Student questions asked in live office hours about this problem
OH 17: HVAC 21
Q: For HVAC-21, when finding mixed air properties along a process line, can I also use the enthalpy proportion directly rather than the temperature proportion?
A: Yes — you can mix states by proportional averaging of any thermodynamic property (temperature, enthalpy, humidity ratio), and they should all locate the same mixed state on the process line. I'll expand on this to make sure I'm answering the full intent of your question.
OH 32: HVAC 21
Q: For HVAC-21, I calculated the CFM and plugged it into the latent heat formula but got a different answer — what am I doing wrong?
A: This problem is about finding the humidity ratio (and psychrometric state) of the room from the supply air conditions and the sensible and latent loads. I like that you started with CFM — let me walk through exactly where that path diverges and how the correct approach picks up from there.
OH 98: HVAC: HVAC-21
Q: For HVAC-21 (degree days problem), can you explain how the design heat loss of 300,000 BTU/hr translates to energy savings when going from 70°F to 68°F indoor setpoint?
A: The design heat loss is directly proportional to ΔT, so reducing the indoor setpoint by 2°F reduces the average driving temperature difference by 2°F relative to the average outdoor temperature. Using the seasonal degree days and the base outdoor temperature, you can calculate exactly how much energy is saved annually by that 2-degree setpoint reduction.