HVAC cooling & heating load calculations on the PE Mechanical exam

Load calculations sit at the center of the HVAC & Refrigeration depth — they feed equipment sizing, airflow, and psychrometrics, so the math here shows up everywhere. The good news: most of it reduces to a handful of standard-air relations you can apply fast once they're automatic.

What you actually need to own

• The standard-air shortcut relations (sea-level, standard air):
  • Sensible: Q_s = 1.08 × cfm × ΔT (Btu/h), with ΔT in °F.
  • Latent: Q_L = 0.68 × cfm × ΔW (Btu/h), with ΔW in grains of moisture per lb of dry air.
  • Total: Q_t = 4.5 × cfm × Δh (Btu/h), with Δh the enthalpy change (Btu/lb).
  • Know that these factors come from standard-air density and must be corrected for elevation or low temperature when the air isn't standard.
• The cooling-load components: conduction through the envelope (U·A·ΔT, or a cooling-load temperature difference method), solar gain through glazing, internal gains (people — sensible + latent, lights, equipment), and infiltration/ventilation (both sensible and latent).
• The heating-load components: conduction losses (U·A·ΔT) and infiltration/ventilation — typically without crediting solar or internal gains (the conservative design condition).
• Sensible heat ratio SHR = Q_s / Q_t, which ties the load back to the psychrometric coil process.

Where people lose points

• Mixing up the factors — using 1.08 (sensible) on a latent or total quantity, or vice versa.
• Forgetting elevation/temperature corrections to the standard-air factors — the exam specifically tests non-sea-level air.
• Dropping the latent side of people and ventilation loads, or the latent portion of infiltration.
• Confusing heating and cooling assumptions — crediting solar/internal gains on a heating load.

How to study it

Drill the three air-side relations until the factor and units are reflexive, then practice full component-by-component load problems — envelope + solar + internal + ventilation — so you don't miss a term under time pressure. Tie every result back to the psychrometric process so the load and the coil are one picture, not two.

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