Planning, Layout, and Estimating
Start with the load, not the square footage
A C-20 project begins with a room-by-room load calculation (ACCA Manual J or an approved equivalent), never a rule of thumb. Sensible and latent loads are computed separately; equipment is then selected (Manual S) to meet both at design conditions, and the air distribution system (Manual D) must be able to deliver that capacity. Know the vocabulary: outdoor and indoor design dry bulb and wet bulb, envelope U-factors, infiltration, internal gains, ventilation air per ASHRAE 62.2, and altitude correction.
Estimating is quantity takeoff times labor unit, plus material, equipment, and subcontractors, then overhead and profit. A takeoff counts every duct size in linear feet, every fitting, refrigerant line length, hangers, sealant, insulation, controls, and permits. Labor units are expressed as hours per unit (per hundredweight of duct, per fixture).
A check figure, not a design method: nominal airflow is about 400 CFM per ton, so a 3-ton system moves roughly 1,200 CFM. Traps on the exam: using square-feet-per-ton shortcuts; confusing nominal tonnage with delivered capacity; forgetting duct gain and blower heat in the load; omitting plan check and permit fees; ignoring a specification that calls out a seal class or insulation R-value.
Worked example: 36,000 Btu/h total load with a 0.80 sensible fraction means 28,800 Btu/h sensible. At 1,200 CFM and 1.08 x CFM x dT, the room temperature difference is about 22 F, a quick reasonableness check on the equipment selection.