#114 – Basic Fire Alarm Heat Detector Layout

by Shawn Lee.

References to NFPA 72 refer to the 2016 edition

Over the last 10 years or so, I have developed a love of doing fire alarm system layouts. It’s second only to my love of troubleshooting systems. System layout is an important aspect of fire alarm work, yet remains one of the subjects that is viewed as difficult, at least according to some of the folks I have spoken with. This article will introduce a few tips on how to do a basic layout for a given area. Before we get into the how it’s done, I’d like to take a little time to discuss why it’s done the way it is.

Heat and Smoke Spot-Type Fire Detectors

We will primarily focus our discussion on heat and smoke spot-type fire detectors. As you know, to make sure a fire is detected in the absolute minimum amount of time as possible, the detectors have a maximum distance they can be placed from the fire. Fire Alarm DetectorsThis distance is determined by the manufacturer with verification from a testing organization such as Underwriter’s Laboratory (UL) or Factory Mutual (FM). After verifying the detectors will perform as the manufacturers state they will, UL and/or FM will list the detectors for use at a specific spacing and those distances will be in the data sheets. Occasionally you can find detectors that have different spacing distances from UL and FM. For simplicity, assume our example detectors are listed for the same distances from all listing organizations.

There are a few things I would like you to keep in mind as you do a layout and then perform your review. For a space with a single detector located in it, the detector’s area of coverage should encompass the entire room. There should be no area or corner that is not covered by that single detector. If there is, then another detector is needed in that space. If the area will have two or more detectors in the area, there should ALWAYS be overlap in the detectors’ coverage. For example, if our space has two detectors, the space in the middle of the two detectors will be covered by both detectors. In an area that is square in shape, and has four detectors placed in it, the middle of the room will typically be covered by all four detectors. This is by design so that the fire will be detected as soon as possible by one of more of the detectors. If you follow the requirements of NFPA 72, you will not go wrong.

Today, we will concentrate on the location and spacing of spot type detectors on smooth flat ceilings. We will discuss different ceiling configurations in later posts. So, let’s begin with a room that has a simple flat ceiling that is 10 feet or less from the finished floor. The area we will be locating our detectors in is a 30-foot by 30-foot room. This room will eventually be filled with typical furniture, papers, wall décor, etc. There will be no hazardous items in the room. We will start with spot type heat detectors.

The Spacing Rules for Heat Detectors are defined in NFPA 72, Chapter 17

The spacing rules for heat detectors are defined in NFPA 72, Chapter 17, specifically Section 17.6.3 Location and Spacing. 17.6.3.1.1 provides the following:

One of the following requirements shall apply:

  1. The distance between detectors shall not exceed their listed spacing, and there shall be detectors within a distance of one-half the listed spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height.

  2. All points on the ceiling shall have a detector within a distance equal to or less than 0.7 times the listed spacing (0.7S).

Let’s break down the first requirement. The listed spacing comes from the manufacturer and will be on the data sheet for the detector. Spacing ranges between 20 feet to 50 feet. I have seen some spacing at 15 feet, but not very often. Regardless of the detector’s listed spacing, the process of properly spacing remains the same. For our purposes, we will go with a heat detector spacing of 30 feet. That means the detector can cover a space 900 square feet (30 feet x 30 feet).

Since our detector spacing is 30 feet, we know that we may not have more than 30 feet between any two heat detectors in this space. In addition, we now know that we may not have a heat detector placed more than 15 feet away from all walls and partitions that extend within 15 percent of the ceiling’s height. For our room with a 10-foot ceiling, this means that if there is a partition in this area that is 8.5 feet or higher, then it is treated as a permanent wall for detector placement. In our example room, there is no partition. Let’s see how a 30-foot detector will be located in a 30-foot by 30-foot (or 900 square foot) room following the requirements of NFPA 72.

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Based upon the illustration, let’s see if we adhered to NFPA 72’s rules for locating and spacing a spot type heat detector.

a. The distance between detectors shall not exceed their listed spacing – there is only one detector in the space as of now, so we have not violated the requirement for spacing, but we need to continue to make sure.

b. …and there shall be detectors within a distance of one-half the listed spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height – one-half the listed spacing in our example is 30 feet ÷ 2 = 15 feet. Our detector is within 15 feet from all four walls and there are no partitions in this area.

So far, we have a code-compliant detector layout for our 30-foot x 30-foot room. Now let’s take a look at the second option of 17.6.3.1.1. It states “All points on the ceiling shall have a detector within a distance equal to or less than 0.7 times the listed spacing (0.7S).”

This is relatively straight forward. Take the detector spacing (S) and multiply that by 0.7. In our case 30 feet x 0.7 = 21 feet. This means that every spot on the ceiling must have at least one detector within 21 feet. If that can be accomplished with one detector, great. If it takes two or more detectors, then we must ensure detectors are placed accordingly. For our example space, look at the illustration below.

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Our detector layout has met the requirement for option #2 as well. If you were to measure out the diagonal from the upper left corner of the room to the center point of the detector, you would get approximately 21 feet. Now we know that if we follow the requirements of 17.6.3.1.1, options 1 or 2, we will have a code-compliant layout every time. Next, we will look at a larger space to see how detector spacing works out.

Our next example area will be 50-foot by 50-foot. Our detector’s listed spacing will remain the same: 30-foot by 30-foot.

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We followed the same rules as stated above: detectors spaced no more than one-half the listed spacing from all walls measured at right angles (15 feet max). Only this time since the room is larger than the previous space we now have more than 15 feet between our detector and the far-right wall. We will have to place a detector no more than 15 feet from the far-right wall and see what we have.

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After placing the second detector, we find we have 20 feet between both detectors. Since our detectors have a listed spacing of 30 feet, we are not violating the requirements of 17.6.3.1.1(1). So far so good. Now we need to place additional detectors so that we make sure this room is properly covered by our detectors. Take a look at the illustration below.

This layout meets the requirements of both options of 17.6.3.1.1.

These two examples are just the tip of the iceberg. However, if you are not able to understand how we do a layout of this type it will be a challenge getting to the more complicated layout configurations involving sloped ceilings, peaked ceilings, joist ceilings, beamed ceilings, and ceilings higher than 10 feet from the finished floor. And don’t get me started with sloping joist ceilings that are higher than 10 feet. Once you get the basics though, it is easy to start applying the requirements of NFPA 72 to the different ceiling types.

Practicing Fire Alarm Detector Basic Layouts

If you haven’t had the opportunity to get familiar with spot type detector layout before now, I would highly recommend practicing a few times a week with basic layouts such as the type we discussed above. Start out with easy rooms. Keep them square or rectangular at first. I would also suggest using graph paper so it will be easy for you to develop a scale such as 1 square = 1 foot or 1 square = 5 feet. Use a ruler so that you can measure out everything to see how it all works.

If you are feeling extra motivated, use an engineering compass to help you see where a detector’s area of coverage actually is. Using the detector’s center point, you set the compass up so that the circle’s radius will be 0.7 x detector spacing and then draw the circle. In the case of our examples, the radius will be 21 feet. Believe it or not, the circle will encompass a 30-foot by 30-foot square area (900 square feet). That is the area of coverage for the 30-foot detector.

As you get the hang of it, start experimenting with moving the detectors closer to the walls to see how that might affect your detector layout. Take a look at the illustration below for an idea of what I mean. This is the same 30-foot by 30-foot room we first did a layout for. This time however, let’s assume the left detector needs to move closer to the far-left wall to avoid an obstruction. As we “pull” the left detector closer to the far-left wall, we now create a condition where that detector will be more than one-half the detector’s listed spacing from the far-right wall. To fix this, we must locate another detector within 15 feet to the far-right wall. Same room, same detector spacing, but we were not able to center the detector. There will be times when you will have to readjust your detector layout based upon what is already on the ceiling.

Read through the requirements of NFPA 72 Chapter 17, Initiating Devices before attempting to tackle detector layout. After you’ve read through Chapter 17 once or twice, then sit down with a bunch of graph paper, a ruler, and an engineering compass and go for it. It is easier than you might think.

 
 

To read the next article in this series, Link to: #119 – Fire Alarm Detector Layout Part 2: Irregularly Shaped Areas