System Design and Sizing
Core concepts
Understand manual load calculations, equipment sizing, duct design, and refrigerant line sizing. Know how building envelope, orientation, occupancy, and internal gains affect load. Familiarize yourself with sensible and latent load components, airflow requirements (typically 400 CFM per ton), and static pressure budgets. Recognize the importance of matching equipment to load rather than oversizing, and understand how line length and elevation affect refrigerant charge and capacity.
Traps in questions
Oversizing is a common distractor: it causes short cycling, poor humidity control, and reduced efficiency. Undersizing leads to inadequate comfort on design days. Questions may confuse total capacity with sensible capacity. Watch for traps about duct sizing-velocity too high causes noise and high static pressure; too low causes poor distribution. Also be aware that adding refrigerant lines beyond manufacturer limits requires adjustments.
Worked example
A 3-ton system typically needs about 1,200 CFM (400 CFM/ton). If a room requires 9,000 BTU/h sensible and 3,000 BTU/h latent, total load is 12,000 BTU/h, or one ton. Rule of thumb: do not size strictly by square footage; verify with a load calculation.
Study tip
Practice converting between tons, BTU/h, and kW, and always check both sensible and latent loads.