What is a common unit of measurement of pressure drop in a forced hot water system?

Foot head is the correct choice because it specifically relates to the height of a column of water that corresponds to a certain pressure. In a forced hot water system, pressure drop is often expressed in terms of how much height of water would be required to exert that pressure, making foot head a practical and relevant unit of measurement for HVAC applications.

This unit is particularly useful in the context of water systems, as it directly correlates to the gravitational effects of the fluid. A pressure drop measured in foot head allows engineers and technicians to easily visualize the impact of system changes, such as pipe length, diameter, and elevation changes, which ultimately affect the water pressure in the system.

While psi, bar, and pascals are all valid units of pressure, they are less common in hydronic heating systems compared to foot head. Psi is often used in a variety of applications, bar is common in metric systems, and pascals is a standard scientific unit. However, foot head provides a more intuitive understanding of how pressure drops in these specific systems.