HVAC System Design, Load Calculation, and Equipment Selection
Start with the load, not the square footage
Every defensible design begins with a room-by-room load calculation. Heating load is dominated by envelope conduction (Q = U × A × ΔT) plus infiltration and ventilation; cooling load adds solar and internal gains and splits into sensible and latent components. Use recognized design conditions (ASHRAE 99%/1% or the locally adopted values), correct for altitude and duct gains, and evaluate each orientation separately because fenestration and exposure change the answer.
The sensible heat ratio (SHR = sensible/total) tells you whether the equipment's sensible capacity matches the space - nominal tonnage alone is meaningless. Oversizing a cooling unit shortens run time, raises space humidity, and invites cycling failures; modern variable-capacity equipment forgives less-than-perfect sizing but does not replace the calculation.
Traps to expect: using a rule of thumb as a substitute for a load calc; forgetting duct losses and infiltration; selecting by nominal tons while ignoring rated sensible capacity at the actual entering wet-bulb and outdoor temperature; confusing total and sensible capacity on manufacturer tables; ignoring altitude corrections on airflow and capacity.
Worked example (conduction): a wall with U = 0.50 Btu/h·ft²·°F, area 400 ft², at a 40°F indoor-outdoor difference: Q = 0.50 × 400 × 40 = 8,000 Btu/h. Repeat for every assembly, then add infiltration, ventilation, and internal gains.
Rule of thumb (sanity check only): residential cooling often lands near 400–600 ft² per nominal ton depending on climate and envelope. If your calculation wildly disagrees, recheck inputs rather than trusting the shortcut.
Match the airside too: selected airflow and static pressure must be achievable by the chosen blower and duct system.