Heaters are used to heat products. Line heaters operating on associated gas or liquid fuel are mainly used as heaters. Line heaters ensure smooth heating thanks to an intermediate heat transfer agent, and can be used to heat an oil emulsion, oil, gas, gas condensate, water, heat transfer agents at oil production, transportation and primary treatment facilities. Fresh water (mineralization of not more than 1 g/l), as well as any non-combustible non-viscous heat transfer agents (ethylene glycol water solution, propylene glycol water solution, antifreeze, tosol cooling liquid, etc.) can be used as an intermediate heat transfer agent.