Phantom Flow for QR Reduction & Attic Systems?

2/24/2025

Is the additional flow described by NFPA 13 27.2.4.2.5 (2019) based on the original minimum design area or the minimum area after modifications?

Say I have a light hazard system with low ceilings. The default minimum area is 1,500 sqft but is reduced to 900 per Section 19.3.3.2.3.1. The calculation on the sprinklers in that 900 sqft area results in 128 gpm total flow (not including the hose allowance).

If the additional flow in 27.2.4.2.5 is based on the original 1,500 sqft, then I would need to add 22 gpm of extra flow on the main to get the minimum of 150 gpm running through the main.

If the additional flow is based on the 900, then I'm already meeting the requirement of 90 gpm, and I can ignore the additional flow requirement.

Similarly, we start at 1500 sqft in a dry attic system, but it needs to be increased to 2,535 sqft minimum due to the dry system and sloped ceiling.

Will the minimum flow as per 27.2.4.2.5 be 150 gpm or 254 gpm?

This sometimes comes up when using specialty attic sprinklers, which allow for fewer operating sprinklers in the calculation. It comes down to the definition of "minimum design area".

Is this the original minimum area, or the modified minimum area?

In my designs I've been defaulting to the larger of the two demands to be conservative, but what is the actual intent of this clause?

I haven't found any discussions relating to the changing of the minimum design area.

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9 Comments

Zachary Bishop

2/24/2025 08:30:59 am

The way this reads to me is the minimum design area as modified. Since, the "minimum" design area required is allowed to be increased / reduced in the situations you mention, it makes sense that the phantom flow requirement would change as well to match the requirements for the current situation.

Pete H

2/24/2025 09:06:02 am

I'd agree with Zachary and go with design area as modified.

Especially in the case of the dry system.

Dan Wilder

2/24/2025 09:12:03 am

It is based on the "Minimum Design Area", so the final area after adjustments. I had to look back to the Ken Isman article that does reference an area reduction (high temp sprinklers for an Extra Hazard classification) which leads me to think it's the final minimum design area, not the starting point.

"Minimum Area" is only referenced in 19.3.3.1.5.1 / 19.3.3.1.5.2 in relation to the 3000ft² area for unsprinklered concealed spaces. "Minimum Design Area" is only referenced in 19.3.3.2.2.3 for EC sprinklers.

I would argue that a specialty design, like attic sprinklers, are not Density/Area based, more special application, and are not required to apply this section.

Jesse link

2/25/2025 08:08:17 am

Yeah I agree with Dan here, especially in the case os special application sprinklers. These are not Density / Area based

Casey Milhorn

2/24/2025 11:59:03 am

I agree with Dan and others. It's based on the final reduction or increase. As far as dealing with listed design approaches that are based on a listed system/product, then no, you would not add phantom flow. These listed design approaches have been tested and approved as listed.

Jack G

2/24/2025 03:30:52 pm

Here s the way i approach it.
I think its a rarity that i use it. Also where you add it depends on which version of 13 you use.
Common sense should also be used.
If you can use " large room rule", or "area reduction method/ qr heads" , phantom flow should not be added. These are acceptable calculation methods.
Where i find myself using it is say you are doing a modification of a small area within a larger area.
Like a strip store, or a light hazard area. So say there are partitions where you cant get in or trace piping. Like licked tenant areas. So say its a 650 ft2 modification. At .1 density the modifiedvarea needs 70 gpm (when i calculate this area only)
The 1500 ft2 normal area requires 150 gpm. ( .1×1500) So phantom flow to be added is 150-70 = 80 gpm.
So i add the 80gpm to my calculated area ( only in the renovated area ) and i add it to the appropriate point ( depending on 13 version) ,, be it branch line or where the main splits.
Saves on finding a lot of keys , retracing system, or entire system calculations.
I thought the 2013 version and appendix explaines it so " our heads dont hurt "! Opinion.

2/24/2025 03:58:38 pm

Looking at you dry attic with sloped roof, this area and sloped ceiling increases is for the “ dry system” and the “ ceiling slope”.
Where it matters is say attic/ and light hazard system below.
If the attic system has a home run supply, say from the riser room
then phantom flow would not need to be added. ( full size main)
So where phantom flow would be used for is to compensate for situations where a smaller area with a higher density is being protected within a larger area, less demanding sprinkler system .
This is relevant in say attics with a smaller design , or attic sprinklers giving a smaller area based on a minimum number of sprinklers over a smaller area. ( adding flow / potential fire load ) even though a smaller amount of sprinklers may be activated .

Jose figueroa

2/24/2025 05:37:58 pm

I read the exact same question on reddit.com.

I have try to understand the concern of the NFPA13 (2019) clause. I quote “ 27.2.4.2.5
Where the total design discharge from these operating sprinklers is less than the minimum required discharge determined by multiplying the required design density times the required minimum design area, an additional flow shall be added at the point of connection of the branch line to the cross main furthest from the source to increase the overall demand, not including hose stream allowance, to the minimum required discharge”

I went straight to the NFPA to understand the intent of this clause. I believe this is thr core issue.

Putting it the simplest way,

Jose Figueroa

2/24/2025 07:11:11 pm

I'm sorry. I posted without finishing my post: If the total design discharge from the sprinklers is less than the minimum required discharge, this means there's insufficient water flow to properly suppress the fire. To compensate for this deficiency, additional water flow must be introduced.

Phantom Flow for QR Reduction & Attic Systems?

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