# Pressure rise in a fire sprinkler system? **URL:** \ **Category:** Factual Questions\ **Created:** [June 19, 2009, 8:47pm UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332 "2009-06-19T20:47:29Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [June 20, 2009, 6:48am UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332/21 "2009-06-20T06:48:56Z")
What you are talking about sounds like entrapment. As the water’s temperature increases the pressure will increase. does not happen on systems with a pump. Also if there is no air in the system. If there was air in system it would compress and the pressure would not increase. You can add a relief valve or an expansion tank. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [June 20, 2009, 6:51am UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332/22 "2009-06-20T06:51:19Z")
> [@mangeorge](#): > > That’s what they’re doing with ours. So when the system cools, the pressure should go negative. When the system cools the pressure should drop, once it is below the supply pressure more water will enter the system. If you are venting off enough water you could end up with a water bill. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [June 20, 2009, 12:52pm UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332/23 "2009-06-20T12:52:49Z")
> [@mangeorge](#): > > Wow! I live a sheltered life. I’ve been at this job for 25 years, and have never had a sprinkler head fail. We’ve lost a few but not since going with the higher rated heads. Our system is normally pressurized to ~200 psi so it isn’t such a big jump to \>350 here but it’s basically the same issue. They must have implemented the same set-up as **NinetyWt** described but I can’t find a copy of the report to verify it and the engineers don’t work on the weekend. > [@Snnipe\_70E](#): > > If you are venting off enough water you could end up with a water bill. The problem with that is one of the alarm conditions is usually water flow. The sprinkler heads have a glass bulb or fusible link holding them closed, when the bulb or link opens and water flows through the sprinkler the flapper on the pressurized side of the system open to allow more water to enter and replenish and at the same time send a fire trouble signal to the alarm panel. This activates the fire alarm and the fire pump in the systems I’ve worked on. Small leaks won’t trigger the water flow alarms but it doesn’t take a lot of flow either. I think the pop-off valves that were previously mentioned act as the expansion reservoir to absorb the pressure changes. I’ve never seen one of them though so I’m not sure about that part. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [June 20, 2009, 1:07pm UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332/24 "2009-06-20T13:07:50Z")
> [@HongKongFooey](#): > > We’ve lost a few but not since going with the higher rated heads. Our system is normally pressurized to ~200 psi so it isn’t such a big jump to \>350 here but it’s basically the same issue. They must have implemented the same set-up as **NinetyWt** described but I can’t find a copy of the report to verify it and the engineers don’t work on the weekend. > > The problem with that is one of the alarm conditions is usually water flow. The sprinkler heads have a glass bulb or fusible link holding them closed, when the bulb or link opens and water flows through the sprinkler the flapper on the pressurized side of the system open to allow more water to enter and replenish and at the same time send a fire trouble signal to the alarm panel. This activates the fire alarm and the fire pump in the systems I’ve worked on. Small leaks won’t trigger the water flow alarms but it doesn’t take a lot of flow either. I think the pop-off valves that were previously mentioned act as the expansion reservoir to absorb the pressure changes. I’ve never seen one of them though so I’m not sure about that part. I knew the pop off was a bad Idea and have never seen one but last night my brain was not comming up with why. But you are right it would trigger a flow alarm. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [June 20, 2009, 3:51pm UTC](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332/25 "2009-06-20T15:51:41Z")
> [@Snnipe\_70E](#): > > What you are talking about sounds like entrapment. > > As the water’s temperature increases the pressure will increase. does not happen on systems with a pump. Also if there is no air in the system. If there was air in system it would compress and the pressure would not increase. > > You can add a relief valve or an expansion tank. I agree. This is the most likely scenario. > [@Snnipe\_70E](#): > > When the system cools the pressure should drop, once it is below the supply pressure more water will enter the system. If you are venting off enough water you could end up with a water bill. I think the amount of water would be minimal. [Previous page](https://boards.straightdope.com/t/pressure-rise-in-a-fire-sprinkler-system/500332.md?page=1)