ANTIFREEZE SYSTEMS
By the Ironsmith Fire Engineering Team · Tennessee · Memphis, Nashville
Antifreeze sprinkler system requirements come down to three questions: what’s in the pipe, which edition of NFPA 25 your fire marshal enforces, and whether the solution passed its annual test. New systems can only use listed antifreeze. Older systems with non-listed fluid may or may not be allowed to keep it, depending on the edition. Every antifreeze system gets tested each year before freezing weather.
Quick facts
- Listed only for new work. Systems installed after September 30, 2012 have to use antifreeze listed for sprinkler use. UL 2901 is the standard those products are tested to.
- The edition decides old systems. The 2020 edition of NFPA 25 required non-listed antifreeze out by September 30, 2022. The 2023 edition dropped that deadline.
- Legacy fluid has limits. Under the 2023 edition, an existing solution can stay only if it tests at or below 30% propylene glycol or 38% glycerin and still protects to the required temperature.
- No topping off with old fluid. Anything added or replaced has to be a listed product.
- Test every year. The test comes before freezing weather. A failed sample, or a fluid nobody can identify, means drain and refill.
- Label it at the valve. A sign at the control valve should name the antifreeze type, the maker, the concentration and the system volume.
- Never automotive antifreeze. And no mixing concentrate on site.
Why antifreeze sprinkler system requirements changed
An antifreeze loop is a small wet system filled with a glycol or glycerin solution instead of plain water. It protects the spots that can’t stay at 40°F: attics, canopies, garages, docks and breezeways. For years, the fix for colder spaces was a stronger mix.
That changed after a 2009 kitchen fire in Truckee, California. A sprinkler holding a glycerin solution above 70 percent opened over a cooking fire, and the discharge flashed. One woman died. Testing by the Fire Protection Research Foundation at UL found that strong glycerin and propylene glycol solutions could ignite when sprayed onto a fire.
NFPA issued safety alerts in 2010 and emergency code changes in March 2011. The rules have kept moving since. That’s why owners are confused, and why the honest answer to “is my system legal?” starts with “which edition?”
What NFPA 13 requires for new antifreeze systems
Listed antifreeze only. Since the 2013 edition of NFPA 13, new antifreeze systems have to use a solution listed for fire sprinkler use. Listed products are tested to UL 2901. Generic glycol or glycerin from a drum doesn’t qualify, and automotive antifreeze never did.
Premixed, not field mixed. Listed solutions arrive ready to use. Mixing concentrate on site isn’t allowed.
Every product has its own limits. Each listing sets a lowest protection temperature and conditions on system size, sprinkler type and hazard. Some listed products can’t be used with ESFR sprinklers or in extra hazard areas. Read the maker’s data sheet against your actual system.
It’s not a one for one swap. A listed fluid can change the hydraulics. Converting an older loop may mean new calculations and, in some cases, piping changes. Pipe material matters too. Not every fluid is approved for CPVC.
The basics still apply. An antifreeze loop needs backflow protection to keep the solution out of the water supply and an expansion chamber to handle pressure swings.
What changed for existing systems
This is where most of the bad advice lives. Three recent editions of NFPA 25 treat older, non-listed antifreeze three different ways.
The 2017 edition
Systems installed before September 30, 2012 could keep non-listed premixed antifreeze until September 30, 2022, as long as it stayed at or below 30% propylene glycol or 38% glycerin. Stronger mixes needed an engineering risk assessment accepted by the authority having jurisdiction (AHJ).
The 2020 edition
Non-listed antifreeze had to be replaced with a listed product by September 30, 2022. If no listed product fit the system, the owner had to switch to another way of keeping the pipe from freezing.
The 2023 edition
The replacement deadline is gone. An existing non-listed solution can stay if it tests at or below 30% propylene glycol or 38% glycerin and still protects to the temperature the space needs. You can’t add old fluid or refill with it. Anything new going in has to be listed.
Which one applies to you
The edition your fire marshal enforces decides. Tennessee adopted the 2021 International Fire Code statewide in April 2025. Nashville’s fire marshal lists the 2024 edition. Each fire code points to its own edition of NFPA 25, and local amendments can change that. Ask your fire marshal which edition they enforce before you decide to keep, replace or convert.
If a page tells you antifreeze is banned, or that every old system had to be replaced in 2022, it’s skipping the edition question.
The annual antifreeze test
NFPA 25 calls for testing antifreeze every year before freezing weather. It’s part of standard ITM on any building with a loop.
Where samples come from. Older editions called for samples at the top and bottom of each system. Newer editions focus on the most remote part of the system and the point where the antifreeze section connects to the wet system. Larger systems need extra test points.
How it’s measured. A hydrometer or refractometer matched to the fluid in the pipe. Listed products follow the maker’s testing instructions.
What happens if it fails. If the concentration is wrong, or nobody can say what’s in the pipe, the system gets drained and refilled with a listed solution.
The test also catches a quieter problem: a sign that’s missing or wrong. NFPA 25 added the control valve sign in 2017. If yours doesn’t say what’s in the pipe, your next inspector will ask.
Other ways to beat the freeze
Antifreeze isn’t the only answer, and it’s not always the cheapest one to keep.
- Dry pipe or preaction systems. Air or nitrogen in the pipe, water held back in a heated room. Our guide to wet vs dry sprinkler systems covers the tradeoffs. If you go dry, you’ll need to drain your low point drains before every freeze.
- Dry sprinklers. Fed from a heated wet system, they reach into a cold space without water sitting in it.
- Listed heat trace. Electric heat on the pipe, supervised so a failure doesn’t go unnoticed.
- Heat or insulate the space. Sometimes the cheapest fix is the building, not the sprinkler system.
The right choice depends on the space, the hazard and what’s already installed.
What to do before the first freeze
Memphis and Nashville don’t get long winters, but they get hard freezes. Here’s the short list:
- Find every loop. Attics, canopies, garages and docks are the usual spots. Older buildings sometimes have loops nobody remembers.
- Read the sign. Type, maker, concentration, volume. If it’s blank or missing, that’s your first answer.
- Pull last year’s test. Check the date, the readings and whether anything was flagged.
- Ask which edition applies. One call to the fire marshal’s office settles it.
- Decide: keep, replace or convert. Base it on the test, the edition and the space.
Fold it into the rest of your winterization checklist. Our fire sprinkler maintenance team tests the solution, checks the sign and backflow, and handles replacement or conversion when a loop doesn’t pass. One call, one accountable team, from the test report to the fix.
Not sure what’s in your antifreeze loop? We’ll test it and tell you which rule applies.
Frequently asked questions
Are antifreeze sprinkler systems still allowed?
Yes. Antifreeze systems are still allowed, but new ones have to use antifreeze listed for fire sprinkler use, tested to UL 2901. Existing systems with older, non-listed solutions may be allowed to keep them or may need to replace them, depending on which edition of NFPA 25 your fire marshal enforces.
Do I have to replace my old antifreeze with listed antifreeze?
It depends on the edition your AHJ enforces. The 2020 edition of NFPA 25 required replacement by September 30, 2022. The 2023 edition lets existing solutions stay if they test at or below 30% propylene glycol or 38% glycerin and still protect to the required temperature. Any fluid added or replaced has to be listed.
How often does sprinkler antifreeze need to be tested?
Once a year, before freezing weather. Samples are pulled from set points in the system and checked with a hydrometer or refractometer. Listed products follow the maker’s test instructions. If a sample fails or the fluid can’t be identified, the system is drained and refilled with a listed solution.
Can I add antifreeze to my existing sprinkler system?
Only listed antifreeze. Topping off a system with non-listed glycol or glycerin isn’t allowed, and automotive antifreeze is never allowed. Mixing listed and non-listed fluids can also make the solution impossible to test accurately. Talk to your contractor before adding anything.
What is listed antifreeze for fire sprinklers?
Listed antifreeze is a premixed solution tested and certified for use in fire sprinkler systems under UL 2901. Each product has its own limits on the lowest protection temperature, system size and sprinkler types. It replaced the older practice of mixing glycol or glycerin to a target concentration.