Fire Up Your Future: Ace the 2026 City of Chicago Boiler Exam!

Heavy fuel oils are very viscous, especially in cold conditions. The heaviest grade contains more waxy content and higher molecular weight, so at room temperature it can be thick enough to resist being pushed through pumps, lines, and filters. Heating is used to lower that viscosity so the fuel can flow smoothly and be delivered to the burner with reliable atomization. If it isn’t heated, the pump can struggle, filters can clog, and combustion may become unstable.

Lighter fuel oils flow much more readily at normal temperatures, so they typically don’t require heating just to pump. Heating them is not usually necessary unless the temperature is so low that they begin to thicken, which is less common for these grades.

So, the essential idea is that the heaviest grade must be heated before pumping to ensure it can be pumped, filtered, and atomized properly for reliable boiler operation.