What type of boiler uses hot gases to heat water in tubes?

A firetube boiler is the correct answer because its design involves hot gases produced from combustion passing through tubes that are immersed in water. This arrangement allows efficient heat transfer from the hot gases to the water surrounding the tubes, effectively raising the temperature and producing steam or hot water.

In a firetube boiler, the heat from the combustion gases is contained within the tubes, while the water that needs to be heated surrounds these tubes. This configuration is typically used in smaller applications where high pressures and steam generation might not be required, making firetube boilers highly effective for specific heating needs.

Other types of boilers, such as watertube boilers, operate differently. In watertube boilers, water flows through tubes that are heated by hot gases outside the tubes, which allows for higher pressures and larger steaming capacities. Electric boilers use electrical resistance to heat water directly, and steam boilers can refer to both firetube and watertube designs depending on their operation, but do not specify the heat transfer method in the same way that a firetube boiler does. Each of these boilers has its own applications and operational characteristics, but when it comes to using hot gases to directly heat water within tubes, the firetube boiler stands out.