Choosing a Water Supply Source for a Residential Sprinkler System

The water-supply source should be established early in residential sprinkler design because it determines how the required flow, pressure, and duration will be provided. Residential sprinkler systems commonly use municipal water, onsite wells, or storage tanks with pumps.

Each source can support a residential sprinkler system, but each creates a different design problem. Municipal service is usually driven by the available water-supply characteristics and pressure losses through the service. Well systems require consideration of sustained well output and available pressure. Tank-and-pump systems require enough stored water for the required duration along with a pump capable of providing the required flow and pressure.

Municipal Water Supply

A municipal connection is typically the most straightforward source from a duration standpoint because the water supply is continuously available. The main design question is whether the service can deliver the required flow at sufficient pressure after losses through the underground service, meter, backflow equipment, elevation, and sprinkler piping are accounted for.

For example, a home with approximately 60 psi of static pressure may still have a tight hydraulic condition if the residual pressure is significantly lower or substantial pressure is lost through the service and sprinkler system. Depending on the available water supply and the calculated demand, additional pressure may be needed even though the home is connected to municipal water.

A municipal supply therefore generally removes the need to provide a dedicated stored volume for the required operating duration, but it does not remove the hydraulic design problem. The designer still needs to confirm that the available water supply can satisfy the required flow and pressure.

Well Supply and Supplemental Storage

A well introduces a different question: how much water can the well sustain while the sprinkler system is operating? A well does not automatically require a storage tank. If it can provide the required sprinkler flow at sufficient pressure for the required duration, it may be able to supply the system directly.

If the well cannot provide the full sprinkler demand continuously, it may still contribute part of the required flow while a storage tank supplies the difference.

For example, if the sprinkler demand is 26 gpm and the well can sustain 12 gpm, the remaining demand is 14 gpm. For a 10-minute duration, the supplemental water volume would be:

(26 gpm − 12 gpm) × 10 minutes = 140 gallons

This makes the sustained output of the well an important design variable. A well that can provide more of the required flow reduces the amount of supplemental water that needs to come from storage. If the well contributes little or no usable flow during sprinkler operation, the stored water may need to provide most or all of the sprinkler demand.

Storage Tank and Pump Systems

Properties without an adequate municipal supply or with limited well capacity may use a storage tank and pump as the primary sprinkler water supply. In this arrangement, the tank provides the required water volume while the pump provides the flow and pressure needed by the sprinkler system.

A storage tank may serve both the domestic and sprinkler systems, or a dedicated tank may be provided for the sprinkler system. The appropriate arrangement depends on the sprinkler demand, available water source, domestic water arrangement, required duration, and local requirements.

The important design consideration is the amount of water actually available to the sprinkler system. Tank capacity by itself does not establish the usable sprinkler supply if part of that volume is reserved for another purpose or otherwise unavailable to the sprinkler system.

Duration Can Control the Tank Size

The required operating duration directly affects the amount of stored water needed. NFPA 13D commonly requires a 10-minute water supply duration, with a reduced duration permitted for certain qualifying dwelling units. Local requirements can also affect the required duration and should be confirmed for the project.

For example, a 26 gpm sprinkler demand supplied entirely from storage would require 260 gallons for a 10-minute duration:

26 gpm × 10 minutes = 260 gallons

If the same 26 gpm demand had to be maintained for 30 minutes, the required water volume would increase to 780 gallons:

26 gpm × 30 minutes = 780 gallons

The sprinkler demand has not changed, but tripling the required duration triples the amount of water that must be supplied.

For a municipal system, a longer duration may not require a physical change to the water supply if the municipal service can continue to provide the required flow and pressure. For a well or tank system, however, a longer duration can require additional storage or other changes to the water-supply design.

Match the Source to the Project Conditions

The water-supply design should be based on the actual conditions of the project. For a municipal supply, establish the available water-supply characteristics and account for the pressure losses between the source and the sprinklers. For a well, determine the sustained output and available pressure rather than assuming that storage is automatically required. For a tank-and-pump system, determine the required water volume and confirm that the pump can provide the necessary flow and pressure.

The same residential sprinkler requirements can result in very different water-supply arrangements depending on the home, available water source, sprinkler demand, and local requirements. Establishing the source early gives the designer a realistic basis for the hydraulic design that follows.