Load Calculations, Psychrometrics, and Equipment Sizing (Manual J, S, and D principles)
Manual J
ACCA Manual J, eighth edition, is the residential load method: room-by-room and block loads from envelope U-factors, glass area and orientation, SHGC, internal gains, infiltration, ventilation, and duct gain or loss. Use ASHRAE design conditions for the actual city - heating at the 99 percent dry bulb, cooling at the 1 percent dry bulb with its coincident wet bulb. New Mexico elevations reduce air density, so capacity tables must be corrected to altitude.
Manual S and D
Manual S matches equipment to the load using manufacturer expanded performance data at design conditions, not nominal tonnage. Check total and sensible capacity and the latent match. Do not exceed about 115 percent of the calculated load, and confirm the equipment SHR is at or below the load SHR so the coil still dehumidifies. Manual D then sizes ducts from the selected blower.
Worked example
Manual J gives 27,000 Btu/h total and 21,000 Btu/h sensible, so SHR is 0.78. A nominal 3-ton unit rated 34,000 total and 25,000 sensible is 126 percent of load - oversized. A 2.5-ton unit rated 29,000 total and 21,500 sensible is 107 percent, within the limit and meeting the sensible load, so it is selected.
Traps
Square-foot rules of thumb; picking tonnage before the load is run; ignoring duct gains in a 140 degree attic; forgetting infiltration in a windy high-desert house; using sea-level airflow assumptions; skipping the wet bulb and then fighting humidity.