Does a Duct Near Ceiling Need Sprinkler Above?

1/31/2024

I understand if a duct is wider than 4-ft (1 m) that there needs to be a sprinkler under the duct.

But what if the duct is just below the ceiling? Does a sprinkler also need to be above the duct?

I have a group of ducts with a combined with of 16'-6" (5 m) with the highest point of the duct 10 inches (25 cm) under the ceiling.

Does there need to be a sprinkler above and below the ducts?

Thanks in advance.

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11 Comments

Anthony

1/31/2024 08:07:30 am

Plain reading of code says yes. You might be able to convince an AHJ that its not necessary in non combustible construction. A few points to keep in mind: the possibility of a fire inside the duct, more probable activation of the sprinkler due to the height, and the protection of the structure in relation to escape time for the occupants.

Patrick Drumm

2/2/2024 12:06:12 pm

Anthony, which code section would you be referring to? My initial thought was the answer to this question is no, because you are covering floor space, but I could be convinced if I had a code reference or specific language to reference.

Patrick Drumm

2/2/2024 12:20:42 pm

Apologies Anthony, with a little more thought I think I understand where you were going. You're probably asking, "If the duct is 10" below the deck and spanning 16'-6", how do you meet the 1" to 12" deflector below ceiling requirement for unobstructed construction, or the 1" to 6" below the structural members up to a maximum distance of 22" for obstructed construction. In that case, how to you meet the deflector requirements? Code dictates heads above and below, but I understand all the commonsense arguments against below.

Glenn Berger

1/31/2024 08:24:02 am

This is a simple case of needing to evaluate the obstruction caused by ductwork with respect to sprinklers to the side of the obstruction.

I cannot provide a specific answer until I know where sprinkles are placed within the room.

Amod kumar Singh

1/31/2024 08:38:43 am

following

Dan Wilder

1/31/2024 08:40:27 am

What hazard and how deep are the ducts?

My thought is since sprinklers would likely be required in the middle of the total obstruction width (maybe you could use 2 on the sides with arm-overs under to get the spacing <15' (4.5 m) pending hazard spacing requirements), but dropping a sprinkler at the ceiling and another in the middle beneath the obstruction should not be too far out of the normal path and may provide a better layout pending branch line direction (it would be completely obstructed . I would keep the adjacent sprinklers close to the side of the duct to minimize the total ceiling level spacing (or lack of).

Common sense would say that the 10" clear (9" more than likely with deflector distance) provides no useable protection so it's omission would not be a loss but getting an AHJ to buy off on that is a whole other process.

I don't think there is a specific section defining this however, 13-19' - 9.5.5.3.1.5 allows sprinkler omission below obstructions that are 24" (<600 mm) or less above the floor or deck, that may be one option to help argue not putting sprinklers above the duct outline.

Another thought may be anything greater than 18" (450 mm) below as that is delineation between use of the obstruction tables and when obstructions are only considered when over 48" (1.2 m) wide. You're providing protection below so nothing above isn't going to reduce total protection.

Third may be calculating the space above the duct and argue that a ceiling pocket allowance allows up to 1000 FT³ (28 m³) (essentially available hot gas collection) without protection so another + as to omission of protection above that duct that is already "addressed", while not specific to this application.

Franck

1/31/2024 09:44:13 am

With the duct only 10 in. below the ceiling, you will have problems to provide your pipes and sprinklers on top of it + hangers...
No to mention the fact that the water distribution pattern from the sprinkler located in this tiny place will not be ideal... Remember the 18 in. rule between the deflector and the top of any storage/combustible element to protect... Dan pointed it out (but then you still need to convince the AHJ).

If the roof is non-combustible and you don't run cables or other combustible element above the ducts, I won't protect it above. But certainly below.
And consider the obstruction from the sides of your ducts for the other sprinklers in the area that will be placed at the ceiling level.

Jesse

1/31/2024 10:23:53 am

It would be good to know the building construction. If this is a NC building and the duct is up at the deck, I don't see a benefit in protecting above it. The duct will inhibit the spray pattern from developing.

Dave

1/31/2024 11:00:48 am

I agree with Franck here (when would I NOT agree with Franck?), You are protecting the floor area, not the top of the duct. Once the obstruction exceeds 4 ft wide, it becomes problematic. Even if you did have the 18 in. to develop the spray pattern above, that is a large shadow area below the sprinklers. Is there any space (or can you negotiate a space) between the individual ducts? I have sometimes been allowed to use the fairly new 9.5.5.3.1.2(2) (2019 ed.) placing a pendent sprinkler between and just below the ducts not more than 3 in. from the edge(s) of the obstruction(s).

1/31/2024 01:46:11 pm

Agreed 100%. Just a note, if you do use that section, the sprinkler needs to be intermediate type. Got caught on that before!

David Rowe

1/31/2024 11:38:57 am

One of the sprinkler vendors did a webinar a couple of years ago addressing this question and I think he made a good point. Where's the heat going to go? Obviously heads above the duct are seriously obstructed, but if the heat bypasses the heads beneath, how long might it take for those heads to eventually actuate? But if the heat gets to a head above the duct, even though it may not provide good coverage, at least we set off a flow switch and get the fire department rolling.

I believe that's the reasoning behind the rules for cloud ceilings. If the area around the cloud exceeds 20% of the total ceiling area, heads are required above and below. If you replace the word "duct" with "cloud" in your scenario, the answer seems much clearer.

Does a Duct Near Ceiling Need Sprinkler Above?

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