What is the duct design friction loss when the available external static pressure is 0.80 inches of water column, with static pressure drops totaling 0.65 inches?

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In HVAC duct design, the friction loss is determined by taking the available external static pressure and subtracting the static pressure drops that have already been accounted for in the system. In this scenario, the available static pressure is 0.80 inches of water column, and the total static pressure drops are 0.65 inches.

To find the duct design friction loss, you subtract the total static pressure drops from the available static pressure:

0.80 inches (available static pressure) - 0.65 inches (static pressure drops) = 0.15 inches.

This means that the remaining friction loss that can occur in the ducts while still operating effectively within the system is 0.15 inches of water column. This figure reflects the pressure loss due to friction in the ductwork itself, which is crucial for maintaining airflow efficiency.

Therefore, the correct answer aligns perfectly with the calculated duct design friction loss.