Where a heating and cooling load comes from

A Manual J load is the sum of conduction, solar, infiltration, and internal gains — not a square-footage rule. Heating omits solar and internal gains.

A house cross-section with every heat-transfer path labeled: conduction through the roof, walls, floor, and windows; solar gain from the sun through the glass; air infiltration leaking in at gaps; and internal gains from people and appliances inside. A load calculation adds all of these paths, which is why floor area alone cannot tell you the equipment size. — Source: ACCA Manual J 8th ed.

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A house cross-section with every heat-transfer path labeled: conduction through the roof, walls, floor, and windows; solar gain from the sun through the glass; air infiltration leaking in at gaps; and internal gains from people and appliances inside. A load calculation adds all of these paths, which is why floor area alone cannot tell you the equipment size.

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