Douglas Krantz - Technical Writer - Describing How It Works
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Why Does It Seem that Dry Sprinkler Systems Only Show Low Air Pressure At Night?

Compressed air inside the dry sprinkler system pipes is what keeps the clapper valve closed. If the air pressure drops too low, the clapper valve opens and the dry sprinkler system floods with water.
The dry sprinkler system is pressurized with air to keep the water out of the pipes. If the air pressure gets too low, the pipes will flood with water. Before the pressure drops too low, the sprinkler system sends a "Low Air" signal to the fire alarm panel to indicate something needs to be fixed.


By Douglas Krantz

Doesn't it always seem to happen? In the middle of the night, someone's fire alarm fire alarm system sends a supervisory signal to the monitoring company. It's a dry sprinkler system low air signal.

You're a fire alarm service technician, and you're on call.

I know. It's the sprinkler company's realm to fix the sprinkler system. The trouble is the supervisory low air signal is a fire alarm signal and you service fire alarm systems - - you get the call.

Turn on the Circuit Breaker

Usually, it's not much that has to be done, but either someone has to go on site or sometime in the near future the dry sprinkler pipes are going to flood. You get dressed, go to another part of the city, flip the circuit breaker back on, do the write up while the compressor fills the air tank, and go home again.

On your way back home you ask yourself, "why does it seem that the supervisory low air signal only occurs at night?"

Temperature Variations

Well, it's not just your imagination. If you kept statistics, you'd discover that by far, the greatest number of times the sprinkler low air signal is sent to the monitoring company really is at night, usually after 11 PM.

There's a reason for this, and it's not because the sprinkler system has it in for you; it's because of variations in outside air temperature.

It's warmer during the day and cooler at night and the dry sprinkler system is often exposed to these up and down outside air temperatures.

The dry pipe also has small air leaks. All dry systems have small leaks or they wouldn't need air compressors to replenish the air on a regular basis. Both day and night, these leaks slowly let air out of the dry sprinkler system.

Day --- Pressure Up; Night --- Pressure Down

During the day, inside the dry pipe, the pressure goes up because the temperature of the air goes up. During the day, even though the temperature has gone up, the air leaks have prevented the air pressure from going up very far. During the day, the compressor isn't needed.

At night, inside the dry pipe, the pressure goes down because the temperature of the air goes down. At night, as the temperature goes down, the air leaks from both day and night push the pressure down faster. At night, the compressor is needed. br>
Here's the problem. If it's at night that the compressor is really needed, and the compressor doesn't work for whatever reason, it'll be at night that the supervisory low air signal is going to be sent to the monitoring company.

Now you know. Sending in a supervisory low air signal only at night to wake you out of a sound sleep, it's not the dry sprinkler system that has it in for you; it's the cool nighttime air that has it in for you.

Post Script

I know, there are other causes for the low air signals than just a blown breaker, but that seems to be the most common.

Also, flipping on the breaker doesn't always fix the problem, but if you make enough 2 AM overtime calls, the owner has incentive to get the problem fixed.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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