A characteristic that made early boilers very inefficient was:

The characteristic that made early boilers very inefficient, specifically the small heating surface, is crucial to understanding their performance. In boiler operation, the heating surface refers to the areas in contact with water that allows heat from the combustion process to transfer to the water efficiently, producing steam. When early boilers had a limited heating surface area, the transfer of heat was minimized, leading to incomplete heating of the water and ineffective steam production.

A small heating surface means there isn't enough area for heat transfer, resulting in longer heating times and higher fuel consumption for the same amount of steam output. This inefficiency not only wasted fuel but also decreased the overall effectiveness of the boiler system, leading to higher operational costs and lower performance in terms of steam generation.

In contrast, factors like the quality of fuels, the design of the internal furnace, and the prevalence of flat surfaces, while they may have contributed to inefficiency in other ways, are not as directly correlated with the fundamental mechanics of heat transfer and steam generation as the area of the heating surface. Therefore, the small heating surface stands out as a primary reason for the inefficiency in the early boiler designs.