Use One 20x20 Sprinkler in NFPA 13R Calculation?

2/14/2023

The installation contractor and I have had an argument about how sprinklers should be spaced and calculated in a 13R system.

I have argued that per NFPA 13R Section 7.1.1.3.1, we should calculate 3 sprinklers in the greatest hydraulic demand area. This area is furthest from the riser and the calculation permits 16x16-ft spacing for the area that has the most sprinklers.

Section 7.1.1.3.1:
For each of the following situations, the number of sprinklers in the design area shall be all of the sprinklers within a compartment, up to a maximum of four sprinklers, that require the greatest hydraulic demand:

The contractor wants to remove a sprinkler in a single room and move a sprinkler 10 feet off of the exterior wall, but in NFPA 13R makes no mention of a single sprinkler proof calculation that could justify such a move.

There is no mention of a single sprinkler calculation in NFPA 13R permitting a spacing of 20x20, and so all sprinklers on the project can be only 16x16 if the greatest hydraulic demand is permitting it.

In NFPA 13D, it makes no mention of a single sprinkler calculation either, but does refer to single sprinkler operations in A.10.1. I did not feel comfortable designing a sprinkler system that was not up to code standards and need an outside opinion on this matter. Thanks in advance.

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10 Comments

Peter Howard

2/14/2023 06:35:55 am

Why can't a residential head that is listed for 20 x 20 spacing (at the correct pressures) be calculated for 20 x 20 spacing?

Ultimately, this does mean that if you have any compartments with more than one sprinkler in it on the same floor, that likely is now your more demanding compartment as it will require more sprinklers to activate.

But 13R's calculation requirements don't dictate sprinkler spacing, the listing of the residential head does.

Anthony

2/14/2023 07:20:06 am

Yes you definitely can calculate only one sprinkler if there is only one sprinkler in the compartment.

You should also calculate several areas if you have several different design bases.13D and 13R use compartment based hydraulic analyses not overall area.

So for your example you would prove that the 20x 20 room works and the 16x16 room works. heck, you can calculate every room for the most efficient design (don't though).

Final thought, do you have a corridor calc? That might be your most demanding calculation as that would probably require 4 heads.

danefre

2/14/2023 07:44:21 am

Contractor is correct. Yes, you can have a 1 sprinkler calculation in 13R if that's the compartment that produces the greatest demand. Also, very common to reduce number of heads and increase spacing for hydraulic purposes. Each compartment is calculated separately and you can either prove all of them or the ones that logically make the most sense.

Example your kitchen has 2 heads at 14 x 14 but the 3rd floor bedroom with 1 head at 20 x 20 is more demanding. Doesn't mean that the kitchen has to be calculated at 20 x 20. And you should show both calculations because it's not evident which one is more demanding.

I have had reviewers take your side thought - not sure where this approach is coming from... there's nothing in the standard that implies that.

Alex

2/14/2023 08:05:40 am

Yes, you can only calculate a single sprinkler head if only one head is protecting the entire compartment. Like others have mentioned, if you have a compartment on the same floor that is protected by (2) sprinklers, I would run both calcs (1 with 2, the other with the single head) to ensure both designs work.

Jesse

2/14/2023 08:15:24 am

Oh how I wish GCs would stick to GC things and let fire people do fire things.

He's almost right. You can do a single sprinkler calc and you can space it 20x20.

But, and this is big but - this may move the hydraulically remote area. A lot of times those unfamiliar with what we do think "remote" area equals physical distance from the base-of-riser. It does not. Well, not necessarily. He wants to change to aa 1 sprinkler calc presumably because it calculates better. But in doing so, you may shift the hydraulically remote area to another area that has multiple sprinklers in a compartment that prove to be more hydraulically demanding.

Dan

2/14/2023 08:33:16 am

We provide this kind of proof calculation on a normal basis in our residential designs. It allows us to provide layouts for the couple different situations we find during install.

Rooms with 1 sprinkler are designed to 20x20 (or 18x18) so any obstruction (ceiling fans) can be kept to a minimum by creating the largest available space for a single sprinkler to be placed. Additional calculations are provided for the largest room with the most sprinklers (typically it's the 2 or 4 sprinkler calc) at a lesser spacing just due to the obstructions in the middle pushing a 2 sprinkler wide layout. I've not had an issue with plan review when providing both calculation options.

Casey Milhorn

2/14/2023 09:03:34 am

Everyone covered it really well, the only thing I would add is that MAKE SURE you are meeting the definition of a compartment per NFPA 13R. Then yes, a 1 head calc could prove 1 head at 20x20, with the rest being at 16x16.

Joshua Bush

2/14/2023 10:49:41 am

I had a similar issue just last week. I had a contractor place a ceiling fan in the main room of an apartment. Because of this change I had to adjust the head to be just over 9'-0" from the farthest wall. For calculation purposes that sprinkler would demand a higher pressure to work properly. I still had 2 other heads in that same room that were spaced at 16'x16'. So, I kept the pressures required for those heads at the 7.0 PSI. Now I have 1 head with a pressure requirement of 16.7 PSI and 2 heads at 7.0 PSI. My plans were rejected from the reviewer with the reasoning that if I increase the pressure of one head in a remote area calculation, all heads have to be calculated at that higher pressure, even if they are spaced in a way that does not require higher pressures to provide coverage. In the end I was told I had to increase the pressure requirements for all 3 heads to16.7 PSI for the calculation to be correct. I was told that I was not able to pick and choose pressures in a remote area calculation. So from my understanding, if I have a multi head room, I must calculate all sprinklers with the pressure requirements of that single head that requires increased pressure. Does this sound correct? I feel like I am either missing something or the plan reviewer is. I'm happy either way. I'd just like to know which is correct so that I do things the proper way.

Cliff Schulze

2/14/2023 12:50:59 pm

The reviewer is partially right. The 7 psi is a minimum but if another head in the same room has a higher pressure, it will drive up the pressure to the other heads. If one head in the room needs 16.7 psi, the other heads in the room are going to see the higher pressure as well. The only caveat is the pressure can be reduced by manipulating the pipe sizes; i.e. larger pipe size to the head requiring 16.7 psi, smaller pipe size to the heads requiring 7 psi.

2/14/2023 03:02:03 pm

What Cliff said is true. The 14x14 will flow more than the minimum due to the 20 x 20 causing it to overflow.

BUT, I don't agree with the reviewer... you're not required to set a higher minimum pressure for the 14x14 to match the 20 x 20. NFPA 13R (2013) Figure A 7.1.1.1 doesn't agree either. The Handbook notes in that section are very informative as well. Check out the Reliable Residential Design Guide too.

Compartment calculations are different than density/area and a lot of "rules of thumb" that reviewers use stem from the density/area method. Yes - the pressure will creep up in a density/area method because by definition you have to calculate branchlines off the same crossmain. You don't have to do that for 13R calcs.

You could have a completely separate 1" riser to the 14 x 14 and it will definitely have a lower pressure than the 20 x 20. Residential jobs very commonly have 1" risers to sidewalls from the floor below.

Use One 20x20 Sprinkler in NFPA 13R Calculation?

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