Downside to Using Sidewall Sprinklers?

MeyerFire University | FX108.71H

By Joe Meyer, PE

RESOURCES

• One-Page Summary [PDF]
• Notes Page [PDF]

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: 8.1.4 Minimum Operating Pressure for a Sprinkler
• NFPA 13 – 2022: 9.5.4.2 Deflector Aligned Parallel to Ceiling or Roof Slope
• NFPA 13 – 2022: 10.3.5.2.1 Sidewall Must be at High Point of Slope

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TRANSCRIPT

Downsides to Sidewall Sprinklers

INTRODUCTION

The deep, dark, dirty downside to sidewall sprinklers. 

In our last segment, we shed light on the positive side of sidewall sprinklers. We covered where we could use them and why we use them in certain situations. 

Today, we're talking about the downsides. 

Sidewall sprinklers have a unique spray pattern that is especially sensitive to where the bulk of the water is thrown out and away from the sprinkler. It has a lot further distance to throw than an upright or pendent would to get the same floor area covered. So as a result, it's especially sensitive to obstructions because blocking the spray just a short distance in front of the sprinkler, could mean a much larger shadow area than if that same obstruction was next to an upright or a pendent sprinkler.  

So, there are really four key downsides that we're gonna discuss today that affect sidewall sprinklers. Just basically things we need to be cognizant of. Not that they're terrible sprinklers but just that we need to be aware of so that we can overcome those challenges. 

PROXIMITY TO A FIRE

Concern #1, proximity to a fire. 

Because a sidewall is placed on one side of a room, it has the potential to be much further away from a burning fuel source than an upright or a pendent would be. 

Let's take for example, a standard spray pendent that has a Light Hazard spacing of 225 sqft, say we're Noncombustible, Unobstructed, Hydraulically Calculated, which we'll cover later. The spacing could be 15 feet by 15 feet (or 4.6 by 4.6 meters). 

A standard spray sidewall sprinkler covering the same area could have a 14-ft spacing across the room.  

That means if a room is 14 feet wide, a pendent or upright could be as far away as 7.5 feet (2.3 meters) from a fire that's growing on one side on the furthest wall. 

That same fire on that same wall would be 14 feet away from a sidewall sprinkler in that room. 

If you look at this in plan view and you do a little geometry using the hypothenuse of the triangle to calculate distances from the fire to the sprinkler, the furthest point here from a pendent or upright would actually be 10.6 feet (3.2 meters) and the maximum distance from a sidewall would be 15.7 feet (4.8 meters), but the point still exists. That's about 50% further away that a sidewall can be as compared to an upright or a pendent. So, a sidewalk sprinkler is allowed to be further away from a fire source, and because of that, there could be a delay in the activation time of a sidewall sprinkler because our hot smoke plume would take a little longer to build up and activate that sidewall. 

Now, one of the ways we combat this delay is by adjusting the allowable height of a sidewall. The height that a side wall can be is much, much more fine-tuned than what we allow for pendents and uprights. We'll cover that in a whole lot more detail later. Just know that because of this potential delay, sidewall sprinklers are gonna be a little bit more particular on the height of the sidewall sprinkler to make sure that they're still responsive enough to a fire. 

SLOPES

Concern #2, ceiling and roof slopes. 

So, we've talked about the distance to a fire source, but what about the ceiling geometry? What happens with a slope ceiling or roofs? 

For sidewalls, slopes make things a little more complicated. If we have a slope ceiling or roof with too big of a hazard, we might not get consistent sprinkler activation and water spray in a very predictable fashion. Again, sidewall sprinklers could be further away from a fire source than operates our pendents. 

So, NFPA 13 provides some limitations on us for slopes. 

First is that with a slope ceiling or roof, that's over 2 in 12. That's 2 units rise versus 12 units horizontal. The sidewall sprinkler must be located at the high point of the slope and positioned to discharge downward. 

So, this takes a lot of design flexibility away in some situations. In a normal flat, smooth ceiling, we could locate a side wall likely on any of the four walls of a room. But if we've got a slope that's over 2 in 12, there's only one wall where we could locate that sidewall sprinkler. It's gotta be pointed down. It has to be at the high point of the slope pointing down the slope. If an entire ceiling or roof has one continuous slope that's over 2 in 12, let's say, to bring light in from a clerestory window down into a fancy architectural space, while this section of code is saying that if we use a sidewall that it would need to be at the high point facing down, that could be problematic. 

Can we feed a sidewall sprinkler from an exterior wall? 

Well, not if we have a wet pipe system and if we bump the pipe and the sprinkler inside the wall, now we have exposed pipe right above a window. 

That doesn't make a lot of sense especially for a fancy architectural feature. We're limited in how we can position sidewalls in some cases, and we're limited in how we can position sidewalls, and in some cases, it can be problematic. 

That compounds when we get into extended coverage sidewall sprinklers. For at least some extended coverage sprinklers, there are limitations in the listing for ceiling slopes. So, we always have to have confidence that the sprinkler that we're using is listed for the situation that we're putting it in. 

And that is the case with slopes. Some sprinklers haven't been tested and listed for specific slopes. So, if the product data for a sidewall sprinkler says that it requires a level horizontal ceiling, well, then we can't use it with the slope. 

COVERAGE

Concern #3, Coverage. 

Because of the nature of the sidewall sprinkler's throw, it's more susceptible to obstructions in front of the sprinkler and lack of coverage immediately beneath the sprinkler. 

This shows up in the obstruction rules for sidewall sprinklers. They're not the same as pendent and uprights. 

Sidewalls need clearance to throw a little bit up and a lot out horizontally. So, we can't have lights or beams or soffits or other obstructions that are going to interrupt that forward throw. The proper spray pattern for sidewalls is especially sensitive and in front of the sprinkler and right next to the sprinkler. 

Let's take a look at an example with our obstruction calculator right on the website (which you have access to); you can see visually how sensitive a sidewall is and how much longer clear space it needs than an upright or a pendent in order to develop its full spray pattern. 

Also, sidewalls kind of have a weak spot right below the sprinkler. What if a sidewall's located within a soit? Well, maybe the building owner doesn't want exposed pipes, so it's concealed within a soffit. Well, that's all great, but how wide of a soffit is too wide before we have to have a sprinkler below the soffit itself?   

NFPA 13 addresses this. Sidewall sprinklers cannot be too far away from the wall where they are mounted, and if they’re in a soffit, that soffit cannot be more than 8 inches (or 200 mm) before a sprinkler is also required below the soffit. That’s all described in Section 10.3.5.1 for Sidewall sprinklers. There’s also some nuances if there are cabinets below the sidewalls as well. 

So, with sidewalls, we have to be especially mindful of obstructions that are in front or near the sprinkler because of the unforgiving nature of its spray pattern. And we also have to be cognizant of making sure that we address the space immediately below a sidewall sprinkler and sometimes behind the sidewall sprinkler if it's projected out from a wall. 

PRESSURE

Concern #4, what about pressure? 

A standard spray pendent or upright only needs to throw 7.5 feet (or 2.3 meters) to each side. Well, if a sidewall is needing to throw almost double that distance across a room, wouldn't it need more pressure? 

Yes, it does.

Now, NFPA 13 Section 8.1.4 gives us some guidance that the minimum operating pressure for any sprinkler is the higher of 7 psi (0.5 bar) or the minimum operating pressure specified by the listing. 

Now standard spray sprinklers are going to stick to that 7 psi (0.5 bar) minimum pressure. But when we get into extended coverage, there, sidewall sprinklers are going to start needing much more pressure than an upright or pendent would in order to cover the same distance.

Why? It’s the pressure needed to achieve that longer throw.  

So, this shows up in the listing for each sprinkler, with the manufacturer’s publish in their product data. If you have extended coverage sidewall sprinklers, they're gonna need higher pressure to cover the same floor area that an extended coverage upright or pendent would. 

SUMMARY

So, in summary, sidewall sprinklers are good for a lot of things, but they do have some drawbacks. What are those drawbacks? Well,
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• Sidewall sprinklers are allowed to be further away from a fire source, which affects the height where we need to fine-tune the position of the sprinklers so that it's not too delayed from activating.
  
  
• Sidewall sprinklers have unique rules when it comes to ceiling and roof slopes, including being at the high point of a slope and throwing down along the same direction as the slope. That's for slopes over 2 in 12.
  
  
• Sidewalls are especially sensitive to obstructions in front of the sidewall or near it. 

Last, in order to cover similar floor areas, extended coverage sidewalls often require more pressure to achieve its spray pattern than an upright or pendent would. 

So, in our next segment we’re going to cover where we’re even allowed to use sidewall sprinklers out of NFPA 13 and FM Global. 

I am Joe Meyer, this is MeyerFire University.