Mechanical Systems
Loads and psychrometrics
Cooling load splits into sensible heat (temperature) and latent heat (moisture); heating load is mostly sensible plus infiltration. One ton of refrigeration equals 12,000 Btu/h - a unit candidates mix up with Btu/h constantly. On the psychrometric chart, track dry bulb, wet bulb, dew point, relative humidity and enthalpy; coil performance is about enthalpy difference, not temperature difference alone.
Equipment and distribution
Know the cast: boiler, chiller, cooling tower, air handling unit, fan coil, VAV box, PTAC, rooftop unit. Constant-volume systems deliver a fixed airflow; VAV varies airflow and needs proper minimums and outdoor-air control. Duckwork is sized by the equal-friction method with velocity caps for noise: roughly 1,000–1,200 fpm in main trunks, 600–900 fpm in branches. Hydronic systems use pump head (not pressure alone) and often primary-secondary pumping. Reheat at terminals wastes energy.
Ventilation and life safety
ASHRAE 62.1 outdoor-air rates combine a per-person and a per-area component - outdoor air is not the same as supply air. Stair pressurization, smoke exhaust, and fire versus smoke dampers (smoke dampers at smoke barriers, fire dampers at rated assemblies) are classic exam territory. Energy recovery ventilators cut OA load.
Traps: confusing supply cfm with outdoor-air cfm; assuming a fan coil provides ventilation; swapping fire and smoke dampers; ignoring duct static pressure and friction losses.
Rule of thumb: nominal 400 cfm per ton (5-ton unit → 2,000 cfm); typical office cooling rule of thumb 1 ton per 400–500 sf.
Worked example: 2,000 cfm in a main at 1,000 fpm needs 2 ft² = 288 in²; round duct diameter = √(4 × 288/π) ≈ 19 in.