Im a 24/7 burner, never an issue leaving it while its going. Funny side story, I occasionally help a friend who installs boilers and furnaces for a living, he started selling these oil boilers that had a pretty large site glass into the burn chamber, we did a large 5 section boiler for this lady, everything went well, 2 days later we're done and cleaned up, turn the boiler on and leave, we get back to his shop and his cell phone rings, the lady said he had to come back and that the fire dept was also going to the house because the boiler is on fire, we drive back to the job a little more briskly, caught the fire chief on scene, the engine already left, asked what happened, the lady said she saw flames, yup, she saw the reflection through the little 2" site glass port hole, even after explaining to her that everything was operating as normal she was still rather unsettled that there would be a fire burning inside her home anytime she needed heat, yes.. people...