Problem: In air handling fan delivers 9,000 CFM against a total pressure of 3 inches water gauge at standard conditions.
Given: 11. In air handling fan delivers 9,000 CFM against a total pressure of 3 inches water gauge at standard conditions; 9...
Approach: So typically when you're changing fan size or speed and you're looking at the change in pressure of power these parameters of s...
Key formula: formula for finding air horsepower and that comes straight out of the reference handbook and since we're going to use...
Result: So in this case I'm going to just make two columns and we can say we have the original and the new and if the original I'll just call that case one...
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Student questions asked in live office hours about this problem
OH 77: HVAC: HVAC-11
OH 120 · May 11, 2026
Q: For HVAC-11, can you use fan law 3C (the Q1/Q2 ratio) instead of the law involving N, avoiding the use of speed entirely?
A: Yes, you can use Q1/Q2 instead of N1/N2 because volume flow rate and rotational speed are linearly related in the fan laws — they are interchangeable. However, Dan notes there is a typo in the reference handbook for the N-based law (exponent shown as 4 instead of the correct 5), which makes the Q-ratio form safer to use since it avoids that error.
Q: In the solution video for HVAC-11, you mention a formula for air horsepower (Q × ΔP / 6356) — where is that in the reference handbook and what's the search term?
A: If I don't say the search term out loud in the solution video, check the key search terms PDF at the beginning of each module, or the key formulas PDF — both are designed to help you find things without memorizing every location. Put them side by side and cross-reference until it clicks.