Problem: A fan in an office delivers 1900 CFM via a 14-inch main duct, which reduces after each branch takeoff to maintain an equal friction loss per unit l...
Given: 1900 CFM via a 14-inch main duct, which reduces after each branch takeoff to maintain an equal friction loss per unit...
Approach: The first branch receives 400 CFM and is located 50 feet from the fan.
Calc: All outlets have a design terminal pressure of 0.3 inches of water.
Calc: We have the fan itself and we know that the system requires 1900 CFM and we know that the main duct initially is 14 inches.
Result: It's very close so we have a friction loss of 0.3 inches of water per 100 feet of duct and in our case we have 200 feet of duct so it's going to do...
Office Hours
5
Student questions asked in live office hours about this problem
OH 19: HVAC 14
Q: In HVAC-14 (equal friction duct design), why don't we recalculate the pressure loss at each reduced flow rate after each branch?
A: The problem statement specified equal friction loss for the entire duct — that's the key hint that the equal friction method applies. Under that method, you design for a constant friction rate throughout, so each duct section is sized to that same friction rate regardless of reduced flow.
OH 20: HVAC 14
Q: In HVAC-14, why didn't you include velocity pressure in the solution when the problem asks for total pressure (total = static + velocity)?
A: The friction loss chart provides total pressure drop directly — velocity pressure effects are already embedded in the chart, so adding velocity pressure again would be double-counting. When you read a pressure drop from that chart, you have the total pressure loss for that duct section.
OH 65: HVAC: HVAC-14
Q: For HVAC-14, I understand the terminal pressure at the last outlet, but why isn't there an additional 0.3 inches WG for each outlet — why only for the last one?
A: You size the fan for the longest (highest resistance) path from fan to the farthest terminal — that path sets the required total pressure. Outlets closer to the fan have more static pressure available, which you handle through dampers or outlet sizing, not by adding their terminal pressures to the fan requirement.
OH 101: HVAC: HVAC-14
Q: In HVAC-14, why didn't we use the equation TP = SP + VP to find the fan's total pressure — could that approach give the same answer?
A: When you know the duct size and flow rate, the friction loss chart gives you total pressure loss directly without decomposing into static and velocity components — it's the intended shortcut. You could use TP = SP + VP and arrive at the same answer, but it's more roundabout.
OH 117 · March 30, 2026
Q: The student was confused about whether the problem was asking for total pressure (static + velocity) or just static pressure, and attempted to calculate both components separately by estimating duct diameters at each branch to find velocity pressure, then summing those with static pressure losses — arriving at 1.37 inches w.g., but unsure if this approach was correct.
A: Dan clarified that when using the friction loss table with the equal friction method, the losses in inches per 100 feet already represent total pressure (static + velocity combined), so there is no need to separately calculate velocity pressure at each branch step. The equal friction method assumes consistent losses throughout the entire run, allowing you to apply a single friction rate based on the main duct size and flow rate without stepping through each branch individually. Dan advised the student to keep the solution simple and revisit the solution video rather than pursuing the more granular approach.