Why do I hear water running in a floor drain near a commercial restroom?

Water that rushes for a few seconds when someone flushes is normal. Restroom fixtures and the floor drain often share the same drain line, so a flush upstream sends water past the floor drain's connection and you hear it through the grate.

A trap primer can also make a short sound. The primer's job is to keep the floor drain's trap full of water. Evaporation, MIFAB explains, "allows sewer gas, pests and bacteria to enter the interior of the building," so primers are used on drains that rarely see water. Different primers fire in different ways:

• Electronic primers run on a timer. Zurn's Z1020 unit is factory programmed to inject water every 24 hours, with a default spray time of 6 seconds. One brief burst a day is the design.
• Pressure-drop primers fire when a nearby valve or faucet on the same cold line opens. MIFAB says the M-500 type "only requires a 3 PSI line pressure drop to activate," so running a sink or flushing can trigger a brief trickle.
• Flush valve primers sit below a flush valve and send a measured amount of the flush water to the floor drain trap. You hear them when the toilet or urinal flushes.

The code treats a primer as one of several ways to protect the seal. IPC Section 1002.4.1 requires trap seals "subject to evaporation" to be protected, and 1002.4.1.5 lets a lavatory or hand sink drain in the same room serve as the primer for an emergency floor drain, trench drain or floor sink. Hearing that lavatory drain run into the floor drain is the design working.

Anything steady, lasting more than a minute or two with no one using a fixture. The usual causes:

1. 1A flush valve or urinal that does not shut off. Commercial flush valves fail by running on after the flush. Our guide to why a Sloan flushometer keeps running covers the diaphragm and bypass causes. EPA notes that some older urinals use as much as five times the federal standard of 1.0 gallon per flush, so a stuck valve adds up quickly.
2. 2A continuous-flow trap primer that stuck open. MIFAB says these primers lack any way to screen debris, which "often results in their clogging and either flowing water continuously, or not at all." Water that never stops at the floor drain can be this.
3. 3A leaking fixture on the shared drain. A toilet flapper, a faucet or a mop sink valve that drips constantly will keep a thin stream running down the line. EPA describes the symptom plainly: if a water meter changes during a period when nothing should be using water, "you probably have a leak."
4. 4Equipment draining nearby. An ice machine, drinking fountain or condensate line that discharges to the same drain adds its own sound. Check what is connected before assuming a leak.

Work from simple to technical.

1. 1Listen at the drain with the restroom empty, then flush each fixture one at a time and note which flush produces the sound.
2. 2Look at every toilet and urinal for water still moving in the bowl after a flush. Close the stop on any that keep running.
3. 3Shut off the ice machine and equipment drains for a few minutes and see if the sound changes.
4. 4Check the water meter before and after a stretch when the building is unused, as EPA suggests for leaks. A meter that keeps moving means water is still flowing somewhere.
5. 5If you have a trap primer, find it and see whether it is the timed, pressure-drop or flush-valve type. A unit that never stops is faulty.

If the water is still unexplained, a plumber can trace the line and watch what the drain does with a camera. A camera inspection shows whether the flow is a fixture or a leak in the line itself, and a camera inspection report documents what was seen.

Water that runs through a stuck valve or a leaking fixture still passes the meter, so it shows up on the water bill. Separately, a failed primer is how a floor drain ends up dry. A dry trap lets sewer odor into a restroom; see why commercial floor drains need a trap primer and dry floor drains and sewer smell.

HQ is licensed and insured and gives free estimates on commercial restroom and floor drain problems, with upfront pricing before work starts.