Water-based fire protection systems that use antifreeze solution need to be inspected, tested and maintained regularly in order to ensure performance. NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, 2014 Edition requires any water-based piping exposed to temperatures below 40 degrees Fahrenheit to use antifreeze solution and recommends the following.

NFPA 25 Chapter 5.3.4 Antifreeze Systems:

  • Testing of the antifreeze solution must be done annually before freezing weather.
  • The antifreeze type shall be determined.
  • If the antifreeze is no longer permitted, cannot be determined, or is not within the requirements of the standard, it shall be replaced.

Determining the type of antifreeze solution present and making sure it is within acceptable concentrations is critical —not only to make sure it prevents system failure due to freezing but also to ensure the solution doesn’t become a fire hazard itself.  When sampling antifreeze solution for annual testing, NFPA 25 recommends a sample be taken from the top and bottom of each system.  Additionally, NFPA 25 requires samples be taken at the most remote portion and the water supply if they are not located at the top or bottom.  If the system is greater than 150 gallons, there must be an additional sample taken for every 100 gallons of antifreeze solution.

If a hydrometer or refractometer is used in the field to determine the concentration of an antifreeze solution, it is imperative that the type of solution be known and the corresponding calibrated meter be used. Table 1 shows an example of how different the specific gravity, concentration and freeze point will be when a refractive index of 1.3867 is measured for an antifreeze solution.  If the wrong meter is used for testing this solution, the wrong concentration and freeze point will be calculated.  It could also lead to the belief that the solution is acceptable in accordance with NFPA 25 when actually it is not.  Note that in this example the propylene glycol solution would not be allowed (it is mixed too rich) while the glycerine solution is acceptable.

Table 1. Example of how different antifreeze solutions can be even if the refractive indexes are the same.

Antifreeze Type

Refractive Index

Approx. Specific Gravity

Approx. Concentration (% by volume)

Approx. Freeze Point (°F/°C)






Propylene Glycol





If the system inspector is not able to perform this test or is unsure of the type of antifreeze, the sample can be sent to a qualified laboratory.  In a laboratory setting, testing in addition to specific gravity or refractive index can be completed to identify the antifreeze solution type, concentration and freeze point.

As of the 2014 Edition of NFPA 25, ethylene glycols are prohibited for use as an antifreeze in water-based fire protection systems.  For systems installed prior to September 30, 2012, NFPA 25 allows the use of only glycerine and propylene glycol which must be mixed at specific concentrations (Table 2). In general, the concentration of antifreeze solution should be limited to the minimum necessary for the anticipated minimum temperature.  If the concentration conditions are not met, the antifreeze must be replaced. 

Table 2. Antifreeze solution requirements for systems installed before September 30, 2012.

Antifreeze Type

Maximum Concentration (% by Volume) Allowed Without Risk Assessment

Maximum Concentration (% by Volume) Allowed With Risk Assessment




Propylene Glycol



Ethylene Glycol 

Not Allowed

Not Allowed

Existing systems installed before September 30th, 2012 that require new antifreeze solution are to use premixed antifreeze solutions that are factory mixed by the manufacturer with quality control testing in place to ensure the solution will remain homogeneous.  New installations of antifreeze systems require the use of a listed antifreeze solution.  However, since no antifreeze solution is listed as of this writing, NFPA 25 does not permit new installations.  Alternate freeze protection approaches other than antifreeze must be taken until a listed antifreeze solution is available.  Once listed solutions become available, the annual testing will be conducted in a similar manner to existing systems.  In extreme climates where the concentration of the solution needs to be greater than what NFPA 25 allows, an alternate approach other than antifreeze must be used to prevent freezing.

For additional recommendations on antifreeze solutions for water-based fire protection systems, please consult NFPA 25.

About the Author:

Grant Lobdell is the Senior Chemist and Lab Manager at Dyne Technologies in St. Paul, MN. Dyne Technologies is a compliance testing company offering testing of firefighting foam and antifreeze solution. Grant can be reached at 800-632-2304 or grant@dyneusa.com.