Plumbing in the Home/Slow Tub Draining



My bath tub literally takes 20 minutes to drain.  But it is totally fine just running water straight down the drain. No issues with showering, only upon opening stopper to drain the tub. All the pipes are fairly new (PVC).

Any insight on causes, troubleshooting to or potential fixes would be much appreciated.  Thanks.

Hi, If you plumbing is new PVC, the root cause of the slow drain is most likely improper venting. All drains need an air vent to work properly. When water is  introduced into the line it displaces the air in the pipes and literally pushes it ahead of the water to a vent pipe that vents to the outside of your home. Conversely, once the water passes the vent opening, air must be drawn back in to break the siphoning effect caused by gravity. With all that being said, a vent must be within a certain distance from the drain depending on the pipe size. for 1 1/2" it must be within 3 ft. for 2" - 5ft and 3"...10 ft. My guess is the installer failed to follow the  venting rules and now the drain is getting air locked and slowing down the tub drain. Since the venting is done behind the walls, it would be difficult to add them now without causing a major expense. If by some chance the vent is plugged, an old trick is to run a garden hose down the vent from the roof but that only works if the tub is separately vented.
If it were a clog it would show up when the shower is used so by your description I would bet against a clog.

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Jay Nicholas


Plumbing & Heating (warm air, hot water & steam heat)


40 years in the business, 35 years as a lic. master plumber in NY State. Retired

Graduated Magna cum laude at the School of Hard Knocks

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Commercial, residential and light industrial. You name it ...I have probably worked on it

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