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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.)

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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.

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    OP may not have thought to complain about the smell. I have more than once walked in to a bathroom in a commercial building where the trap in the floor drain obviously was dry, and nobody seemed to care. (i.e., if it stinks in here, that must be somebody else's problem.) I had not heard about "primer" before today. Smart idea. I hope the inventor was rewarded. – Solomon Slow Sep 30 at 14:54
  • OTOH, OP said, "It's easier to hear in 'our' bathroom when a person in the other bathroom flushes." So, maybe not a primer at all. – Solomon Slow Sep 30 at 14:57
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    Building has lost that "new building" smell, but it hasn't (thank god) yet developed obnoxious noxiousness. No, the drain doesn't smell. I asked a female co-worker about it and she said she'd noticed the same from her side. If we flush, she hears water flush down the drain on her side, so I dunno, maybe the system is set up to prime both sides, maybe there's single drain system. Anyway, I accept that this is the most likely reason (prime the trap). – CGCampbell Oct 1 at 12:35
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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.

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    The designers my have assumed the maintenance staff would mop the floors into the floor drains more often than they do, so failed to provide a trap primer. – Ecnerwal Sep 29 at 17:59
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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...

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