Domestic meter sizing for combined domestic/fire sprinkler system

[RTson]

Basic question. In California, when sizing a domestic water meter for a single-story, single-family residence with a fire sprinkler system, do you size the meter for the larger of the fire system flow (29 gpm) and the domestic flow (32 gpm), or do you size the meter for the combined flow (61 gpm)? I can't seem to find a clear answer in the CPC, CFC, NFPA 13D. Thanks!

 

[TravisMack]

You do the sprinkler demand plus a 5 gpm allowance through the meter. That is in the CFC, I believe.

 

[RTson]

Thanks, Travis.
The fire sprinkler system designer calculated 24gpm for fire plus 5 gpm allowance for domestic for a total of 29 gpm.
Our domestic calculations per CPC give us 32 gpm.
When we size the meter, do we size for 32 gpm or for 56 (32 + 24) gpm?

 

[cdafd]

I do not speak California, even though I grew up there

903.3.5.1 Domestic Services​

Where the domestic service provides the water supply for the automatic sprinkler system, the supply shall be in accordance with this section

903.3.5.2 Residential Combination Services​

A single combination water supply shall be allowed provided that the domestic demand is added to the sprinkler demand as required by NFPA 13R.

https://up.codes/viewer/california/ca-fire-code-2022/chapter/9/fire-protection-and-life-safety-systems#903.3.5

 

[fel3]

I haven't looked at NFPA 13 in a while (my hard copy is dated 2002), so I looked it up. It turns out that there are now three flavors of NFPA 13. The regular flavor of NFPA 13 is for both life safety and fire protection and is primarily used for commercial/industrial/institutional applications. To reduce fire sprinkler costs for residential applications, NFPA developed NFPA 13D and NFPA 13R, which are primarily for life safety. NFPA 13D only applies to one- and two-family dwelling units and manufactured homes and NFPA 13R is for multi-family buildings. See https://www.nfpa.org/news-blogs-and...03/12/comparing-nfpa-13-nfpa-13r-and-nfpa-13d.

So, the governing standard that the 2022 CFC should have referenced for your case is NFPA 13D and not NFPA 13R as per the link from cdafd. IMHO, the CFC should have referenced both NFPA 13D and NFPA 13R, but only for their intended applications, so it seems to be an error in the CFC. Unfortunately, I don't have these NFPA standards so I can't give you any of their requirements. You will need to track down these standards.

Per the link I provided, "The main goal of an NFPA 13D system is to provide life safety by preventing flashover for the first 10 minutes of the fire, which allows occupants time to escape while also allowing the system to be installed at a substantially lower cost than an NFPA 13 system." During this short escape window, you should expect the sprinklers to be on and probably to have some amount of domestic demand, but how much I don't know. I would think such guidance is in NFPA 13D, but if it's not then check with the fire department that has jurisdiction. The worst case would be 32 gpm domestic + 24 gpm sprinklers + 5 gpm allowance = 61 gpm, as you stated, but perhaps you only need to include part of the domestic demand. On the municipal side (my expertise), we usually analyze water systems for Maximum Day Demand Plus Fire Flow rather than Peak Hour Demand Plus Fire Flow.

 

[jhnblgr]

Typically in my experience the fire and domestic are two different systems due to the flow rates and back flow prevention measures involved. These flow meters normally only can operate at certain rates and likely if you opt for the sprinkler flow rate the residential flow will not be measured accurately or at all. Also back flow devices operate at certain minimum flows. I think another thing is that the fire line gets locked open to prevent being shut off accidentally if the domestic system requires repairs. Sometimes code or standard are the minimum and not what should be considered the best design.

 

[RTson]

Thanks to everyone who replied. It looks like if this were a 13R system, it would be the 56 gpm. But since this is a 13D system, the requirements are unclear.

 

[cdafd]

I believe Travis is correct above

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[cdafd]

6.2.4​

Where a water supply serves both domestic and fire sprinkler systems, 5 gpm (19 L/min) shall be added to the sprinkler systemdemand at the point where the systems are connected, to determine the size of common piping and the size of the total water supply requirements where no provision is made to prevent flow into the domestic water system upon operation of a sprinkler. For multipurpose piping systems, the 5 gpm (19 L/min) demand shall be added at the domestic connection nearest the design area. This demand may be split between two domestic connections at 2.5 gpm (10 L/min) each

California One- and Two-Family Dwelling Fire Sprinkler Installation Code 2022 based on the NFPA 13D, 2022

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