Sprinkler Req'd Below Duct 18-inches from Floor?

8/5/2024

What are the code references for obstructions greater than 4 feet wide (ductwork) but located only 18" from the floor?

We have a mechanical room with several large ducts 5 ft wide x 3 ft tall. The ducts are only 18" from the bottom of the duct to the finished floor.

Additionally, there is a second level of a similar-sized duct that is mounted above, which leaves a 12" space between the top of the lower duct and the bottom of the upper duct.

Would we be required to install two levels of shadow protection under each duct, or are there provisions that allow sprinklers to be omitted if there is limited space below the duct?

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7 Comments

Anthony Crispo

8/5/2024 07:52:38 am

NFPA13- 19&22ed 9.5.5.3.1.5
2016 - 8.5.5.3.1.5
Sprinklers shall not be required below noncombustible obstructions over 4 ft (1.2 m) wide where the bottom of the obstruction is 24 in. (600 mm) or less above the floor deck.

That being said if you're a contractor and your bid documents require it you may be protecting under low ducts anyway.

Glenn Berger

8/5/2024 08:12:11 am

Thanks for the reference. I wish that NFPA folks would provide a commentary why they chose that number (24 inches in this case).

Anthony

8/5/2024 09:45:45 am

I would guess 2' is too short for storage as a bent/squatting person usually wouldn't have their face at 2' A.F.F. so it'd be unlikely to have a box of stuff. Similar to occupancy in an emergency/fire, it's an unlikely area to hide. Also NFPA generally choses round numbers for ease of use.

Dan Wilder

8/5/2024 01:54:44 pm

Took a bit but:

Committee Statement
Committee Statement: At some point it becomes ineffective to install sprinklers under obstructions that are close to the floor. The spray pattern of the sprinkler can't develop and the fuel load is so small that supplemental sprinkler protection isn't necessary.
Response Message: FR-90-NFPA 13-2013 The proposed value of 12” above the floor was revised to 24” based on minimum 18” required for sprinkler discharge pattern development plus deflector distance below obstructions.
Public Input No. 474-NFPA 13-2013 [New Section after 8.5.5.3.1.1]

Peter Howard

8/5/2024 08:22:33 am

From NFPA 13 (2016 edition):

8.6.5.3 Obstructions that Prevent Sprinkler Discharge from Reaching Hazard.
8.6.5.3.1 Continuous or noncontinuous obstructions that interrupt the water discharge in a horizontal plane more than 18 in. below the sprinkler deflector in a manner to limit the distribution from reaching the protected hazard shall comply with 8.6.5.3.
8.6.5.3.2 The requirements of 8.6.5.3 shall also apply to obstructions 18 in. or less below the sprinkler for light and ordinary hazard occupancies.
8.6.5.3.3 Sprinklers shall be installed under fixed obstructions over 4 ft wide. [There's your code path]
8.6.5.3.4 Sprinklers shall not be required below obstructions that are not fixed in place, such as conference tables.
8.6.5.3.5 Sprinklers installed under open gratings shall be of the intermediate level/rack storage type or otherwise shielded from the discharge of overhead sprinklers.
8.6.5.3.6 The deflector of automatic sprinklers installed under fixed obstructions shall be positioned no more than 12 in. below the bottom of the obstruction.
8.6.5.3.7 Sprinklers installed under round ducts shall be of the intermediate level/rack storage type or otherwise shielded from the discharge of overhead sprinklers.

--

That's the code path for standard spray upright/pendent sprinklers.

It's 8.7.5.3.2 for standard spray sidewalls. 8.8.5.3.2 for extended coverage upright/pendents. 8.9.5.3.2 for extended coverage sidewalls. 8.10.6.3.2 for residentials uprights/pendents 8.107.3.2 for residential sidewalls.

CMSA and ESFR have different rules.

I see no provision allowing for the omission, but if you find one, I'd be interested in reading it.

Franck

8/5/2024 09:21:39 am

This is a quite unusual situation.

You need to protected underneath the obstruction if there is something to protect.

If there is absolutely nothing underneath the obstruction that may be combustible or flammable, I don't see the benefit of providing additional sprinklers.
Some may say that you may find some storage one day undernath the ducts, but there is another rule in NFPA 13 asking to provide a minimum distance of 18 in. between the top of storage and the sprinkler deflector, which is again not possible in that configuration. So no possible storage allowed.

The only reason where you may need sprinklers underneath is if you are expecting a spill of flammable/combustible liquids with a pool fire...

Jack G

8/6/2024 12:27:38 am

I agree with Anthony . 24 inches the requirement. Believe me after a few years and maintenance is done there wil be filters, motors, and boxed replacement parts everywhere.
I believe the sprinkler is to be 3 inches minimum inside the duct.

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Sprinkler Req'd Below Duct 18-inches from Floor?

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