HVAC · Systems-and-Components · Problem 28PDFSolution in PDF ↓
HVAC · Systems-and-Components · Problem 28
Problem & Solution
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Student questions asked in live office hours about this problem
OH 49: HVAC: Systems And Components-28
Q: I found Q_total going from outdoor conditions to the saturation condition and subtracted Q saved by the enthalpy wheel — is this approach correct?
A: Take a look at the correction note below the video for context, but your general approach of finding total load and then accounting for what the enthalpy wheel recovers is conceptually sound. If you got the same answer, you're in good shape — let me know if a specific step didn't add up.
OH 96: HVAC: Systems And Components Module #28
Q: How do I know which given information matters in ERV word problems? You said the saturated supply temperature isn't needed — should I always look for enthalpy values first?
A: For a dedicated outdoor air unit with an enthalpy wheel, the core calculation is the enthalpy difference between entering outdoor air and the post-wheel condition — so yes, enthalpies are your primary targets. Some given values are there to set the scene or as distractors; with practice you'll get faster at recognizing which ones the energy balance actually requires. The saturated supply temperature wasn't needed here because we were solving for energy recovery, not the final supply condition.
OH 98: HVAC: Systems And Components Module #28
Q: Is my approach correct? I found the energy avoided by calculating the actual enthalpy difference from the numerator — I tend to overthink enthalpy wheel deltas.
A: You nailed it — I looked at your work and saw no issues, and you got the same answer. Don't overthink this one.
OH 106: HVAC: Systems And Components Module #28
Q: I calculated cost per ton-hour without the enthalpy wheel, then with it, and took the difference as savings — is this how simple payback is computed for an enthalpy wheel vs. the approach in problem 27?
A: Simple payback is always initial cost divided by annual savings, regardless of how you calculate the savings. Whether the savings come from an enthalpy wheel delta, reduced chiller load, or any other source, the denominator is the same — annual cost avoidance. Your approach of comparing cost with and without the wheel to get the annual savings is exactly right.