HVAC · Systems-and-Components · Problem 4

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Student questions asked in live office hours about this problem

OH 19: HVAC: Systems and Components-4

Q: For Systems and Components 4 (technology room), why isn't ventilation taken into account even though the problem doesn't explicitly say to ignore it?

A: The problem says it's an unoccupied technology room — that's my signal that ventilation (driven primarily by people requirements) isn't the focus here. In real practice you'd include minimum ventilation, but without occupants or explicit ventilation requirements, ignore it for this problem.

OH 40: HVAC: Systems and Components-4

Q: For the filtration problem (Systems and Components 48), does 'the new system will increase static pressure by 1.5 inches WG' mean the new pressure is 2 × 1.5 = 3.0, or that it adds 1.5 to become 2 + 1.5 = 3.5 inches?

A: It's the latter — 'increase by' means additive, so the new total pressure is the original plus 1.5 inches. 'Increase by' is never multiplicative unless the problem says 'increase to' or 'double.'

OH 43: HVAC: Systems and Components-4

Q: For Systems and Components 4, why use the rule-of-thumb Q = 4.5 × CFM × ΔH instead of Q = ṁ × ΔH using mass flow rate from the entering specific volume — is it because Cp of air doesn't change much?

A: I'm going to answer with a question: did you try both approaches and compare the results? If you get the same answer, that confirms they're equivalent; if you get a different answer, that's a signal about when the rule of thumb breaks down — that self-check is worth doing.

OH 98: HVAC: Systems and Components-4

Q: For Systems and Components 4, I calculated 180 CFM using the electrical equipment room ventilation rate (0.06 CFM/ft²), and understand the answer choices are far apart — but shouldn't HVAC systems always include minimum outside air?

A: Minimum outside air is primarily driven by occupant requirements, and this space is unoccupied — without people, the ventilation rate is typically negligible. If the answer choices were close enough that ventilation mattered, that would signal the problem expects you to include it.

OH 105: HVAC: Systems and Components-4

OH 119 · April 28, 2026

Q: When the sensible heat is already given in the problem, why use the total heat gain rule of thumb for infiltration rather than computing latent separately — wouldn't that double-count sensible?

A: There are three distinct loads: the internal sensible equipment load (given directly), the infiltration sensible load from hot outside air, and the infiltration latent load from humid outside air. The total heat gain rule of thumb handles both infiltration components together without double-counting the equipment load. Since the question asks for total cooling load in tons, all three are simply summed in BTU/hr and converted.

Q: For Systems and Components 4, why don't we account for outdoor area ventilation (0.18 CFM/ft² from table 9.3.7.5), and why is the ventilation load not considered?

A: The unoccupied technology room designation is the clue to ignore ventilation — no people means no occupant-driven fresh air requirement. For unoccupied spaces, the ventilation load is generally treated as zero unless the problem gives explicit ventilation requirements.