High Sprinkler Demand from Many Small Rooms?

6/3/2025

We have an office building where we have performed hydraulic calculations for a light hazard occupancy, using a design density of 0.10 gpm/ft² over a remote area of 1,500 ft².

However, our calculations require 20 sprinklers to cover this area, which is huge. Typically, there are only 7 sprinklers (at -215 ft² per sprinkler) that are sufficient for 1,500 ft².

The issue comes because the office has many small rooms, each with its own sprinkler, increasing the total number within the remote area. The output is higher at 439 gpm with 20 sprinklers.

How can we correctly define the remote area to bring down the overall flow bust still comply with NFPA 13?

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15 Comments

Peter H

6/3/2025 06:09:36 am

What's the ceiling height of the office building and are you using quick response heads?

If the ceiling height is under 20'-0" and you are using quick response heads, refer to 11.2.3.2.3.1 (NFPA 13 2016 Edition) to see if you can apply a design area reduction as per code.

MOHAMMAD MASHIRUL HAQUE

6/4/2025 09:01:29 am

Dear Pete H,

The design area reduction is not permitted because the sprinkler heads used as standard response. Additionally, the AHJ is asking for the area/density method, the room design method is not permitted.

I have calculated the sprinkler demand within the remote area based on the individual coverage areas of each sprinkler (e.g., 103 sqft, 115 sqft, 92 sqft, etc.). Sprinklers located within small compartments have also been omitted. However, the AHJ has said that the calculation is incorrect.

Brett

6/3/2025 06:38:23 am

The quick response area reduction is the obvious answer as Peter H has suggested. Another potential option is to stick with a 1,500 sq. ft. area and omit sprinklers in compartments that are 55 sq. ft. or less (section 27.2.4.7.2 in the 2019 edition of NFPA 13). If you're using the 2013 edition or earlier, the 55 sq. ft. area isn't in there. It used to just say "small compartments".

Glenn Berger

6/3/2025 08:11:03 am

Actually you possibly have correctly identified the hydraulically most demanding remote area. Whether it is obstructed construction or many spaces within the remote area, you will see a higher required flow rate because of the number of potentially operating sprinklers.

You want the calculations based upon the real design not the ideal (for sprinkler flow) design.

danefre

6/3/2025 08:11:30 am

Great responses above. Try both of those before my suggestion.

Check out this Meyerfire blog post: https://www.meyerfire.com/blog/hydraulic-calculations-failing-here-are-ideas

Put in the actual spacing for the sprinklers. I have a feeling your most demanding is already at 7psi so that might be moot. In light hazard the method is area of room divided by number of sprinklers.

There are zero-hydraulic-loss flexible braided hoses available.

"Balance" the calc. Find the heads that are overflowing and connect them to a different branchline and/or try to make them more demanding.

Last resort is try a K4.2 sprinkler. Need to swap all of them as you can't use the K-factor to balance calcs. This might create future confusion but it's an option.

Dan Wilder

6/3/2025 08:11:33 am

Using small room rule and using the actual coverage of each sprinkler (vs just defaulting to a max sq footage) should be enough to allow the remote area to work.

Other option is to calc the full area, omitting sprinklers in the less than 55 sq. ft. rooms, but still applying the actual coverage areas to the sprinkler demands.

If, on the rare chance these have automatic or self-closing doors (no rating required) and the rooms are rated for at least 30 minutes, room design could be utilized.

Max B

6/3/2025 08:12:33 am

Can you use room design rule? Is every head tagged with correct pressures/flow numbers for the area its covering?
Can you use k4.2 head?

Brian

6/3/2025 08:38:36 am

Also look into 23.4.4.6.2 (2013) that lets you omit sprinklers in small rooms for calculation purposes.

Jesse

6/3/2025 09:14:31 am

As Glenn indicated, seems you found the true hydraulically remote area. A couple suggestions:

First consider ceiling height. You may be able to reduce the size of the remote area using QR sprinklers and claiming a corresponding QR reduction

Second, is Room-Design method an option?

Juan Leon

6/3/2025 09:48:18 am

Use residential 3.0 heads, use the small room rule and loop the branch lines at the end.

Anthony

6/3/2025 12:44:50 pm

How are those allowable in a full 13 building that is clearly office space?

Chad

6/3/2025 09:56:36 am

Yeah I would be surprised if you cannot use a QR reduction to drive that down a bit.

Louie Lincoln

6/3/2025 11:40:29 am

Can you reduce the area to 900 sqft.

James E Art Fire Protection Engineer (FPE)

6/4/2025 06:15:54 pm

California Adopting Ordinance allows the Quick Response reduction ONLY for Light Hazard.

Kevin

6/5/2025 09:21:06 am

Max B noted a K=4.2. These can be used with K=5.6 as NFPA 13 does not have a restriction about mixing K factors. I'm in NY and still using the 2016 NFPA 13:8.3.4.
We have run into this frequently and reducing the K factor is a simple way to reduce GPM.

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High Sprinkler Demand from Many Small Rooms?

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