Which of the following is a disadvantage of water tube boilers?

Water tube boilers do require more floor space compared to fire tube boilers. This is because water tube boilers are designed with multiple tubes that contain water and are positioned in a way that allows for better heat exchange, often resulting in a larger overall footprint. The arrangement often necessitates more horizontal and vertical space to accommodate the essential components, such as headers, drums, and support structures, which contributes to the increased space requirement.

In contrast, other options focus on characteristics that typically favor water tube boilers rather than disadvantages. For example, they are generally known for their ability to be assembled quickly in the field, making them advantageous in certain settings, and their initial costs can vary considerably based on design and manufacturer's factors, often being lower in some contexts. Additionally, while they may have fewer openings for air filtration, this is not universally seen as a disadvantage or a significant factor in their overall footprint.