How to Measure Sprinkler Distance from Diffuser?

5/22/2024

NFPA 13 Section A.9.4.2.5 provides additional detail for temperature ratings of sprinklers located near horizontal hot air diffusers.

However, I cannot find anywhere that says specifically where the measurement is to be taken from.

Is the 2'-6" measured from the edge of the diffuser in all directions, or should it be measured as a 2'-6" radius from the center of the diffuser?

Thanks in advance!

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14 Comments

Pete H

5/22/2024 07:18:13 am

I've always gone edge with these measurements as it was more strict, but I'm curious to see if anyone has a different interpretation.

Anthony

5/22/2024 08:07:17 am

I agree the edge of the diffuser. think of an atypical diffuser that was 3' in diameter. You'd want to start that measurement at the edge of the diffuser not the middle. The air will be 'just' as hot at the edges as the center.

Todd E Wyatt

5/22/2024 08:15:14 am

From the "Enhanced Content" for NFPA 13-2022 9.4.2.5 per NFPA LiNK :

ASK THE AHJ

Q : When measuring the distance from a sprinkler to a horizontal HVAC diffuser, would an inspector measure from the near edges of the sprinkler and diffuser or from the center of the sprinkler and diffuser?

A : Although NFPA 13 does not directly address this issue, other provisions of the standard imply that the distance should be measured from the center of the sprinkler to the edge of the diffuser. For example, see Figure 9.4.2.5, 9.5.3.1, and Figure 10.2.6.1.1.3(A).

Casey Milhorn

5/22/2024 08:19:02 am

Great question and I think this should be something that gets cleared up in NFPA 13 (in my opinion). Some AHJs are ultra legalistic (which I can appreciate) and it benefits everyone to have this defined. For example, if you were to install a pendent, center of tile (within 1") but you ended up 11" away from edge of diffuser, does that now mean you need to install intermediate temp for just this one sprinkler? I know NFPA allows intermediate throughout as an allowance, but some AHJs don't allow, or the project specs can say otherwise. It would be great to see it defined as measured to the edge of the diffusers opening. That would allow for that 1 or 2 inches of variance of being center of tile.

Casey Milhorn

5/22/2024 08:47:45 am

Oh, and I will also point out a couple of issues we've experienced personally. Some of the extended coverage heads are listed in only 135 degree when used at 20x20 spacing. Well apparently, us, and one other contractor were using the 135 per the listing (in this specific brand). We actually had two sprinklers activate on two different projects where the sprinklers were spaced at 12" away from the diffuser. When tested, the diffusers were putting out between 120 and 125 degree air. it appears over time, the sprinkler bulb was weakened enough to eventually fail (that or the temp was higher at certain times).

Dan Wilder

5/22/2024 08:33:55 am

Well, the 2025 language indicates from the side (new language) as approved via PI 161-NFPA 13-2022:

Horizontal Discharge: "Semicylinder or cylinder with 2 ft 6 in (750 mm) radius from edge in direction of flow extending 1 ft 0 in (300 mm) below and 2 ft 6 in. (750 mm) above"

This also aligns more with the Figure 9.4.2.5 (Unit Heater) and the Figures A.9.4.2.5 (a) through (d) for fireplaces...at least from the edge.

For me, I use intermediate sprinklers throughout any residential occupancy just to allow the fitters much for flexibility for placement but will likely need to address intermediate temp sprinklers (in lieu of ordinary temp) for commercial spaces if I lose a full tile in all directions around any diffuser. Previously I just kept the 1' clear and called it good (in commercial at least).

I do wonder, when it's 115°F in the summer here, do I need to treat exterior doors as possible heat sources now....? hmmmm

5/22/2024 09:58:47 am

I agree with this approach as well. We have had no issues from plans reviewers, inspectors, insurance companies, or any other AHJ's... until this year, inspectors are requesting 2'-6" from the edge of the diffuser which forces us to the next tile, then possibly forces more sprinkler heads due to spacing if the HVAC contractor is not willing to work with us. Never had this issue in past years. Of course there is the option of intermediate temperature sprinklers installed 13" from the edge of the diffuser. Then that causes issues with the insurance companies. always has to be that one jurisdiction that like to cause headaches for no reason than to stand out amongst the rest.

Steve Cooper

5/22/2024 09:25:56 am

You would measure off the edge of diffuser because heat can come out of a corner. Because heat is being pushed out the entire diffuser you need to measure from the edge. I like to add a large rectangle around diffusers on my plans to prevent me from accidentally getting to close to them.

Chris Heyer

5/22/2024 09:36:49 am

The edge would make sense

Robert W Hughes

5/22/2024 11:31:08 am

The original language says "heat source" and was then conflated with all diffusers. In my opinion, the original language was not intended for office spaces with typical HVAC, but for open ceilings, gas heat, etc.

When "heat source" becomes any diffuser, say in an existing 3 story office building, suddenly no tile surrounding any diffuser can be occupied by a QR 155 sprinkler.

Some AHJ's, allow, and some companies around here do, intersperse intermediate temp heads wherever necessary when "2'-6" from edge of diffuser" is being specifically enforced for acoustical tile (or similar) ceilings. Even sprinklers within the same large compartments.
Well, there goes Response Time Index. Very important if not critical to proper system function.
Unless almost every head within a remodeled suite is changed to intermediate temp, a contractor.is in a pinch point But QR 155 is still the proper standard for light hazard office space.
So what gives?

Jesse

5/22/2024 01:58:19 pm

I've always done so from the edge in the direction of flow

Mike Russell

5/22/2024 03:52:48 pm

We have run into this issue quite a bit and have had some success arguing most HVAC systems in a typical office do not / cannot discharge air above 100 deg F (ref 2019 13-9.4.2.5(9)), which allows ordinary temp heads to be used throughout. This sometimes requires a letter from the building owner to convince the AHJ.

That said, we have a general company policy where we try to use Intermediate temp heads throughout whenever possible or allowed. Partly because it can reach temps over 115 deg F here and we cannot control the temp that heads are exposed to in the back of a delivery truck. While some manufacturers do have temperature sensors on the packaging, not everyone does.

...temperatures over 120 deg from a diffuser is crazy hot! Never seen that.

Mike link

5/22/2024 04:51:21 pm

Why not standardized with smoke detectors vs diffusers and measure from the edge.

Troy

6/2/2024 10:02:45 am

When considering sprinkler proximity to a diffuser in a commercial office, be sure to ask the building engineer or mechanical contractor if the HVAC system is used for heating or cooling. Most high rises use radiant heat at the perimeter for heat and use the HVAC system (aka the diffusers) for cooling only.

How to Measure Sprinkler Distance from Diffuser?

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