Load Calculation and System Design
The Manual J framework
Load calculation estimates how much heat a building gains or loses under design conditions. Conduction through assemblies uses Q = U × A × ΔT, where U = 1/R. Infiltration and ventilation add sensible and latent load; internal gains come from people, lights, and appliances; solar gain depends on orientation, glazing, and shading. Manual J covers the load, Manual S covers equipment selection, Manual D covers ducts, and Manual T covers air distribution.
Design conditions and selection
Design conditions come from published outdoor and indoor design temperatures for the location. Equipment is selected to meet the calculated sensible and latent loads at those conditions, not by square footage rules. Trap: the popular "one ton per 500 square feet" shortcut ignores insulation, windows, and climate and routinely oversizes systems. Another: a unit matched to total capacity but short on sensible capacity will not hold temperature on a design day.
Altitude and derating
Gas heat input derates at altitude, and equipment capacity changes with entering air conditions; manufacturer tables must be consulted rather than nameplate ratings alone.
Traps to watch
Do not mix total and sensible loads. Do not ignore duct losses in unconditioned space. Do not size to the largest room-by-room number without checking the whole-house block load.
Worked example
A wall measures 20 ft × 8 ft = 160 ft², is insulated to R-13, and sees a 30°F design ΔT. U = 1/13 = 0.0769. Q = 0.0769 × 160 × 30 ≈ 369 Btu/h. Summing every assembly, plus infiltration and internal gains, produces the room load that drives the duct design.