The _____ has water on one side and gasses of combustion on the other:

The heating surface is the correct answer because it is defined as that part of a boiler or heat exchanger where heat is transferred from the hot gases of combustion to the water. On one side of the heating surface, water is present, while on the opposite side, the hot combustion gases flow. This setup allows for efficient heat exchange, facilitating the heating of water, which is a crucial step in steam generation.

The other options do not accurately describe this specific component. The water surface typically refers to the level of water within a boiler and does not encompass the relationship with combustion gases. The steam surface describes where steam and water interact but does not specifically relate to the heat exchange process with combustion gases. The fire side usually refers to the area where the combustion occurs, which does not directly denote the interface combining water and combustion gases. Thus, the heating surface distinctly characterizes the area critical for heat transfer in heating systems.