Douglas Krantz - Technical Writer - Describing How It Works

Why Are There False Waterflow Alarms?

Even though a waterflow alarm is caused by water flowing into a sprinkler system, a false alarm is usually caused by water leaking out of the sprinkler system. Because water has leaked out of the system, water coming back in will look like a waterflow, causing an alarm.

A check valve allows water to flow freely in one direction, and is supposed to prevent any water from flowing the other direction
Standard in a sprinker system is a check valve. The check valve allows the water to enter the sprinkler system, and is supposed to block the water from going back. However, a piece of rust can prevent the valve from fully closing, allowing the water to slowly leak back.

By Douglas Krantz

The question was asked "Intermittently, we are having a false waterflow alarm problem. For a while, it was OK, but we did the annual waterflow test and since then we get at least once a week a false water flow alarm --- always around 2AM. What can we do?"

False Waterflow Alarms are the Symptom

Waterflow alarms are usually caused by water flowing past the waterflow switch. The problem, though, is not really with water being pushed past the waterflow switch; the problem is with leaks in the system combined with changing water pressures from the city and inside the building.

City Water Pressure

Outside the building, the water pressure inside the city water pipes is not steady; it's always changing. Things like greater daytime water use by neighborhood residents, daytime industrial water use, and even hydrant testing by the fire department decreases the daytime city water pressure.

Reducing this water use at night increases city water pressure.

Air Tank

The sprinkler system inside the building is a bunch of pipes used to carry water from the fire sprinkler room to all parts of the building, but it's also a long, thin air pressure tank. It's an air pressure tank because when servicing or testing, air has to be let in to let the water out of the sprinkler system.

Unless someone takes the time to "bleed" the air out of the of the high points of the sprinkler system, the air never comes out. Often, at the top of the sprinkler risers, bleeder valves aren't even installed; air just stays in the system because it can't be bled out.

Leaks Will Lower the Pressure

Inside the sprinkler system, the pressure can change. Put more water in, the air is compressed more and the pressure increases; take water or air out (through leaks), the air is compressed less and the pressure decreases.

The leaks can be in:

There could also be a problem with the alarm retarder (time delay) inside the waterflow switch.


At the beginning of the sprinkler system is a check-valve. This valve allows water to flow into the sprinkler system and should not allow water to flow back out. In essence, the check-valve should keep the pressure inside the sprinkler system at the highest level that city water pressure ever reaches. Once the building's sprinkler system attains that higher city water pressure, no water should, in theory, be pushed into the sprinkler system.

Remember though, the problem with regular false waterflow alarms is not with water getting into the sprinkler system, the problem with regular false waterflow alarms is with water leaking out of the sprinkler system.

If the check-valve leaks water back out of the sprinkler system, even at a slow rate, when the city water pressure increases, water flows past the waterflow switch to replace the leaked-out water.

Without taking the check-valve apart, however, the leakage problem inside the check valve is difficult to confirm.

Drain Valve

This valve is only opened for testing and maintenance of the sprinkler system. If, after testing, the drain valve leaks, it slowly lets water out of the system. When the city water pressure increases at night, water to replace what leaked is pushed past the waterflow switch.

A leak in a drain valve is easy to confirm; see if water is dripping out of the drain pipe. Even a small leak here can be important.

Normal Leaks

No sprinkler system is perfect; all sprinkler systems leak --- at least for a little while.

For instance, air can leak out of micro-cracks and junctions in the pipes, and the pressure will slowly drop. After the air has fully leaked out, though, this is no longer a problem.

Retarder - Pneumatic Dashpot Time Delay

An air damper or dashpot is built into common fire alarm waterflow switches. So a short term pressure change won't set off the alarms, the dashpot delays the activation of waterflow alarms. It's not a leakage problem, but mis-adjustment of the dashpot has caused a lot of false waterflow alarms.

The dashpot retarder can be adjusted to delay the waterflow switch alarms up to 90 seconds. Because water has to flow to set the time delay on the dashpot, usually the sprinkler service company adjusts these. If there's a question on how the time delay should be, the AHJ (Authority Having Jurisdiction) or Fire Marshal should be consulted.

Long Term Problems with the Dashpot

The pneumatic (air damper) dashpot is a rubber diaphragm, a small air chamber, and an adjustable air valve. Over years of time, the rubber diaphragm will dry out so it leaks air, decreasing the time delay. If the retarder is set for too short of time in the first place, or if the rubber is drying out, the retarder won't delay alarms long enough for short bursts of water and the 2:00 AM false alarms will occur.

Logging Pressure Gauge

As a last resort for troubleshooting, install a Logging Pressure Gauge in the sprinkler system. Over time, it will show what is happing to the pressure. If throughout the day or week the pressure is slowly decreasing and at 2:00 AM on a regular basis, the pressure suddenly increases the water or air has to be leaking out.

Water Movement

Remember, in most cases, the problem with regular false waterflow alarms, isn't with water flowing; that's just replacement water. In most cases, the problem with regular false waterflow alarms is with water or air leaking out of the sprinkler system; replacement water is required to make up for the leaks.
Life Safety
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