by Grant Lobdell, President, Dyne Fire Protection Labs
Diesel fuel degrades over time due to oxidation, contamination (e.g. water), and microbial growth. As it degrades, insoluble particulates and sludge accumulate, which can clog fuel filters and foul diesel engine injectors leading to, at best, poor performance or, at worst, catastrophic fuel system damage and failure. For this reason, the 2026 edition of NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, requires periodic testing of diesel fuel:
8.3.4.1 Diesel fuel shall be tested for degradation at least annually.
A.8.3.4.1 Biodiesel fuel might need to be tested for degradation more frequently.
While no such exception is explicitly made in the standard, an argument is often made that this testing is not necessary if the pump is run (no-flow test) weekly for a minimum of 30 minutes as required by NFPA 25. The theory behind this argument is that the fuel will be burned up well before it gets old. However, consider the following from NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection:
11.4.1.3.1 Fuel supply tank(s) shall be sized for a minimum of 12 hours of engine run time based on the fuel consumption rate requirements of the engine as indicated on the manufacturer’s published data, plus 5 percent volume for expansion and 5 percent volume for sump.
What’s more:
11.6.4.1 The fuel storage tanks shall be designed and installed so that they can be kept as full as practical at all times but never below 66 percent (two-thirds) of tank capacity.
In other words, the tank should constantly be topped off well before it’s empty, resulting in a mixture of fuels of different ages. The old fuel is then never completely consumed, just diluted.
It’s also important to consider that contaminates, whether they be from fuel degradation or the tank itself (corrosion), can accumulate over time, significantly reducing fuel lifespan. For this reason, when deficient fuel is found, not only does the fuel need to be replaced, but the tank needs to be cleaned as well according to NFPA 25:
8.3.4.2 If diesel fuel is found to be deficient…the fuel shall be reconditioned or replaced, the supply tank shall be cleaned internally, and the engine fuel filter(s) shall be changed.
8.3.4.2.1 After the restoration of the fuel and tank…the fuel shall be retested every 6 months until experience indicates the fuel can be stored for a minimum of 1 year without degradation beyond that [which is] allowed.
Therefore, unless you are fully consuming all of the fuel AND cleaning the tank before each refill, periodic testing of your fire pump’s diesel fuel is necessary.
Questions? Contact us today by calling (800) 632-2304 or emailing dyne.lab@nfpaglobal.com.
©Dyne Fire Protection Labs 2026
