Air Conditioning, Heating, Ventilation, and Air Distribution Systems
Air Conditioning, Heating, Ventilation and Air Distribution covers the air side of comfort systems: load estimation, duct design, airflow measurement, psychrometric coil performance and ventilation. Questions often hinge on the sensible heat equation and on reading static pressure correctly.
Core rules
- Sensible capacity: Qs = 1.23 × L/s × ΔT (SI) or Qs = 1.08 × CFM × ΔT (imperial, sea level, standard air). Total adds latent: QL = 3.0 × L/s × Δg/kg, or 0.68 × CFM × Δgrains.
- Nominal airflow is about 400 CFM per ton (roughly 50 L/s per kW), but verify against the equipment table for the actual static pressure and entering wet bulb.
- Residential loads in Canada are calculated to CSA F280; ventilation rates come from ASHRAE 62.2 for dwellings or 62.1 for commercial buildings.
- Total external static pressure is measured across the air handler with a manometer, both sides, then compared to the fan performance table.
Duct design and balance
Use equal-friction or velocity-reduction methods, keep trunk velocities moderate with radiused fittings, and give every system a return air path. Fit balancing dampers at branches and size filters for low pressure drop. Fan laws: flow varies with speed, pressure with the square of speed, and power with the cube.
Worked example
A system delivers 400 CFM with a 20°F temperature rise across an electric furnace: Qs = 1.08 × 400 × 20 = 8,640 Btu/h. A 5 kW heater is 17,060 Btu/h, which at 400 CFM should give 17,060 ÷ (1.08 × 400) = 39.5°F. A measured 20°F rise means roughly half the elements are open, or actual airflow is far below 400 CFM. Check element amp draw and static pressure.
Traps
- Using 1.08 at altitude without correction.
- Confusing total external static with the fan's internal pressure drop.
- Setting airflow by guess rather than the fan performance table.
- Ignoring latent load: a coil that satisfies dry bulb can still leave the space at high humidity.