One easy way to vent a new drain in the basement is to tie into an existing drain vent. The first thing you should do before making any modifications to your plumbing is trace the sewer vent from your rooftop pipe, through your attic and, potentially, down to your basement. Ensure your basement has not already been plumbed for drain ventilation, as tying into this existing vent will save you a lot of time and labor.
Sewer vents are a necessity for all household drains, as moving water in a pipe creates an area of high pressure ahead of the water and low pressure behind it. Typically sink, tub and toilet drains will vent to the rooftop. This vent is a metal or plastic pipe, about 2 inches in diameter, that allows air to be pulled in from outside the drain system to keep water flowing freely within the drain. Basement sewer vents, located so far from the main household sewer vent, can be a bit more tricky. There are a few things to keep in mind when deciding where to put sewer vents in a basement.
Existing Vent Pipe
The stack vent is the main vent for your home. It is the large-diameter pipe that leads to the rooftop vent. The household drain that requires the most ventilation is the toilet, so the stack pipe will most likely be someplace close to your main bathroom. If this stack vent runs down to your basement, venting basement drains entails tying into the stack. You will need to cut the pipe and, depending on whether it is PVC or metal, cement or weld a T-coupler into the stack to accommodate your new vent pipe.
Bathtubs often have larger vent pipes than smaller sinks and floor drains. Depending on your local code, you are usually able to tie in a new drain vent to an existing bathtub vent without issue. You will want to check your local municipality's plumbing code to ascertain the required vent pipe diameter for the distance you need to travel horizontally to reach the bathtub vent from your basement drain.
Wet venting is one method of ensuring adequate airflow in drain vents that have long horizontal runs. If, for instance, your drain needs to be tied into a stack vent that is on the other side of your basement, wet venting may be an option. Wet venting entails tying into an existing vented drain line somewhere between your new drain and the vent. This allows air to be pulled in from the drain line when a vacuum is present, ensuring that the drains flow freely. For a wet vent to function properly, all fixtures must be on the same floor.
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