Combustible liquid

Flash point is the dividing line. A flammable liquid in DOT Class 3 has a flash point of not more than 60 C (140 F), while a combustible liquid falls above 60 C and below 93 C. For U.S. domestic ground transport, DOT also lets a flammable liquid with a flash point at or above 38 C (100 F) that meets no other hazard class be reclassed as a combustible liquid. Diesel fuel's flash point varies by grade, so check its SDS and shipping description rather than assuming. Many heating oils and some glycol solvents fall in the combustible range, though specific values vary by product.

Lower volatility does not mean safe. A combustible liquid can behave like a flammable one if it is heated, sprayed as a mist, or soaked into rags and insulation, where a large surface area allows vapor to form quickly. Hot processes, hot work nearby and warm storage rooms all raise the risk.

Definitions are not identical in every system. OSHA HazCom and the GHS use flammable liquid categories 1 to 4, and category 4 covers liquids with a higher flash point that were traditionally called combustible. Local fire codes and DOT rules also use their own thresholds, so a given liquid may be handled under more than one label.

Storage rules usually relax compared with flammable liquids but still apply. Limits on container size and quantity, proper ventilation, and keeping ignition sources away remain sensible practice. Check the fire code that applies to your site.