Hydraulic Calculation for 1 Sprinkler on Domestic?

6/25/2026

GC asked us to add (1) head to a dentist's office closet where they store the nitrous tanks. Not a big deal, done it before tapping off the domestic line. The AHJ asked for calcs to prove it will work. The existing building is not sprinklered.

What are your thoughts on this?

Is there a way to prove it's workable?

Is there a standard that stipulates what is and what's not acceptable?

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12 Comments

Pete H

6/25/2026 06:05:29 am

Well we're definitely in a grey area. But AHJ made the request.

My advice: Do it as a 13D calc (2 most demanding sprinklers in a compartment) There is clearly only one in the compartment (and entire building).

Make sure to include domestic loss in your calculations. There are charts in 13/13R/13D that should show you the equivalent losses to add to the system riser for the domestic load (you count the number of sinks, toilets, showers, etc. and add up the losses).

Then offer that calculation.

It still might not be workable depending on the hazard the AHJ expects the nitrous tanks to be considered for.

Brian K. Fain

6/25/2026 08:29:12 am

A word of caution, we have been required to install a separate backflow preventer for this one sprinkler head in the past.

Kelley Stalder

6/25/2026 08:40:54 am

Isn't a backflow preventer required anyway for a dental office?

Brian

6/25/2026 08:45:54 am

Yes. They had one on the domestic feed (which we tapped into), but the AHJ required a separate one for our sprinkler head and piping.

Ben F

6/25/2026 08:32:32 am

I would think this falls under the limited area sprinkler system requirement in the IFC, which would require a calculation per NFPA 13. Not sure where you are located or what's locally adopted, but 2024 IFC section 903.3.8 lists the requirements. My concern would be if this is classified as a med gas room, which would be OH2. Limited area sprinkler is limited to LH and OH1.

Glenn Berger

6/25/2026 08:44:59 am

I actually like the AHJ asking this comment. Just want to make sure that there is sufficient pressure such that water will flow from the sprinkler.

Where is this sprinkler located with respect to other plumbing fixtures? If the sprinkler is located at the high point on the facility than there could be a problem. If the sprinkler is located in the basement with plumbing fixtures above, then no issues.

Blaine

6/25/2026 08:51:51 am

I’ve always thought that you had to do the whole building and can’t just put in a sprinkler wherever you wanted. Ref NFPA 13 2022 1.3.2. For me it came up when I wanted to put some residential sprinklers in my shop where I had easy access to the water lines in the overhead.

Jack G

6/25/2026 11:20:54 am

Tyco makes a domestic water shut off valve (RSV-1) installed after the split to the single sprinkler. It prioritizes the flow to the fire system so the full flow and pressure is available.
I would use a meter to perform a flow test at the tap, using the system gauge at the bfp for the static and residual..
Then you can use the full water supply.

Wes

6/26/2026 08:04:27 am

I believe the Tyco RSV-1 is only listed for residential use.

Jon N

6/26/2026 11:26:09 am

As a former AHJ, we required many "limited area sprinklers" to protection a localized hazard (boiler rooms, paint booths, flammable liquids rooms, etc.). These would typically be 1-2 sprinklers off the domestic supply. Since this was potable water, they would often use copper lines. Most often these were "pipe schedule" designs - up to two sprinklers supplied by 1-inch copper per NFPA 13 (2016 edition) - Section 23.7.2.2.1.

If you knew the pressure in the water supply, you could probably calculate the system, but I am not sure why that would be necessary. It is assumed that this is a very small room (closet-sized?) and 1-inch copper supplying 1/2-inch orifice sprinklers would be more than adequate.

Again, if the pressure is known (maybe add a gauge at the connection point to the water supply), calculate the friction loss through the copper pipe and fittings, and include the sprinkler flow at the minimum pressure - 7 psi for most (and there you have it, simple hand calculation). 10 by 10 spacing = 100 sq. ft. 100 sq. ft. times 0.15 (OH-1) = 15 GPM. (15 GPM / K5.6)^2 = 7 PSI.

Mark Harris

6/30/2026 02:31:33 pm

Comments have covered most of my thoughts and Jon N well summarized what we did long time ago when BOCA code required sprinklers in a specific area (plus small basements).

One thing we did have to check was that the water meter was same size as piping. Could have a 1" service pipe but 5/8 meter was installed.

David L Miller

7/1/2026 10:55:06 am

RE: NFPA 101:9.7.1.2

"Sprinkler piping serving not more than six sprinklers for any hazardous area shall be permitted to be connected directly to a domestic water supply system having a capacity sufficient to provide 0.15 gpm/ft2 (6.1 mm/min) throughout the entire enclosed area."

Because the requirement has a specific density requirement it must be hydraulically calculated.... Now, find a domestic supply in a building small enough not to require sprinklers that can supply six sprinklers at Ordinary Hazard Group 1 density (no Hose Stream required). Oh, and trace all of the piping and fittings all the way back to the city main where a flow test was done. May be less costly to fire-rate the room since the code requirement is usually either fire rate the room or provide sprinklers.

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Hydraulic Calculation for 1 Sprinkler on Domestic?

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