If there is a leak and an ignition source, then it is easy to start a fire. Keep in mind that in most cases where a fire has begun when a flammable mixture has been present, the ignition source is not found. A sealed system with no oxygen (or oxidizing agents) present cannot catch fire unless a leak occurs.

Many thermal fluids also normally operate above their flash point and require that special precautions are taken in case of a leak. If there is a possibility that the normal degradation of your fluids can result in a lower flash and/or fire point, then it may be prudent to take the same precautions as if normal operation were above the flash point. The other option is to consider changing the thermal fluid to one that does not degrade in such a manner as to reduce the flash point.