Is Trapped Water in a Dry System Acceptable?

8/27/2025

Hi all, we are working on a project where we have installed a dry preaction system, with a main pipe along the ceiling, which has a conventional upright protecting the upper sections of the area, while pendent sprinklers protect the lower parts of the area.

Pendents are standard response sprinklers. Length of the pendent is around 16 inches (400 mm).

I would like to understand a few things related to the dry system:

Are these systems pressure tested pneumatically or hydraulically, or both?

If tested hydraulically, water will be drained, but the pendents will still be filled with water, correct?

Is this acceptable?

The temperature is the area that should be above 68°F. The pipe is galvanized. Thanks in advance.

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14 Comments

Franck

8/27/2025 08:10:52 am

Definitely not acceptable as indicated. Trapped water will increase corrosion and also sediment accumulation at pendent sprinklers that may prevent their proper operation.
I had a similar situation and requested major modifications.

Franck

8/27/2025 08:13:44 am

Only possible solution would be te provide return bend for the pendent sprinklers and remove the water by removing all pendent sprinklers after any ITC testing or sprinkler activation.

Dan Wilder

8/27/2025 08:31:13 am

To just address the questions, all references are to NFPA 13-25 Edition but these have not really changed in several editions.

Testing: Yes to both
Hydrostatic-29.2.1.1 - 200 PSI for 2 hours without loss (may be also be 50 PSI over working pressure if working pressure is above 150 PSI)
Pneumatic - 29.2.2.1 - 40PIS for 24 hours with no more than 1.5 PSI loss.

Filled with water - Yes. The hydrostatic test will do this no matter what.

Acceptable? - Depends. You stated this is an area that does not freeze and as such, does not run the risk of ice. The reason for the preaction is to avoid water above...so the drops being filled with water defeats that purpose.

Typically, there are return bends on the pendent sprinklers (8.2.2.1.1(3) and a means to drain the drop (a tee and plug). Galvanized piping...meh, not a fan.

Anthony

8/27/2025 08:34:02 am

Dan has a great answer here. I'd like to add there is a provision to test the system with air only during freezing times of the year.

Glenn Berger

8/27/2025 08:40:52 am

Excellent responses so far. The one comment to add is why are dry pendents not being utilized to prevent water from being trapped in the drops.

BTW - It is a pet annoyance of mine when I hear that someone is using a dry preaction system as is stated in the question.

D. Melandon

8/27/2025 09:22:56 am

If the pendants are the only way to protect the area in question you could/should install dry pendant assemblies that will not fill with water unless the specific head activates.

Gary Sims

8/27/2025 10:23:02 am

I agree with all of the comments above. I would ask, "Why is a preaction being installed?" We typically specify them for areas of high value commodity/ equipment, or of course freezing scenarios. If the desire/ intent is to avoid water and/ or freezing, then you have the answer: Install dry-type pendent sprinklers to avoid any possibility of even a limited amount of water discharging into the space, and as mentioned above, to avoid corrosion, scaling, sediment, etc.

Nick Vaccaro

8/27/2025 10:24:23 am

I work at an airport and ran into a similar problem during a retrofit. Dry pendants were not an option due to the length of the drop and complexity of the existing equipment beneath. We are going to try using dry flex heads for the drops. This will be the first time we use these. Does anyone have any experience on the pros and cons of these?

Brett

8/27/2025 12:05:52 pm

Pros - They are the only option in many unique situations. They're great for calcs since there is no extra equivalent length required for the hose.

Cons - $$$

Dan Wilder

8/27/2025 01:48:14 pm

Awesome product, just some limitations on available K-Factors.

Watch the bend radius and ability to drain at the top of the flex hose. We have had the most success (least issues?) with the longer hoses and making the drops closer to the ceiling.

Jose R Figueroa

8/27/2025 03:55:03 pm

I must remind FM that water in the drop fit will freeze, even in Texas. I saw it happen at my church. It completely defeats the purpose of a Dry Pipe system.

FM Global Data Sheets, especially Data Sheet 2-0, “Installation Guidelines for Automatic Sprinklers,” specify that only upright or dry-type sprinklers (such as dry-pendent, dry-upright, or dry-sidewall sprinklers) should be used on dry-pipe sprinkler systems. Standard pendent sprinklers are not recommended for dry-pipe systems unless they are specifically designed as dry-type pendent sprinklers. Sstandard pendent sprinklers may hold water, which can freeze and cause system failure. Dry-type pendent sprinklers, like the Model ESFR-17 or TYCO Series DS-1, are FM Approved for such uses when installed according to the relevant FM Loss Prevention Data Sheets.

Fred Walker

8/29/2025 01:59:35 pm

Technically, this can be done as the previous commenters indicated. My caution as others have indicated is unless you are somewhere other than south FL, TX, or CA, then freezing is a possibility. My experience has been the greatest sprinkler freeze losses occur in the southern US where folks say it "hardly ever get that cold here" Careful analysis of the National Weather Service Climate Data should be done to ensure the dry-bulb temperature never goes below 32 degrees because even a 1% chance means there is recorded data that freezing does occur.

Jose R Figueroa

8/29/2025 03:27:48 pm

In 2021, Texas experienced one of the most significant natural catastrophe losses due to freezing.

Jeff

9/7/2025 09:52:55 am

Depending on the accessibility of the drops, it may be possible to “crack open” the drop, then let a Dropmaster suck out the water.

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Is Trapped Water in a Dry System Acceptable?

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