What is the Existing System's Design Criteria?

8/7/2025

This is a backflow preventer retrofit project where I'm trying to figure out what the system's original design criteria was.

I am working with an existing 7,300 sf warehouse area within a larger building, which includes offices and a workshop. It is a marine operation, and the products stored are primarily spare boat parts, propellers, shafts, brackets, housings... large metal objects stored in wood crates or cardboard boxes.

I have determined this to be Class II Commodities, stored no higher than 20'.

I am thinking that, based on NFPA 13-2019, Table 21.4.1.2 and Figure 21.4.1.2(b) curve B, I need a density of 0.3725 gpm/sf over a 2,000 sf area. I would also be required to have a maximum protection area of 100 sf based on Table 10.2.4.2.1(d).

Here's where it gets tricky - the branches are spaced roughly 15' apart, and heads are spaced 8' or more along the branch line. Some are as far apart as 11'-6". The heads are K8.0 solder-type. Because of the K-rating, this shouldn't be a pipe schedule system.

Is anyone aware of an alternative storage protection method that this system may be designed to?

The installation date is 1986. Any thoughts would be greatly appreciated. I do not know what density I should be applying to this area in my calculations. Thank you very much!

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6 Comments

Pete H

8/7/2025 06:42:57 am

Without the original installation plans and what you have said, there is no way to be certain:

The earliest "Free Access" NFPA gives me on NFPA 13 is 1991 edition, so I'm already off by at least five years on this comparison... but in Table 4-2.2 "High Piled Storage" is just noted to have a maximum of 100 square feet. I'm not yet seeing the current restriction of no spacing exceeding 12'-0" between heads (except between bays of 25'-0" where it can go up to 12-6)

Wait, Section 4-4.1.1 does have the maximum distance for sprinkler spacing in high-piled storage as 12'-0".

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Okay, so at least by 1991 you couldn't have 15-0 between branch lines in high piled storage.

-

Honestly, I have a feeling this was designed as "miscellaneous storage" and OH2 and just poorly enforced for decades. It's unfortunately more common than you think.

Anthony

8/7/2025 07:16:13 am

Without a set of plans or hydraulic placards its very hard to determine the "original design criteria." In these cases engaging an AHJ and showing a flow test and the backflow loss pressures can be a good place to start. This can be effective if there is a ~3psi drop at a reasonable flow rate that's a rounding error in most hydraulic measuring methods.

Jesse

8/7/2025 08:13:07 am

The spacing makes me lean to this having been a different occupancy at one point and then someone coming in and making it storage without assessing the capability of the AS system. If I had a dollar for everytime I've seen that, I'd have a bunch of dollars.

I would highly recommend a design survey and then reverse engineer the system to ascertain the actual density provided.

Glenn Berger

8/7/2025 08:46:19 am

The installation date was 1986, so the design date was possibly a few years earlier. Sprinkler system coming up on 40 years in service.

You are in the timeframe when storage was covered by the NFPA 231 series of standards. When criteria was a lot more basic than it is today.

I am with Jesse on this one and wish you luck.

Jerry Clark

8/7/2025 10:55:58 am

Your storage qualifies as Class II Commodities—primarily metal parts packaged in wood or cardboard, with noncombustible or limited-combustibility packaging. According to NFPA 13-2019, Table 21.4.1.2, the required CMDA (Control Mode Density/Area) design density for such commodities stored up to 20 feet is 0.3725 gpm/sf over a 2,000 sf area. Additionally, Table 10.2.4.2.1(d) limits the maximum coverage area per sprinkler to 100 sf.

However, your sprinkler arrangement presents challenges. The heads are spaced 8 to 11.5 feet apart along branches that are 15 feet apart, which means some sprinklers protect more than 100 square feet—exceeding the maximum allowed for hydraulically calculated storage systems at the necessary density.

The existing K8.0 solder-type heads are standard spray sprinklers, not storage-specific (CMSA or ESFR) models, which were not available in 1986. As a result, the original system likely does not comply with current storage protection standards.

Because the system was installed in 1986, it followed an earlier edition of NFPA 13, which required lower densities for storage. Current editions of NFPA 13 allow some flexibility for existing systems—particularly under Chapter 26—but do not grant blanket exemptions. Significant changes in commodity class, storage height, or use generally still require system upgrades.

One approach is to evaluate the system under the provisions for modifications to existing systems. The current code allows “evaluation/modification” for previously installed systems, but sprinkler coverage and density must at minimum meet the requirements in effect at time of original installation. Density cannot be decreased below the intent for that commodity class and arrangement.

It is also possible that your system was installed as a pipe schedule design even with K8.0 heads, since the 1980 edition of NFPA 13 and later only permitted new pipe schedule systems for light and ordinary hazard occupancies. However, local codes in effect at the time of construction may have referenced an older edition, so you should try to verify the applicable standard for your jurisdiction.

The best next step is to consult with your AHJ. If a full NFPA 13 retrofit is not practical, discuss possible alternative risk mitigation strategies (such as additional fire detection, limiting storage height, widening aisles, or enhanced manual fire response plans). Ultimately, acceptance of any alternative is up to the AHJ, and decisions should be formally documented.

Jack G

8/7/2025 10:51:36 pm

Class 2 commodities in the 80 s was under NFPA 231, 231 c ( for racks vs palletized) .
I m assuming storage on pallets.
There were 2 curve charts, on for 20 ft high storage, standard temp heads and another for high temperature heads.
The second chart allowed for a 25% reduction in area but not less than 2000 sq ft.
There was a percentage chart for less than 20 ft storage.,
Looking at my archive copy the density would be about a .17 over 2000 sq ft .( not .3725) Which was very common density. I d try to sell the customer on 100 sq ft spacing ( more sprinklers ) for greater tenant flexibility.
I can’t count how may 130 s I ve changed to 100 sq ft 10 years later for a new tenant.
Densities less than .25 allowed 130 sq ft and 15 ft max spacing.
Greater than .25 was 100 sq ft. 12 ft spacing except for 25 ft bays.
I m not sure why the new rules / density would be so high for class 2 but I ll look into it. My off the top of my head guess is ( new) it should be around a .23. But I ll look.
Regardless the system is grandfathered unless the storage has changed.
A trip to the building department and reviewing the CO docs will tell you exactly.,

What is the Existing System's Design Criteria?

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