A: Yes — if you start from around the 4-minute mark in the solution video, that's exactly what we do there. Take another look and it should answer your question; if there's something specific that still doesn't land, let me know and I'm happy to go deeper.

A: Yes, that's equally valid — this is a good example of two different but correct approaches landing in the same place. The slight deviation between our values is just chart reading precision, not a methodological error.

A: The state is defined by 90°F dry bulb and 80°F wet bulb — not 80% relative humidity. If you re-run the psychrometric calculation with 80°F wet bulb, you'll get the 76.7°F dew point used in the solution. Those two 80s look identical but represent completely different states.

A: For an enthalpy wheel (energy recovery device), efficiency equals the ratio of actual to maximum change in temperature, humidity ratio, or enthalpy — so all three can be used interchangeably. But this problem is an air-to-air heat exchanger, which is only a sensible heat recovery device — only temperature ratios apply, not enthalpy. That distinction between sensible-only vs. total energy recovery is the key to setting up the efficiency equation correctly.

A: No — an air-to-air heat exchanger is an HRV (heat recovery ventilator) that transfers sensible energy only, so effectiveness must be expressed in terms of ΔT, not ΔH. The enthalpy-based formula applies to ERVs (e.g., enthalpy wheels), which transfer both sensible and latent energy. Enthalpy values are only introduced at the end if the question asks for an outlet enthalpy, not to define the heat transfer process itself.