HVAC Fundamentals
HVAC Fundamentals
HVAC Fundamentals covers the physics behind the equipment: heat transfer, psychrometrics, tonnage, efficiency ratings, and load-based airflow.
Core rules
- One ton = 12,000 Btu/h. Sensible heat changes dry-bulb temperature; latent heat changes moisture. Total heat = 4.5 × CFM × enthalpy difference.
- 400 CFM/ton is a nominal rule of thumb, not a design value. Actual airflow comes from the equipment table and the sensible load.
- Efficiency metrics: SEER2 for cooling, HSPF2 for heat pumps, AFUE for furnaces. Higher is better, but ratings are tested under standard conditions.
- A load calculation (Manual J) establishes room-by-room sensible and latent loads before equipment selection.
Worked example
An air handler moves 1,200 CFM. Supply and return enthalpy differ by 6 Btu/lb. Total capacity = 4.5 × 1,200 × 6 = 32,400 Btu/h, or 2.7 tons. If the same system were judged only by dry-bulb ΔT, the latent portion would be missed and the unit could appear oversized or undersized.
Traps
Using 400 CFM/ton as a code rule; selecting by square footage instead of a load calculation; confusing SEER2 with seasonal cost; and ignoring altitude corrections to the 1.08 and 4.5 constants.