Q: What is the difference between a wet and dry sprinkler system?
A: Contrary to what the names might suggest, wet and dry sprinkler systems both use water to extinguisher the fire, the main difference is how the system causes the water to flow out of the sprinkler heads. A wet sprinkler system is pretty simple: heat opens the sprinkler heads and water automatically discharges from them. The system’s pipes contain water and are connected to a water supply which allows the system to work immediately once the heat triggers it.
A dry sprinkler system is a little more complex. In a dry system, the pipes contain air, or rather nitrogen under pressure. This pressure holds back water, much like someone’s finger over a running hose keeps the water from coming out, even though it is trying to flow. The air pressure works like someone’s finger over the hose, until heat pops the sprinkler head. Once the head opens, the pressure is released and the water is sent flowing through the pipes. It does not work instantaneously like the wet system, but is a much better option for areas susceptible to freezing temperatures, where wet systems often freeze and damage pipes.
Q: So are wet and dry sprinkler system heads different?A: Yes. Wet sprinkler systems allow the water to sit right against the opening of the head, which is why the water will flow immediately once the system is activated. In a dry system, the sprinkler head has an extended nipple with a seal above it, which prevents water from resting in it until the system has been activated.
Q: If they are different, why are you using dry heads on my wet sprinkler system?A: Dry sprinkler heads can be very helpful for use in areas that are covered by a wet sprinkler system but in danger of freezing. Preventing the water from resting directly against the head, as it does in a wet system, can sometimes be enough to keep the system working correctly in those isolated areas where conditions could lead to freezing. Examples include freezer rooms or outdoor areas without heat. The business may generally be able to use a wet system without issue, but have problems with these specific areas freezing or becoming damaged due to temperatures. The dry head can provide a little space between the water and the elements and thus keep the system operating correctly.
Q: Why are dry heads so expensive and why aren’t the ones I need in stock?A: Dry heads are expensive because they are all custom made to fit the exact specifications of your business’s unique system. Each one is made to order and can vary in length, orientation, temperature, finish, escutcheon type (also known as the trim plate), and response. This makes it impossible to keep every single type of head on hand, but the exact heads needed for your fire sprinkler system can always be ordered so that the system will continue to function properly.
Q: Why do my dry sprinkler heads need UL tested?A: Dry sprinkler heads that have been in service for 10 years need to be tested to ensure they are still operating correctly. Since they are designed to be holding back pressure, and the proper function of the system relies upon the powerful release of that pressure, the heads becoming weak can cause leaks which can make the system ineffective. The best comparison is to think of springs. If a big spring was placed on a desk, then a large book placed on top of it and left there over a decade, the spring would not bounce back the same way after the book was removed. In the same way, the sprinkler heads need to be tested to find out if the pressure has made them ineffective or if they still have enough strength to function properly.
Rather than replace all the heads, a section of them is taken as a sample, per NFPA guidelines, and sent to a UL lab to be tested for effectiveness. Those heads taken as samples will need to be replaced. If the heads pass the test, then the rest can remain the same and will need to be retested after another ten years. If the test fails, all the heads will need to be replaced to ensure the system will continue to work properly.
Fire sprinklers systems can be the most effective fire suppression tool when functioning properly, as they respond quickly to increasing temperatures and can put out fires where they start, before they have a chance to spread. Understanding what is required to keep them operating can help you understand the importance of having them inspected and maintained on a regular basis.
Have a fire sprinkler question we didn’t answer? You can always give us a call, we are here to provide for all of your fire protection needs.