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Why do I hear water flowing in a floor drain near commercial bathroom fixtures?

Home Improvement Asked on March 8, 2021

I work in a government agency building, less than 10 years old, built to some kind of ‘green’ standards.

When I am in a men’s bathroom, either at a urinal or a commode, when someone flushes a toilet, either in the men’s, or even in the women’s next door, it sounds as if water is being flushed down the floor drain at the same time. (In fact, it sounds like the flushed waste is going down a drain, although that is (obviously) not true.)

Why is this, and what is happening to cause this?

Per a comment: Yes, the sound is coming from a floor drain

(It happens in all bathrooms on all floors of the building. The audible whoosh/splash sounds immediately following hearing a toilet flush. It’s easier to hear in ‘our’ bathroom when a person in the other bathroom flushes, but it also happens when our toilets/urinals are flushed.)

3 Answers

The sound you hear is the “primer” releasing a “squirt” of water into the floor drain. The primer is activated when a nearby water valve is activated. In your case it’s the urinal valve.

The floor drain has a trap that blocks the smell from the sewer lines from flowing back up the sewer lines and into the room and building.

Primers are placed on drain lines and traps because they do not get a regular flow of water to keep the trap full.

Because you didn’t complain about a sewer smell, I know your primer is working.

Correct answer by Lee Sam on March 8, 2021

A floor drain should have a trap just like every other fixture has. The trap should stop both odors and sounds escaping from the drain plumbing. If you can hear sound of water coming up through the floor drain, a possible cause is that the trap is not filled with water. This can happen due to evaporation if no water is added to the trap for a long time, and it can happen if inadequate venting causes the water to be sucked out of the trap.

Try pouring a quart of water down that floor drain and find out whether the problem is resolved.

Answered by Greg Hill on March 8, 2021

Another method I have used is to add a quart of vegetable oil after you fill the trap with water. The oil does not evaporate, and keeps the trap full...

Answered by mario on March 8, 2021

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