Small 580 sqft Dry System Need Phantom Flow?

3/21/2025

I have a small 580 sq. ft. attic, fed by a dry system, next to a wet system in a retail outlet mall. They are 2 separate systems at the valve assembly. The ceiling for the dry system is 10-ft higher than the wet system.

Would a phantom flow be required here?

If so, would it be added at the valve assembly, where they meet?

Thanks in advance.

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8 Comments

Peter Howard

3/21/2025 06:44:51 am

Yes.

And while that location would 'probably' be fine as to where to where to add it. The technical answer is where your line feeding the smaller space meets the common piping, so the T at the bottom of the riser is likely the spot.

Assuming the dry system is a light hazard and the attic is not sloped, the normal minimum design area would be 1950 square feet with heads delivering a .1 density.

So you need to make sure you have a minimum 195 gpm flow demand. Unless you have a sloped roof. Or unless they're using the attic for storage. Or some other variable I'm not thinking of.

Anthony

3/21/2025 08:00:36 am

It's always worth an ask not to do phantom flow. An e-mail to the AHJ costs like 35min, writing a strong e-mail with a couple screen grabs showing the area scope.

Mike Morey

3/21/2025 08:12:02 am

I always suggest evaluating whether or not room design method is applicable/legal. Only density/area method calcs are subject to phantom flow. If the areas are adequately separated I call out room design method and do not add the phantom flow.

3/21/2025 08:30:04 am

If the wet system and the dry system are protecting the same floor area. Yes.

If they do not share the same floor than I THINK no. I've always likened the phantom area approach to a rack calculation balance without the pressure requirement in some situations (ie. a tail end IT Room preaction systems is surrounded by a wet sprinkler system that is "balanced in" at the last main outlet feeding the wet system before the tie in to the preaction valve without the pressure requirement.) The examples within NFPA 13 for phantom flow are for sprinklers off of the same system, so interpretation by AHJ and engineering staffs will vary for this situation.

Jesse

3/21/2025 08:50:07 am

By the letter of the code, yes phantom flow is required. However, as its only applicable for density / area calcs, I wonder if there is the case to make for room-design method.

Also, I wonder if an alternative means of protection such as a small anti-freeze system off of the wet-system would work.

3/21/2025 09:40:22 am

Adding to what others have said- yes, I believe technically by the letter of the code it would be required. I am unaware of any real way around this other than the room design method or getting the AHJ to approve without. Most AHJ's I've worked with do not require this even if you don't call out the room design method. I'd be willing to wager most of them have not even heard of it.
Either way, the letter of the code also states that the flow shall be 'added at the point of connection from the branch line to the cross main furthest from the source' (13 2016 Ed 23.4.4.2.5). To me this means- assuming you have a 'cross main' feeding branch lines or arm overs in the attic rather than just one single line- the junction of the last branch line and the cross main is where this flow should be added. If it is just one line, I would think adding it at the point of connection to whatever is considered the 'main' would be correct. How much difference does it make? Probably not much, but adding flow further from the source will create a higher demand than adding the flow at the base of the riser.

Jack G

3/21/2025 05:02:13 pm

I ve been calculating sprinkler systems for 60 years. I’ve seen and done all the iterations and changes.
I am not a big fan of safety factors and or phantom flows.
Pipes should be sized so that the pipe size controls the flow. ( the system doesn’t have a brain) it’s a function of opening up sprinklers.
So if the branch line is sized properly and you try to push 10 more psi and additional flow thru it —- guess what, the calculation will exceed the water supply just about all the time, due to the pipe sizing.
( try doing a supply calc — because that’s actually what is going to happen when you open the remote area up—- and compare it to your demand calc)
If you are going to add phantom flow and or safety factors, increase the density( .11, .22 etc) or whichever density works say to get A .2 over 1500– 300 gallon system demand.

Jack G

3/21/2025 05:10:47 pm

I ve done hand calcs, peaked grids, ( anyone ever use a slide rule on calculations) TI-52 programs, done them on a telephone modem to a mainframe ibm-9300.
Todays programs make it easy to do what I m saying above and show you the available density ( branch or system must be larger to accept the additional flow and pressure) not just at the split.
The rule as to where it is added is faulty in my opinion.

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Small 580 sqft Dry System Need Phantom Flow?

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