Air Total Heat (4.5 Rule)

Enthalpy is the psychrometric chart's way of putting sensible and latent heat on one axis: a pound of air at 80 °F/50 % RH holds about 31.4 BTU, the same pound leaving a coil at 55 °F saturated holds about 23.2, and the 8.2 BTU/lb difference is the whole job the coil did. Multiply by mass flow and you get total capacity — no need to split it. The constant is the simplest of the family: 4.5 = 60 min/hr × 0.075 lb/ft³, pure mass flow, because enthalpy already carries the per-pound energy.

Test-and-balance crews live on this one. Take entering and leaving wet-bulb readings, look up the enthalpies, multiply 4.5 × CFM × Δh, and you have the real delivered tonnage — the number that settles arguments about whether a rooftop unit is doing what its nameplate claims. A 2,000 cfm unit across a 5 BTU/lb drop is 4.5 × 2,000 × 5 = 45,000 BTU/hr, or 3.75 tons, or 13.2 kW on this page. The trap: wet-bulb accuracy. A sling psychrometer read 1 °F wrong shifts enthalpy about 0.5 BTU/lb, which on this example is a 10 % error in capacity — measure carefully or measure twice.