Problem: a warehouse located in a damp climate uses two-pipe heating fan coil units to maintain a maximum relative humidity of 60%.
Given: 200 CFM fan coil units; 1.08 CFM delta T
Approach: So this is an interesting situation we have a warehouse and they're using heating fan coils to keep the relative humidity from ...
Calc: So this is an interesting situation we have a warehouse and they're using heating fan coils to keep the relative humidity from ...
Calc: This might be the 80% curve and maybe this is the 60% curve, something like that.
Result: The best answer choice is A.
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Student questions asked in live office hours about this problem
OH 29: HVAC 5
OH 120 · May 11, 2026
Q: For a warehouse problem using two-pipe heating-only fan coil units to control humidity, why do we use only the sensible heating capacity rather than calculating enthalpy difference between both states?
A: Heating-only fan coil units add only sensible heat, so the process on the psychrometric chart is a horizontal line at constant humidity ratio — no humidification or dehumidification occurs. The relative humidity drops not because moisture is removed, but because warmer air has a greater capacity to hold moisture, so the same absolute moisture content represents a lower relative humidity at higher temperature.
OH 121 · May 25, 2026
Q: Can the total heat rule of thumb (4.5 × CFM × Δh) be used to calculate sensible-only heating loads, or is the logic flawed when there is no latent component?
A: As long as the process line is truly horizontal on the psychrometric chart (i.e., humidity ratio is identical at both state points), using the enthalpy method captures sensible heat only and is valid. However, for a purely sensible process, the sensible heating rule of thumb (1.1 × CFM × ΔT) is simpler and preferred; a ~15% discrepancy between methods likely stems from density/specific volume differences at the elevated dry bulb temperature.
Q: Can you explain the rule of thumb for Q_water and Q_air — what's the underlying logic, and what's the alternative approach using the PE reference handbook?
A: The rule-of-thumb equations (Q = 1.08 × CFM × ΔT for air; Q = 500 × GPM × ΔT for water) are derived by folding the specific heat and density of each fluid at standard conditions directly into the formula. If conditions deviate significantly from standard, fall back on the full mass-flow-rate approach (Q = ṁ × Cp × ΔT) for better accuracy.