Correct Height for Sidewalls?
NOTES & SUMMARY
CODE & STANDARD REFERENCES
Correct Height for Sidewalls?
So, what is the correct height for sidewall sprinklers? In our last segment, we talked about spacing rules for sidewall sprinklers in the two-dimensional plane. But what about vertically? Where do we locate sidewall sprinklers vertically?
In this video, we're covering the height of a sidewall sprinkler under the rules of NFPA 13.
CHALLENGES WITH SIDEWALLS
Now, sidewalls differ from pendents and uprights in that they're located near a wall that can be further away from a fire's hot smoke plume. That can be a challenge when we're concerned about the time to sprinkler activation.
So, in our series on sprinkler types and orientations, we talked about a few key downsides to the use of sidewall sprinklers. Downsides for sidewalls include (1) the potential distance that a sidewall could be from a fire source, because of its spacing rules, (2) unique challenges with ceilings and roof slopes, (3) the sensitivity of sidewalls to obstructions when those obstructions are near the sidewall sprinkler, and lastly, (4) greater potential pressure needs when we apply the same density over the same coverage area as a pendent or upright might have.
Now, because of these challenges with sidewall sprinklers, NFPA 13 only allows their use in a limited set of situations. We go into much more detail on what those are in our segment about where sidewall sprinklers are allowed. So, check out that video for more information there.
Of these four challenges, we tackle our concern about activation time by fine tuning the height at which a sidewalk sprinkler is allowed to be. We don't want sprinklers too low below a smoke plume where activation could be significantly delayed.
But we also don't want sidewall sprinklers too high either. Let's start there.
THE DEAD-AIR ZONE
If a sidewall sprinkler is too high in elevation, where it could be in the corner of a ceiling where a wall and a ceiling meet, there’s a chance it could be in a “dead air zone” where there’s no air movement.
What is that dead-air space?
Well in NFPA standards, which is not just NFPA 13 but also is NFPA 72 for fire alarm systems as well, this dead-air space is detailed as a triangular zone that’s 4-inches (100 mm) wide by 4-inches (100 mm) tall.
Why not within that space?
Well, when air moves towards a wall, you kind of have this natural buffer between the air that's already near that wall and the hot smoke that's moving over into that area. That buffer acts to slow and sometimes stop smoke movement where it wouldn't move all the way to a sprinkler or to a smoke detector or even a heat detector if it's located up in that pocket. That's what we're referring to as a dead air zone. It's not that air will forever not move or that it wouldn't work its way there. It's just that it's delayed because there's so little air movement right up in that pocket.
The upper corner of a room where the wall and the ceiling meet essentially acts as its own little air pocket. Well, that air mix over time, sure, but the slower or completely still air that's in that corner will significantly delay the activation of a sprinkler or a detector, and we certainly don't want that. We want predictable and timely activation.
When we have pendent or upright sprinklers, we're required to keep those a minimum of 4 inches (100 mm) away from a wall for this reason, and also to keep the spray pattern from the sprinkler from affecting itself.
When we have sidewall sprinklers, which are supposed to be mounted onto or near a vertical surface, this dead air pocket issue is always going to come into play.
In NFPA 13, this is addressed by having a minimum height that a sidewall sprinkler needs to be below a ceiling, and that is 4 inches (100 mm) minimum. We have to be 4 inches (100mm) down from a ceiling.
REQUIREMENTS IN NFPA 13
Chapter 9 of NFPA 13 is our starting point for sprinkler location requirements. But for this topic, Chapter 9 is pretty simple. It directs us to the chapters for the sprinkler type for type-specific requirements.
So, our standard spray sidewall sprinklers, we would go to Chapter 10, which is for standard spray. Then down to section 10.3, which is for sidewall sprinklers. Here we work our way down to 10.3.5, which is deflector position. Then, 10.3.5.1, right at the top, is the distance below ceilings. We found it.
Here, the code tells us to locate sprinklers no less than 4 inches (100 mm) from ceilings and no further than 6 inches (150 mm) from ceilings. So here we have a narrow zone of 4-6 inches or 100-150 mm down from ceilings. This is the best position to “optimize” sprinkler performance, as the commentary would say. Earlier activation, but it's low enough that it's out of that dead-air zone. However, as the next section details, sidewall sprinklers are allowed to be in a zone 6 to 12 inches (150 to 300 mm) down below ceiling, or in a zone that's even further down, 12 to 18 inches (300 to 450 mm) below non-combustible and limited combustible ceilings when they're listed that way.
Why is that?
WHY NOT 4-6 INCHES (100-150 MM)?
Well, there will certainly be situations where we'd like to use sidewalls, but the original 4-6 inches down from a ceiling, (100-150 mm) could be problematic. Perhaps we have a shallow soffit or maybe we have obstructions or maybe a coffered ceiling, and we'd like to have those sidewall sprinklers just a little bit further down in elevation. This allowance and code gives us more flexibility as long as the sprinkler is listed for installation in that manner.
The listing is the approval process that has to follow testing and verified performance of a sprinkler in that situation. So, the listing is saying that it's been tested for that situation.
These listings are gonna be specific to a sprinkler model and it'll show up in the manufacturer's published product data.
EXTENDED COVERAGE & RESIDENTIAL
Let's take a step out of the standard spray chapter and see what other requirements would apply for different types of sidewall sprinklers.
Chapter 11, which is for Extended Coverage, gets us down to Section 11.3 for Extended Coverage Sidewall Sprinklers.
If we work our way past the code section, where it lists where we're allowed to use Extended Coverage Sidewalls and go past the spacing rules, we now get to subsection 11.3.5 for the distance below ceilings.
Here, we have very similar requirements to Chapter 10. In fact, the verbiage is pretty much exactly the same. We should be 4 to 6 inches (100-150 mm) below the ceiling, but we're afforded a little more leniency downward if the sprinkler is listed for distances further down and it's non-combustible or limited combustible construction.
What about residential sprinklers?
Well, residential sidewall sprinklers are covered in Chapter 12. Chapter 12 is structured a little different from Chapter 10 and Chapter 11, but if we scroll all the way down to 12.1.8, which is deflector position, then to 22.214.171.124, we have the same original requirement to locate sprinklers 4-6 inches (100-150 mm) from the ceiling unless the listing allows for greater distances.
So, if a listing allows a sidewall to be, say, 6-12 inches down from a ceiling (150-300 mm), then what’s wrong with locating them down that far?
Well, the activation will be delayed. That's the fundamental downside. But if the sprinkler has been tested and listed for that height, then I don't think we're gonna lose any sleep over that.
But from a design perspective, we can get a little penalized within the listing of the sprinkler itself. When we lower a sprinkler in elevation to potentially say double or triple the distance down from the ceiling, well, that'll delay the time that the glass bulb will break, or fusible link will separate and release water from the sprinkler. It'll take more time for heat to build up and work its way down in elevation past the sprinkler, heat up the sprinkler and activate it.
The fire at that point, at that delayed point and the heat massing in that space, well, they're both gonna be greater for the sprinkler. So, what the sprinkler's experiencing is a slightly larger fire and a hotter space that it now has to fight. All because the sprinklers has a slightly lower elevation and because of that delay in the activation.
So, what is the penalty? What is the penalty that we're getting in that product data? In some cases, not always, but in some cases, sidewall sprinklers that are installed at a lower ceiling elevation or require greater pressures in order to meet their listing. The K factor is staying the same so if you have a greater pressure, you also will have a greater flow. There's a larger fire in the space and there's more heat buildup within the space so more water is needed to cool and suppress that same fire. Therefore, because of that, we have greater flow and pressure needs for the sprinkler to be installed lower in elevation.
As a real world example, here’s a product data sheet for the Reliable k5.8 horizontal sidewall sprinkler. This is a residential type sidewall sprinkler.
Then you’ll see here, below information about the sprinkler itself, that we are given the coverage areas for the sprinkler, and the minimum flow and pressure that is required for each of these coverage areas, and, they’re grouped specifically based on the distance from the deflector to the ceiling.
This kind of chart is not uncommon for sidewall sprinklers. We have to supply them with a minimum pressure of 7 psi (or 0.5 bar), or the pressure that is required by their listing, whichever is greater.
Let’s look as if we have a 14-ft x 15-ft room (4.3 x 4.6 m). We look at the coverage area on the first ccolumn, and have to select the coverage to the next step up, so that’s 15-ft x 15-ft (or 4.6 x 4.6m). We’re given minimum flow and pressure needs, but on this far right column, we get the deflector to the ceiling distance.
Now, if we are locating this sprinkler, say 4 inches down from the ceiling (100m down from the ceiling), then our minimum pressure is 10.7 psi (or 0.74 bar).
Now, if we locate this same sprinkler further down, let’s say it’s 12 inches (300 mm) down from the ceiling, now our pressure requirement gets all the way up to 17.1 psi (or 1.18 bar).
That might not sound like much of a difference, but remember this is the minimum flow and pressure needs for a sprinkler. This comes into play at our most-remote sprinkler and compounds as we work our way or calculate our way back upstream.
Now, as a designer, if we have the option to have lower pressure and flow needs for our most remote sprinkler, we're gonna take that advantage where we can have it. We want the system to be as efficient as possible. So just by locating the sprinklers in our zon 4 to 6 inches (100 to 150 mm), we're allowing ourselves to be more efficient. We have less strenuous requirements in the listing of the sprinkler. So, there's a big advantage to that.
Now, there's a caveat here I don't wanna skip over. Looking at the minimum pressure and flow needs for a sprinkler at the coverage area is one part of the task. When we do hydraulic calculations, we need to satisfy those minimums, but we also need to be sure separately that we're achieving the density that's needed for the space.
So, to do that, we take our density multiplied by the protection area that the sprinkler is covering and and get our minimum flow that that sprinkler needs to provide. So, if this is used in an NFPA 13R system and our density is 0.05 gpm/sqft (2.0 mm/min), and we multiply that by our protection area of 15 ft x 15 ft (or 4.6 by 4.6 m), we're gonna get a minimum flow that's actually gonna be less than this listing. So, this listing will be the driver.
But if we have an NFPA 13 system with a density of 0.10 gpm/sqft (or 4.1 mm/min), and we multiply that by our protection area, that minimum flow is gonna be more than this listing, so that's gonna be a driver. It's the density in the area that are gonna drive the minimum requirements for the sprinkler.
So, our first check is to make sure we meet the sprinkler's listing. The second check is to make sure we actually achieve the density from our design criteria. Those are two separate considerations, and we'll cover that in a whole lot more detail later. I just don't wanna skip over that while we're looking here at the product data, that we also need to be cognizant of the actual density that we're providing for the space
The sprinkler, even at a higher elevation, could have higher pressure and flow requirements than what's published here in the product data, depending on the design criteria for our space. Again, plenty more to cover there. I just didn't wanna skip over it now because that's a big caveat when we're looking at minimum pressure and flow requirements for sprinklers.
So, Joe what you’re saying is if we can keep all of our sidewalls 4-6 inches (100-150 mm) down from a ceiling, that's the way to go, right?
Well, if it's possible, if it's achievable, sure. That's the simplest approach. That is what the commentary is saying is our optimized location. But we do get into situations where that's just not possible.
Let's say we have an exterior balcony that has exposed wood construction underneath. Let's say the balconies built with solid lumber like 12 inches (300 mm) deep lumber. And we don't want a whole dry sprinkler system just to serve the balcony. Instead, we want to use dry sidewall sprinklers.
Well, here we kind of get ourselves into a tricky situation because the depth of the joists would suggest that we're out of line with the listing of our sprinkler. This is a relatively common situation that could result in an installation where you have a sprinkler that's not in conformance with its listing.
Now, at the time of this publication, I'm not aware of a dry sidewall that is listed to be located more than 12 inches (300 mm) down below a ceiling. It's possible that one exists and I just don't know about it. If you know more information than I do, make sure to comment below this video.
But the short rundown is that if we located a dry sidewall sprinkler beneath the structure on this exterior balcony, it would be a non-conforming installation. We're locating that sprinkler too far down below the deck, or we're locating that sprinkler over 12 inches (or 300 mm) down from that deck or ceiling to be in accordance with this listing.
However, this is a rare situation, but we actually get some relief from the International Building Code for some situations like this. Now, not all projects are under the IBC, but if the IBC were to apply here, Section 903.3.1.2.1 under NFPA 13R standards, we’re actually allowed to use, we’re permitted to use sidewall sprinklers under balconies and decks with deflectors 1-6 inches (25-152 mm) below structural members, so that's below the bottom of those wood joist and allowed a maximum distance of 14 inches (356 mm) below the deck of open wood joist construction. This is a rare case where a building code is actually working to overrule some of the limitations that we have specifically when we're under NFPA 13R and the IBC.
So other than unique situations like this, our normal mode of operation would be to first try to locate the sidewall sprinkler 4 to 6 inches (100-150 mm) down from a ceiling. And if that's not possible, then to check the product listing and make sure we can achieve the greater demands of the sprinkler itself while still designing in accordance with its listing.
So, what is the correct height for sidewall sprinklers?
Well, in general, our best location is to locate sprinklers 4 to 6 inches (100 to 150 mm) down below the ceiling. That's the optimized location that has the least strenuous requirements.
If we can't be in that zone and we have a non-combustible or limited-combustible ceiling, we move the sprinkler down in elevation until we can find a height that works, being sure to check against the product data for the sprinkler model to make sure it's still within a sprinkler's listing, and we're meeting the needs that go along with that listing.
In our next segment, we'll run an exercise on a sidewall sprinkler layout. Thanks for tuning in.
I’m Joe Meyer, this is MeyerFire University.
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