Wet vs Dry Sprinkler types of systems from the NFPA (National Fire Protection Association):
- Wet sprinklers are the most common type of sprinklers. In this system, the sprinkler piping is constantly filled with water. When the temperature at the ceiling gets hot enough the glass bulb or fusible link in a sprinkler will break. Since the system is already filled with water, water is free to flow out of that sprinkler head. Contrary to what Hollywood would have you think, not all sprinkler heads will operate at once in this type of system. The temperature around that specific sprinkler head needs to be high enough to break the glass bulb or fusible link that is holding water back. Once that happens, water will immediately start flowing from only that head. Wet pipe sprinkler systems are the most reliable and cost effective. Therefore, they should be the first type considered when selecting a sprinkler system. However, there are times when a wet pipe sprinkler system may not be appropriate. One of the major factors in determining if a wet pipe system can be used is the temperature of the space can be maintained above freezing.
- Dry sprinklers are very similar to wet pipe systems with one major difference. The pipe is not constantly filled with water. Instead, the water is held behind a dry pipe valve, usually some distance away from where the sprinklers are located. Like a wet pipe system, when the temperature at the ceiling becomes hot enough, the glass bulb or fusible link of the sprinkler breaks. However, in this case, water isn’t immediately available because the pipe is not water filled. Instead, air is released from the now open sprinkler head. This creates a drop in pressure causing the dry pipe valve to open and water to fill the system. Water will then flow from the open sprinkler head. A dry pipe system is a great option for unconditioned spaces, or locations where the temperature of the space cannot be guaranteed to be high enough to prevent water in the system from freezing.
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