A "Double-Looped" vs. Gridded System in Attic?

3/17/2025

For a fire sprinkler system, if you have a loop running around a typical hip-roof attic, and you have a center riser, can you tie it into the loop in two locations?

NFPA 13 doesn't really discuss when a looped system turns into a gridded system.

None of the branch lines would be connected, so in my opinion this would be acceptable as long as you meet the discharge time through the remote inspector's test connection.

What are your thoughts? Is this a gridded or double-loop systems?

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9 Comments

Pete H

3/17/2025 06:33:27 am

As long as no branch line is directly fed from two sources, I would say it is not a grid and still is just a weird "double loop".

I agree with you that the major test would be discharge time at the inspector's test connection.

I am basing this on:

NFPA 13 (2016)
3.4.6 -Gridded Sprinkler System. A sprinkler system in which parallel cross mains are connected by multiple branch lines, causing an operating sprinkler to receive water from both ends of its branch lines while other branch lines help transfer water between cross mains.

3.4.7 - Looped Sprinkler System. A sprinkler system in which multiple cross mains are tied together so as to provide more than one path for water to flow to an operating sprinkler and branch lines are not tied together.

Anthony

3/17/2025 08:06:34 am

I'd argue that you have a "compound main" in a looped system. As long as there isn't a way for air to get trapped then you should be fine.

Loops become grids by virtue of the sprinklers on the branch lines connecting form main to main. if the sprinklered pipes don't connect main to main then you still have a looped system.

Casey Milhorn

3/17/2025 08:13:02 am

Yeah, probably stretching it a little bit. Many moons ago I had several projects where it made sense to create a double loop setup in a dry attic scenario. From memory, they varied from 500 gallons to 900 gallons. We ended up with accelerators AND exhausters on a few of them, and on one in particular we installed check valves at the fork on the first loop, just to meet water delivery times. Basically that allowed one side to be the path of least resistance and trapping the air in the other half of the loop, which kept it from being filled by water during the initial trip. Might be worth running this through Tyco's water delivery time program, but I'm not sure it can handle a double loop? Good luck!

Dave L.

3/17/2025 10:53:07 am

Ha, also done it (check valves) many moons ago. Now NFPA requires such check valves to be located in a heated enclosure and 1/8 in. dia. hole drilled in the clapper. (8.2.4.6 in the 2025 ed.)

Glenn Berger

3/17/2025 08:14:25 am

Yes, you can tie your riser into the loop at multiple locations. However, you did not indicate that the system is/was wet-pipe or dry-pipe. Since you asked about gridded systems, there is some implications that you do have a dry system and then you just need to ensure that air can be exhausted from the system and will not cause additional time delays.

The "double-looped" configuration is commonly used to move the most remote area to achieve more favorable calculations.

Dan Wilder

3/17/2025 08:27:45 am

As you discuss discharge (delivery) time, I'm thinking this is a dry (likely) or preaction (not likely) system.

By the letter, tying in mains together creates a loop/double loop/compound system that is acceptable within a dry or preaction system. The restriction is when tying in branch lines is the restricted means of pipng.

With that, the most I have been able to install and get signoff from, is a single compound loop (figure "8" is you will). Plan review and inspectors are not as approving, even with items like an NFSA EOD/AFSA or NFPA Tech Question response that typically is not firm in the affirmative for this situation.

As the FDT program can't handle a compound system for calcs after a point, that only leaves field testing after install which is a huge $ liability vs just making a second valve and splitting the system at the design start.

Jack G

3/17/2025 11:50:31 am

A loop is allowed( 2 paths) a grid is not ( more than 2 paths).
You should probably calculate “ shifts “ —-calculating adjacent branches to determining the most hydraulically remote area, ( like you would do for a gridded branch line ) . I ve found the physically remote is not always the hydraulic remote area.
Air will definitely be pushed and pulled thru the loops , and will cause a sputter ( direct flow/ stream will take much longer)
I would install checks ( even though they are not required ) with drum drips to cut down on the sputter and trip time.
As an apprentice many moons ago, I recall tripping/ testing a new looped dry system. ( my first one actually) We heard a huge whoosh. ( about 95 psi street pressure)
The first office drop was not installed. I had enough time to pick up a drop, wrench, and ladder, and ran to it— and installed it , hand tight before there was water. Now I was much quicker—— but it did take a while for the water to get there. So the inspectors test time was less than a minute, but the head fairly close to the feed that created the loop took about 3.5 minutes I had determined at the time)
I believe your trip time will be greatly increased.

Robert Hughes

3/17/2025 12:53:44 pm

It is a looped system with two feeds.
Have as many feeds as necessary to facilitate calcs, timing, whatever. Only redundand when unnecessary.

Jesse

3/17/2025 06:48:41 pm

Agree this being a "loop" and not being a grid. I'd be interested to know the overall size of the system and the trip time.

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A "Double-Looped" vs. Gridded System in Attic?

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