Facts to Know About Water Meters and Fire Sprinklers

More fire sprinkler systems are being installed in residential homes, making it vital to understand how water system components function alongside them.
Facts to Know About Water Meters and Fire Sprinklers
Residential water meter

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More fire sprinkler systems are being installed in one- and two-family homes and townhouses, so it’s important to understand their requirements. It’s also important to understand the limitations of common components installed in the water supplies to these systems, such as pressure reducing valves, check valves and water meters.

The skinny on water meters

A water meter typically determines water usage by measuring displacement or velocity, and the technologies used for these two techniques vary.

The most common meter used in residential applications is the positive displacement (PD) meter. While PD meters are very accurate for smaller sizes, such as 5/8-inch and 3/4-inch, that accuracy tends to decline as the meter size increases to 1-inch and larger. Due to this decrease in accuracy, water purveyors tend to prefer the smaller meter sizes to avoid lost water and revenue (typically from slow leaks or running toilets).

Meter setters or yokes in fire safety systems

The purpose of meter setters or yokes in a meter assembly is to elevate the meter to a height that makes it more convenient for reading and servicing. While helpful for water purveyors, they can be troublesome for fire sprinkler systems. They can have very small pipe or tubing incorporated into their makeup which, on its own, can seriously restrict the pressure and flow. On top of that, there are multiple options for fittings and valves that can be used in a meter setter or yoke, almost all of which are small sizes and have significant pressure losses as well.

Depending on the components used and the manufacturer, a 3/4-inch meter setter can lose as much as 25 psi of pressure at standard residential sprinkler system flow demands. As residential sprinkler system installations continue to increase, the use of these devices will need to be seriously reconsidered. Alternative means to achieve the same results will be imperative.

Intermittent and continuous flow

PD meters have very similar characteristics across brands and models. Most have similar ratings for flow restrictions based on American Water Works Association (AWWA) standards.

That said, there are two important numbers regarding the flows available from any residential water meter: 1.) the safe, maximum “intermittent” operating range; and 2.) the recommended maximum rate for continuous flow. The maximum intermittent flow listed is for brief periods of flow. The typical 10-minute duration expected for a residential sprinkler flow, or even 30 minutes of a higher flow, fall well within the “intermittent” category.

Therefore, when evaluating water meters for a fire sprinkler system, look at the maximum intermittent flow rate, not the continuous one. While there can be some variation, the most common maximum flow rates for each size of meter are: 20 gpm for a 5/8-inch or 5/8-inch by 3/4-inch meter; 30 gpm for a 3/4-inch meter; and 50 gpm for a 1-inch meter.

Flow rates with residential sprinkler systems

The National Fire Protection Association (NFPA) 13D Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes requires the system be sized to provide flow for the two most hydraulically demanding sprinklers located in the same compartment.

The most commonly used residential sprinklers are 4.9 K-factor pendant sprinklers. The minimum pressure and flow for this sprinkler is 7 psi at 13 gpm for spacing up to 16 feet by 16 feet. So the most common demand for two sprinklers flowing is going to be 26 gpm. Of course, this can vary depending on the requirements of the system and the person doing the design and hydraulic calculations. Increasing the spacing of the sprinklers or using sidewall sprinklers can increase the flow demand significantly beyond 26 gpm.

On the other side of the spectrum, there are low-flow sprinklers with smaller K-factors that will protect 12-foot by 12-foot or 14-foot by 14-foot areas with flows as little as 8 gpm and 10 gpm. This does well at reducing the flow to as little as 16 gpm but will sometimes mean a significant increase in the number of sprinklers and, thus, the cost of the system.

If residential sprinkler systems are going to gain wider acceptance, it is paramount to keep the costs low without sacrificing effectiveness. This is the driving force behind many of the requirements in 13D.

Putting it all together

Let’s put the system demand information together with the maximum flow rate of residential water meters. A typical demand of 26 gpm would be perfectly fine with both a 3/4-inch meter and a 1-inch meter, but it exceeds the maximum flow of a 5/8-inch meter.

A solution would be to use low-flow sprinkler heads to lower the system demand at or below 20 gpm. In many parts of the country, this is a great solution as long as the increase in the number of sprinklers is not excessive.

In some cases, such as two or more dwelling units sharing a supply line and meter or throughout the state of California, a problem arises. In both of these situations, an additional 5 gpm must be applied to the underground supply. (Shared supplies require the additional gpm at the point between the water source and where they split, while the state of California requires the entire supply.)

This additional 5 gpm puts the total system demand through the meter at a bare minimum of 21 gpm, which exceeds the maximum allowed rate of a 5/8-inch meter and will require sprinklers to be spaced no more than 12-foot by 12-foot apart. On that same line of thought, the “standard” system demand of 26 gpm could cause issues in the instances where 5 gpm has to be added, as it puts the total demand at 31 gpm, which exceeds most 3/4-inch meter maximum flow rates.

Is this a problem or not?

Research has been done by the Fire Protection Research Foundation (FPRF) of the NFPA to prove that meters are capable of flowing well beyond their published maximum rate of flow. In the report “Residential Fire Sprinklers — Water Usage and Water Meter Performance Study,” published in February 2011, the general conclusion is: It’s perfectly fine to exceed the maximum flow rate during the control of a fire event, as long as the appropriate amount of pressure loss is factored into the hydraulic calculations.

What if the unthinkable happens?

Consider this scenario: A single-family home is fully protected with a 100 percent compliant NFPA 13D sprinkler system. In the middle of the night, a fire breaks out and manages to grow beyond what the system is capable of controlling. Maybe a sprinkler was damaged at some point during or after installation. Maybe the homeowners inadvertently put up something that obstructed the sprinkler closest to the fire, allowing it to grow out of control.

Then the unthinkable happens. Someone dies in the fire.

Scott Futrell of Futrell Fire Consult & Design Inc. has extensive experience with investigating and inspecting fire protection systems before and after system failures.

“Investigators are going to look at any equipment used outside of its listing and would likely flag the use of a meter beyond its listing. You can certainly cite the paper published by the FPRF as evidence for the use of the meter beyond that maximum-rated flow, but it’s not a guarantee,” says Futrell.

In fact, an investigation into a fire death will typically flag and thoroughly look into many possible reasons for the system failure and all the possible parties involved will be interviewed, including the component manufacturers.

“Manufacturers are definitely going to distance themselves and will never recognize the use of their device beyond what is stated in their literature,” says Futrell.

Call any meter company technical support line and this will quickly become apparent. No matter what supporting evidence you provide from studies showing meters can flow more than what is published, the support staff will never admit this is true.

The issue of liability

This brings us to what ties everything together — liability. Who is responsible? Contractors, designers, engineers, etc. all carry insurance to protect themselves from liability in case something goes wrong.

They could argue that the Authority Having Jurisdiction (AHJ) was the approving authority: that it approved the plans showing the meter and the flow rates. Shouldn’t it be held responsible?

Some would say yes. Others would tell you it will almost never fall back on the AHJ. Whether you’re the designer, engineer, installer or someone else, the big question is going to be risk versus reward. Is the possibility of a system failure low enough to save money and go with a smaller meter? Money usually ends up being the deciding factor. Water purveyors tend to charge large fees to upsize water meters from their standard installation sizes. Having a standardized water tap size and meter helps keep costs down and means purveyors can minimize the amount of inventory on hand as well as the tools needed for installation. The larger-sized meters start to lose efficiency, which can result in unaccounted for water and lost fees.

The good news is that some very viable options for water meters can keep almost every party involved happy (and protected). Most important to know is that a 1-inch meter is not always necessary for a residential sprinkler system. In fact, meters larger than 1 inch are almost never necessary for residential applications. Standard 3/4-inch meters are more than capable of handling the flow of a typical 13D sprinkler system; and when you use the manufacturer’s published pressure-loss charts, they don’t even have a significantly higher pressure drop as compared to their 1-inch counterparts.

One manufacturer of a commonly used meter actually makes a 5/8-inch and a 5/8-inch by 3/4-inch meter, rated at a maximum flow of 25 gpm; as well as a straight 3/4-inch meter, rated at 35 gpm. Other manufacturers are pursuing advancements in metering technologies and are also able to allow for higher flows than older-style meters.

As technologies continue to improve and meter efficiencies increase, even for the larger-size meters, many of these problems will go away. We can just hope that water purveyors and AHJs have the correct information and are willing to expand their options enough to provide a reliable water source for these important, life-safety sprinkler systems.

About the Author
Melisa Rodriguez is the supervisor for Fire Safety Design Services at Uponor. She is a professional engineer with more than 10 years in the industry and an NICET Level IV certification in fire sprinkler design. She received her bachelor’s degree in mechanical engineering from the University of Minnesota and is a professional member of the National Fire Sprinkler Association (NFSA), the National Fire Protection Association (NFPA) and the American Fire Sprinkler Association (AFSA). She is also the vice chair of the Communications Subcommittee for the NFSA Future Leadership Committee, an alternate member of the NFPA Residential Technical Committee, and the Governor’s Council liaison for the Minnesota Chapter of the Society of Fire Protection Engineers. She can be reached at melisa.rodriguez@uponor.com.



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