Water Supplies for Sprinkler Systems

Sprinkler Systems Quick Courses



Automatic sprinklers are the most reliable and effective fire protection devices available today — provided they operate correctly. To determine if a sprinkler system can adequately protect a property, underwriters should be familiar with the components, operation, and testing of the systems, as well as potential causes for failure.

A sprinkler system consists of the following components:

  • A water supply and fire department connection
  • Underground and aboveground pipes
  • Valves and drains
  • Alarms and test connections

Every sprinkler system requires at least one automatic water supply of adequate pressure, capacity, and reliability. The water supply needs to be capable of supplying all sprinklers that operate during a fire and providing water for fire department operations during the fire.


Types of water supplies

Water supplies to sprinkler systems can consist of one or more supply sources:

  • Public/private mains
  • Elevated storage tanks (gravity tanks)
  • Ground-level suction tanks
  • Embankment-supported fabric tanks
  • Static storage—such as a lake, pond, or reservoir—with pumps
  • Pressure tanks

A hydrant connects to public water supply

Static storage for an office building

Public/private water supply mains

The most common and least expensive water source for sprinkler systems is the public water supply system. If the public water supply is the only source of water for the sprinkler system, it must be reliable and maintain adequate pressure and volume at all times.

To establish the capabilities of water supply systems, Verisk field representatives or other designated officials can conduct hydrant flow tests. The objective of the flow test is to determine the quantity in gallons per minute (gpm) and pressure available at a specific location. We can conduct the tests during public protection gradings, or the local fire department or water company can conduct the tests. The testing process can identify closed valves and changes in the carrying capacity of mains.

A fire hydrant connects to the public water supply system

A fire hydrant connects to the public water supply system

Elevated storage tanks

An elevated storage tank (or gravity tank) of adequate capacity and elevation is an acceptable source of water for a sprinkler system. The capacity required depends on the intended use of the tank and the number of gallons available from the tank. Standard sizes of elevated steel gravity tanks range from 30,000 to 500,000 gallons.

It’s very important that water stored in the tank doesn’t freeze. During cold weather, water in the elevated tank may become unavailable because of an ice plug in the riser. Such a plug could burst the pipe.

There are three basic methods of heating tank water:

  • Gravity circulation of hot water
  • Steam coils inside the tank
  • Direct discharge of steam into the water

The type of heating method depends on the tank’s height, construction, size, and shape—and on the lowest temperature of exposure.


Ground-level suction tanks

Suction tanks supplying fire pumps are another acceptable source of water for a sprinkler system. A fire protection engineer or similar expert sizes the tank based on the total volume of water required to meet the demands of the fire sprinkler system for the duration of time the engineer expects the sprinklers to operate. The engineer measures the demand of the sprinkler system in gallons per minute and the duration in minutes. If there's a way for the fire department to tap into the supply, the engineer adds that volume of water to the tank size. If a fire pump pressurizes the system, the pressure requirements of the fire sprinkler system and the fire department’s use are primary considerations for pump selection.

Suction tanks are usually made of steel, and standard sizes range from 50,000 to 1,000,000 gallons. To prevent freezing and subsequent pump failure, such tanks circulate heated water or use embedded coils.

Embankment-supported fabric tanks

Embankment-supported fabric tanks can also function as suction tanks. The tank is usually composed of a reservoir liner with an integral flexible roof. A specifically prepared excavation supports the tank with embankments of earthen berms.

Tank capacity ranges up to 1,000,000 gallons in increments of 100,000 gallons. The tank can be located underground with the top of the tank at grade level or above ground with the earthen berms supporting the entire tank. To prevent freezing, many of the tanks use a water recirculation system with a heat exchanger.

Static storage with pumps

Water for sprinkler systems can also come from natural or man-made bodies of water, such as wells, lakes, ponds, or reservoirs. Those static sources need to deliver the water in the volume and duration required by the system and the anticipated hose streams. And the water must be available 24x7x365.

To ensure that water is available for firefighting, a hydraulic engineer evaluates the water source. The engineer conducts a detailed study of the water source to determine if the required volume and capacity will be available, even during a 50-year drought in the area.

If the water comes from a dynamic source, such as a well, engineers must “proof” the water. They conduct tests to ensure that the water supply can keep up with the demand and draw of the fire pump. Safeguards must also be in place to make sure the pump won’t clog with debris, fish, or silt.

Hydraulic engineer evaluating the water source

Water supply source for sprinkler system

Pressure tanks

Pressurized water tanks also supply water to sprinkler systems. They're often located in rural locations where city water is unavailable and the demand on the sprinkler system is relatively light. Tank capacity is generally 5,000 to 20,000 gallons. Two-thirds of the tank is water, and one-third is air or inert gas. The air pressure in the tank is approximately 100 psi (pound-force per square inch).

Pressure tanks have very limited capacity and are designed to suppress room fires. Once the water in the tank is gone, there's no more water available to the sprinkler system. Well pumps, which don't have sufficient capacity to keep the sprinkler system flowing at the designed rates, usually fill the tanks. Monitoring of the water supply is critical. For a pressure tank to work properly, it's important to monitor and maintain tank water level, water temperature, and air pressure.

Ensuring adequate water pressure

In addition to a water supply that has the capacity to supply all sprinklers that may activate in a fire, the water supply must flow at an adequate pressure.

In locations where the water pressure isn‘t high enough to provide for fire protection needs or no public water supply exists, a fire pump boosts the water pressure. Fire pumps don‘t create water but simply move the water through the pipe more quickly because of the pressure increase they develop.

Electric motors or diesel engines can power fire pumps. Pumps operate automatically and should contain alarms indicating whether the pump is running or has become inoperative because of power failure or other conditions.

The design of fire pumps is specifically for fire service. Each pump has a nameplate that states the rated flow (gpm), rated pressure (psi), rated speed, and other information specific to that pump.

Fire pump powered by an electric motor
(Photo courtesy of APi Group, Inc.)

Fire pump powered by a diesel engine

Fire department connections

Sprinkler systems may require additional water pressure during a fire. Fire department connections, also called Siamese connections, are located on the outside wall of the building or near the street. Such a connection allows the fire department to connect its pumper directly into the building’s sprinkler system to increase water pressure and add water to the system. In a larger fire with many sprinklers operating, that would help maintain higher pressure to the activated sprinklers.


Fire department connections must be:

  • Easily located
  • Accessible for use by the fire department
  • Clear of debris
  • Properly connected to the sprinkler system
  • Maintained in working order at all times

Fire department connection with caps in place

Fire department connection without caps and a bottle in a hose connection

Water supplies for sprinkler systems summary

A well-designed sprinkler system is a highly effective method of minimizing property damage from a fire. But simply installing a sprinkler system doesn't guarantee adequate fire protection. Water supply is a critical component.

For example:

  • A reliable water supply is required for the system to be effective.
  • A water supply needs appropriate capacity, quality, pressure, and temperature.
  • Regular maintenance and testing of water supply sources is vital to ensure that the water supply is adequate at all times.

Verisk has trained field staff to evaluate the design, installation, and condition of sprinkler systems anywhere in the country.

Report: Sprinkler Assessment Report

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