In my June article, I discussed the various types of smoke detection and the requirements for their spacing as NFPA 72 outlines. In all cases, the code requires the manufacturer’s published instructions to be followed. The code also allows other spacing to be used depending on factors such as ceiling height, different conditions other than “normal,” or when different response requirements need to be met.
Most contractors may know the code requirements but do not understand the correct detector type for all applications nor the limitation for the detector types.
For example, the spot-type smoke detector has a maximum allowed spacing of 30 feet on center and 15 feet from a wall; however, does this spacing work for all applications? The short answer is no. The spacing should first be determined based on the detection goals of the owner.
Let’s assume the owner wants to detect a smoldering fire. A smoldering fire, by definition, will have very little “thermal lift,” meaning there is little heat to force the smoke to rise quickly to the ceiling and reach the detector. Air conditioning systems and high ceilings (e.g., greater than 12 feet) will also affect the chances of the smoke getting to the detector while the fire is in a smoldering state. This is where your knowledge of how smoke detection works based on the type of fire will be important.
If the owner is adamant that they want to detect a smoldering fire and you know the conditions of the space will work against that goal, you will need to advise the owner of that consequence. If you don’t and you install the spot-type smoke detector on a high ceiling or in a room with high air flows, there is a better-than-even chance that the owner will hold you responsible when the fire happens and grows well beyond the smoldering stage before detection.
Your options to meet the owner’s requirements could include closer spacing of spot-type smoke detectors or the use of active-air-sampling-type smoke detectors (sometimes called aspirating detectors).
One cardinal rule of smoke detection is the higher the ceiling, the larger the fire will have to be before detection. Said another way, it is good design and installation practice to never install a spot-type smoke detector on ceilings greater than 15 feet. Any application above a 15-foot ceiling height would require, at a minimum, the linear beam smoke detector. Again, as the ceiling height increases, the fire size at detection will increase. Also, with higher ceilings, there is a strong chance that the smoke will stratify before it gets to the detector until the fire gets hot enough to overcome the temperature at the ceiling. Furthermore, the higher the ceiling, the more likely air flow from the HVAC system will impact the smoke, both by dilution and pushing the smoke away from the detector.
High air flow has a major impact on smoke detection and often occurs in clean room applications. The active air sampling or aspirating-type smoke detector is used almost exclusively for this application, although there is at least one very sensitive spot-type smoke detector that has been used in this application. Although there is a table in NFPA 72 2016 that recommends closer spacing of standard spot type smoke detectors based on the air flow, I would not recommend spot-type smoke detectors for this high airflow environment.
A duct detector is a passive air sampling detector in that the HVAC system must be operating to push the diluted smoke to the sampling tubes. Duct detectors are never used for area smoke detection. The primary goal of duct-type smoke detection is to prevent recirculation of smoke from the fire area to other areas of the building.
So as a rule of thumb and a quick recap, here are my recommendations with regards to smoke detector types and applications:
- Spot-type smoke detectors on 30 feet or less spacing—all spaces with ceiling heights 15 feet or less
- Linear beam smoke detectors—all spaces with ceilings greater than 15 feet
- Active air sampling smoke detectors—all spaces with high air flow or spaces with high ceilings and a goal of early detection
- Duct-type smoke detectors—all applications where the goal is to prevent the recirculation of smoke via the HVAC system
I have yet to discuss heat detection, but as you might guess, heat detectors are not considered life safety devices as they only operate when the fire progress to a high heat flaming stage or is growing rapidly.
In my next article, I will look at notification appliances and discuss the concepts of audibility, intelligibility and visibility.