition of the tank.