Include Exhaust Sprinklers in Paint Booth Calc?

4/25/2025

NFPA 33, 2018 Section 9.6.2.1 reads "The sprinkler design area shall not be required to exceed the area of the booth or room in which spraying or resin application is conducted."

My project has a large downdraft paint booth for buses. There are two concrete pits (4-ft height, 3-ft width, 80-ft long) beneath the paint booth that are covered by open grate flooring.

(1) Does the above reference mean that the exhaust duct sprinklers do not need to be included in the booth hydraulic calculation?

I have a large paint booth, and it has 4 exhaust ducts.

(2) Is sprinkler protection required within the pit?

Do these heads have to be included in the booth calculation or can they be considered heads under obstruction?

If sprinkler protection was within the pit, the bags had to be regularly changed, which creates an issue for the end user and adds a large demand to the calculations. I think it would be unlikely that the sprinklers would operate in the concrete pit, as the overhead system should adequately protect the booth itself.

Thanks in advance.

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10 Comments

Glenn Berger

4/25/2025 08:46:21 am

- Sprinklers within the Pit -- Yes they are required. Without more specific details of the interface between the paint booth and the pit, I cannot provide a proper response to the inclusion in the hydraulic calculations question.

- My experience with paint booth exhaust systems is that the duct is an extension of the booth itself. Most likely the sprinklers in the exhaust duct will need to be included in your calculations.

Colin link

4/25/2025 10:57:27 am

I'm also very curious what the consensus is here. I recently completed the shop drawing design for a large military pain spray booth. For my project, I calculated the booth separately from the duct based on this:

9.5.1.2.1 - The sprinkler design area shall not be required to exceed the area of the booth or room in which spraying or resin application is conducted.

And I calculated the exhaust duct separately per this section:

9.5.1.6* Duct Protection. (6)*The system demand shall include the discharge from the hydraulically most remote adjacent sprinklers in a common 30.5 m (100 linear ft) area of duct (horizontal and/or vertical).

There is NO WAY the water supply the gave me will support both the spray booth and the exhaust duct calculated together. But this annex material has me worried:

A.9.5.1.6 Water supply requirements for most industrial paint spray operations should be adequate to supply all automatic sprinklers in the spray area. Loss experience has shown that fires starting in the exhaust duct can spread to the spray booth and that fires starting in the booth can spread to the exhaust duct.

The project is currently under review, so I'll see how it goes, but not sure what my course of action will be if they ask for booth and duct to be included in the same calculation.

Jack G

4/25/2025 11:49:37 am

It has been my experience that the booth , the area in the floor , and filters , also exhaust ducts will require sprinklers. ( in the hydraulic calcs )
If the water supply is a problem, ask for clarification. I ve had AHJ s only ask for sprinklers up to the filters be calculated, and exhaust ducts separate, their reasoning , I think , is the filters would contain the fire to the booth.
What did the A&E show, or denote ?
Maybe a change order ?

4/25/2025 12:08:51 pm

Yes, that's been my past experience too. However, in this case, either the EOR didn't include exhaust sprinkler with the booth in their BOD calculations, or the EOR messed up on their hydraulic calcs. And without express direction from either NFPA 33 or the A&E plans, my quickest route to find out what really happened is to submit a design that the water supply is capable of handling, which in this case is a separate calc for booth and exhaust, meets the requirements of NFPA 33 (which doesn't expressly say to include both booth and exhaust together), and see what they say. If they accept it, then we're good. If they don't, they'll have egg on their face for proposing a system design in the A&E documents with an inadequate water supply, and they'll have to fix it on their end, with a substantial change order likely. Will keep this thread posted on the result.

Brett

4/25/2025 01:07:25 pm

In my opinion section 9.6.2.1 is there to clarify that you don't need to calculate the booth in combination with the ceiling level sprinklers above the booth.

Section 9.6.3 - "The water supply shall be sufficient to supply all sprinklers likely to open in any one fire incident without depleting the available water for use in hose streams."

9.6.3 doesn't explicitly say to include the ductwork in the calculations, but the annex does support that idea as mentioned above by Colin. In most cases I think that the ductwork should be included in the calculation unless there is some sort of separation put in place that will stop fire in the booth from spreading to the ductwork or vice versa.

Jay

4/26/2025 11:18:33 am

See the definition of Spay Area in NFPA 33. Yes, sprinklers are required in all those portions of the spray booth that are included in the definition of Spary Area, and they are all included in the calculation.
Couple interpretations from NFSA and AFSA to note from my recent experience with this:
You are not required to add phantom flow to the EH2 booth calculation because the standard species the design area is only the booth.
The intake/makeup air duct does not require protection because it is specifically excluded in the definition of Spray Area.

4/28/2025 11:07:18 am

UPDATE: I just got the review back on my above mentioned DOD spray project. The EOR approved my hydraulic calculations and is NOT requiring us to calculate the booth sprinklers and the exhaust sprinklers at the same time. So, much like the wording in NFPA 33, it appears this is not a simple cut and dried situation, with an easy 'YES' or 'NO' answer. The FPE reviewing my plans for this project has over 40-years of experience in every aspect of fire protection and specializes in DOD work, and special hazard applications, so it's not a matter of the reviewer overlooking or missing an obvious error. The booths they're using may have some prevention means for fires spreading into the ducts, such as dampers, but I don't have much info on the booth design. Anyway, I got the answer I was hoping for. I'm hoping this

robb k

4/30/2025 08:12:52 pm

I have had The DFPE rule the other way... The problem with the his remarks required us to include the calc... and include a 11k factor head per UFC tabbles for EH1 whereas the NFPA only required 8k - the head was 17 ft higher than the design area - and that was also the most remote - it totally canned the project becuase our scope could was required to use an existing pump - due to this ruling itfailed to provide the flow and pressure - due to this one head driving the calcs way above the existing capabilty of the 750gpm pump... i tried to argue that this is not a design area sprinkler, as well as an 11k head in a 30inch round duct was sever overkll (flooding the duct) - the area density being completely carried over into a duct fell on deaf ears.

4/30/2025 08:17:23 pm

oops ufc table for extra hazard was 11k, NFPA 33 for in-duct was 8k

Chris

4/29/2025 12:40:54 pm

Adjacent topic, work with your structural engineer to ensure that the ducting can handle the pooling water in the ducts.

In my experience with DOD paint booths, I did not include the flow of the ducts, just the spray area itself.

Include Exhaust Sprinklers in Paint Booth Calc?

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