HVAC · Psychrometrics · Problem 8PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 8
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Problem: Psychometrics problem 8.
Given: 000 feet above sea level, a one-ton fan coil unit, discharge is 54 degrees dry bulb 52 degrees wet bulb supplier, mai...
Approach: And then we have our FCU overhead supplying air into the room and returning air from the room.
Key formula: equation for total heat transfer, where we want to include both the sensible and the latent, would be 4
Calc: And this is happening at high elevation, so we're going to have to adjust our approach to accommodate for the fact that we're 5...
Calc: The room is 74 degrees and 50% humidity and the supplies 54 degrees dry bulb and 52 degrees wet bulb.
Result: So there's a sensible heat ratio that's not 1.
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Student questions asked in live office hours about this problem
OH 80: HVAC: Psychrometrics #8
OH 120 · May 11, 2026
Q: Why do the heat equation rule-of-thumb constants change at 5,000 feet elevation (4.5 to 3.74 for total heat, 1.1 to 0.92 for sensible heat), and does using the 5,000-foot psychrometric chart automatically correct for elevation when calculating heat gain?
A: The rule-of-thumb constants are baked in for sea-level air density, so at higher elevation the lower density changes those constants — the adjusted values (3.74 and 0.92) are found directly in ASHRAE handbook section 9.17. If using the 5,000-foot psych chart instead of rules of thumb, you must calculate heat as ṁΔh (mass flow rate times enthalpy difference), where mass flow rate equals density times volumetric flow rate, with specific volume taken from that chart to manually account for the elevation-corrected density.
Q: When reading the psychrometric chart for air at 54°F DB and 52°F WB, I get an enthalpy of 23.4, which leads to ~520 CFM, but using 23.3 gives ~510 CFM—will the actual PE exam really have answer choices this close when small chart reading errors cause different solutions?
A: You're right that these answers are too close, but since you fully understand the concepts and are within a couple percent, you should mark it correct and move on—there may be a handful of problems like this on the exam, so just do your best and don't lose sleep over it. Remember, practice problems aren't scored; what matters is that you understand the fundamentals.