Increasing the efficiency of small hot-water boilers using crude oil as fuel was achieved by designing a high-efficiency 3-MW boiler with an external remote economizer. By reducing flue gas temperatures and optimizing the boiler layout, a 94% efficiency value for crude oil combustion was achieved. The design includes a three-way boiler and a two-way remote economizer, allowing for easier servicing and maintenance. This design solution improves the reliability and efficiency of liquid fuel combustion in small-sized boilers.