Why can't a vacuum system be converted to a gravity pumped return system?

The choice that indicates the smaller pipe sizes would not have sufficient capacity accurately reflects a critical aspect of converting systems. In a vacuum system, the pipes are designed to operate under lower pressure conditions, which allows for the efficient movement of fluids without the constraints imposed by gravity.

When considering a gravity pumped return system, the dynamics change significantly. Gravity systems rely on larger diameter pipes to accommodate the flow of liquids, which must overcome gravitational resistance and maintain adequate flow rates. If the existing vacuum system uses small diameter pipes, they may restrict the flow needed in a gravity system, preventing proper drainage and circulation of the fluid. Thus, the capacity of the smaller pipes can hinder effective operation of a gravity return system, making this option the correct answer.

In contrast, issues related to boiler feed pumps, space for the return system, and specific operating pressures are less central to the main structural and fluid flow challenges presented by switching from a vacuum system to a gravity-based system. These factors may play a role but do not directly address the fundamental requirement for adequate pipe capacity in ensuring efficiency and functionality in a gravity return system.