Problem: in office floor has dimensions of 120 feet by 200 feet with 12 foot ceiling height.
Given: 20 in office floor has dimensions of 120 feet by 200 feet with 12 foot ceiling height; 4 feet wide by 6 feet high for...
Approach: There is one window with dimensions 4 feet wide by 6 feet high for every 8 feet of external wall.
Calc: The overall heat transfer coefficients for the walls and windows are 0.2, B2 per hour per square foot degree F and 0.7 with the...
Calc: We're going to attempt to draw 3D.
Result: That's very close to answer choice D 150,000.
Office Hours
3
Student questions asked in live office hours about this problem
OH 30: HVAC 20
Q: For HVAC-20, I used efficiency = (ṁ × ΔH_actual) / (ṁ × ΔH_ideal) with outside and return air enthalpies to find the leaving air enthalpy — does this fail because there's only sensible heat transfer (no latent)?
A: That approach is conceptually sound but doesn't quite fit this problem's setup — the challenge is correctly defining what 'ideal' enthalpy change means for this specific device. Let me explain what the air is actually doing here and why the efficiency definition needs to match the process.
OH 31: HVAC 20
Q: For HVAC-20 (air-to-air heat exchanger / HRV), can the effectiveness calculation use dew point temperature or humidity ratio, and what's the relationship to latent heat?
A: For an HRV (heat recovery ventilator), effectiveness is based on sensible heat exchange only — dry-bulb temperature change, not enthalpy. If you're using humidity ratio (which implies latent heat transfer), you'd be treating it like an ERV (energy recovery ventilator), which is a different device — make sure the effectiveness definition matches the equipment type.
OH 42: HVAC: HVAC-20
Q: For HVAC-20 (office area cooling load), why didn't you include heat from people — the problem said to ignore equipment and ventilation but didn't mention people?
A: Good point — I should add that clarification to the problem statement. My intent was to have you calculate only the external thermal loads (solar, transmission) and exclude all internal gains including people; I'll update the problem to make that explicit.