Dead-End Length in a Fire/Domestic 13D System?

2/15/2024

For NFPA 13D systems which are "multipurpose" or "flow-through" combination sprinkler and domestic systems, it has been troubling me as a fire code official not knowing the maximum length that a dead-end or arm-over section of pipe can be in order to be deemed safe from contaminating the potable water in a home.

NFPA doesn't seem to weigh in on this, that I know of.

What is the maximum length for a dead-end or arm-over section of pipe in a combined sprinkler/domestic system?

I've been searching for studies, contacting other AHJs, asking plumbers; but no one can give a definitive answer.

From what I understand, there are no documented cases of illness from residential sprinkler systems, but I would rather not rely on this as a way to measure safety. I've seen a few sources say (and show) that water only circulates into the dead end at 1.5 x the diameter of the pipe going across. So if water isn't circulating, it seems like there's a risk for contamination.

If contamination is not a risk for whatever reason, why have any maximum at all?

I would love to get to the bottom of this and I look forward to hearing from others who have a deeper understanding. Thank you.

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4 Comments

Ivan Humberson

2/15/2024 12:27:09 pm

I am quite certain NFPA codes do not regulate that aspect. I would think that such requirement would be up to the local plumbing code. That is typically where you will find maximum dead-end lenghts in potable water piping.

Joe F

2/15/2024 01:15:47 pm

Great Question we all would like to know the science behind this requirement.
Our local AHJ's in Western Washington limit the dead-end arm-overs or branch lines lengths to a maximum of 36". It is unknown where this distance originated from.
Our flow through designs typically consists of connecting to or suppling the cold water to each toilet, and some jurisdictions even require the fire sprinkler system to supply the cold water to the washing machine for additional purging of any stagnate water.
The additional piping cost often outweighs the savings on installing a backflow preventer and designing a standalone wet system.
We prefer flow through/ passive purge systems because it will prevent accidental shutting off of the standalone fire sprinkler systems. However, the dead-end limitations can become a design and installation nightmare on some larger custom homes.
We hope someone can provide proof of a documented problem with longer dead-end lines or will complete an inadept study on this issue.

2/15/2024 02:43:24 pm

It seems that it would be prudent for all stakeholders involved with these systems to push for a scientific study to determine what the answer really is. I wonder what it would take to have this done so that it could be added to the code. I'm actually surprised that it hasn't been done already.

Mark Cacchione

2/21/2024 09:23:32 am

In Illinois, at least in my jurisdiction, this is covered by the Illinois State Plumbing Code because it is part of the domestic system. According to our Plumbing Inspectors the maximum length of a dead-end in a system not protected with a backflow preventer is 2 feet.

Dead-End Length in a Fire/Domestic 13D System?

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