Calcs Required to Replace K3.9 with K4.2?

5/20/2026

I have a campus housing situation with a few thousand reliable 3.9k heads that are no longer made, on 16x16 spacing, requiring 17 gpm and 19 psi per the cut sheet.

We are looking to replace these with 4.2k sprinklers, same 16x16 spacing, is 16 gpm and 14.5 psi per the cut sheet.

Question is, I know the 4.2K falls just over the allowed 5% deviation, but these 4.2k sprinklers are more "efficient", so would they require running new hydraulic calcs to verify, or would it be safe to swap out as is?

Thank you in advance.

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10 Comments

Pete H

5/20/2026 06:07:49 am

In terms of buereaucracy/AHJ's: Yeah, you still need to recalc. You changed the flow characteristics of the system.

In terms of realism: Assuming the safety factor of the existing calcs wasn't "barely making it", there's no real reason to assume this calc won't produce remarkably similar numbers. You could likely ask to be "grandfathered in" and justify this as a "relocate" if you're up-front with the AHJ about what you're trying to do.

anthony

5/20/2026 12:09:05 pm

As usual I agree with Pete here. Engage the AHJ early here you MIGHT get an allowance for a head swap. Worst case if you need to recalculate the system as other have said and you're back to the place you started form.

Dan Wilder link

5/20/2026 07:54:42 am

This is a two fold question....

The first much less addressed factor that may be your biggest support for or require the calc is the water supply evaluation. Proving the water supply is as good or better than the original install along with your conclusions above for a lesser pressure and flow demand may be enough to allow the change out without full hydraulic calculations. It does work the other way and could support the requirement for calcs.

As to the change out itself, 13 & 25 say the same thing but do not provide a path to omit calculations. Even with the sections provided, as Pete said you are changing the characteristics of the system thus requiring proof.

How you would comply with the system hydraulic placard that is required to be installed is a final thought in the process. Provide it now for the new sprinklers would be the best optic...writing them up later would not look the greatest, or even worse another company writes them up for it.

13-30.3.6
25-5.4.1.3.2*

Jesse

5/20/2026 08:34:49 am

Anytime we're changing the hydraulic characteristics of a system I'd want to calculate it to prove the larger origifice sprinkler will work,

Glenn Berger

5/20/2026 08:40:34 am

As others have said; recalculation required. With the higher k-factor, at the same minimum pressure, you will required more water for the system and therefore a greater pressure at the source to meet the new demand.

RayJ link

5/20/2026 09:39:53 am

In Springfield, IL if you change or relocate more then 20 heads, you must re-calculate and submit everything just as if it is a new installation.

Dave L.

5/20/2026 11:20:43 am

See also NFPA #13 (2025) 30.3.6 -
Added to the 2016 edition:

"When replacing residential sprinklers manufactured prior to 2003 that are no longer available from the manufacturer and that are installed using a design density less than 0.05 gpm/ft2 (2.04 mm/min), a residential sprinkler with an equivalent K-factor (±5 percent) shall be permitted to be used, provided the currently listed coverage area for the replacement sprinkler is not exceeded."

Can you find a K-factor closer?

Dave L.

5/20/2026 11:22:00 am

(I guess I'm not sure exactly what kind of sprinklers you are replacing)

Jack G

5/20/2026 07:13:21 pm

So are we talking about a 2 or 4 head residential sprinkler calculation ?
Yes supply new calculations.
Is there a residential tank and pump? Is the supply enough gallons for the new head ( calculations) 7 minute or 10 minute duration or 30 minute duration. Around 1-2 gpm more required ( overflow)

James Art, FPE

5/22/2026 12:26:44 pm

"Next Larger K Factor" NFPA 25 says this:

5.4.1.3.2*
Where replacing residential sprinklers manufactured prior to 2003 that are no longer available from the manufacturer and are installed using a design density less than 0.05 gpm/ft2 (204 mm/min), a residential sprinkler with an equivalent K-factor (± 5 percent or the next larger K-factor) shall be permitted to be used if the currently listed coverage area for the replacement sprinkler is not exceeded.

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Calcs Required to Replace K3.9 with K4.2?

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