Manual J
The ACCA industry-standard load-calculation procedure for sizing residential HVAC equipment. A real Manual J uses your home's actual measurements and construction details — not square footage alone — to pick capacity. An oversized or undersized system costs comfort and money for its entire 15–20 year life.
The longer answer
Manual J software takes inputs for every exterior surface, window, door, and infiltration rate, then calculates peak heating and cooling loads in BTU/h. The output drives equipment selection — not the label on the old unit, not a rule of thumb, not what the neighbor has.
Manual J also informs ductless zone design: each head gets its own room load, and the sum sets outdoor-unit capacity. Oversplitting zones (too many heads for the load) creates short-cycling; undersplitting leaves rooms uncomfortable. The calculation is the design document, not the floor plan.
The Seattle angle
Glendale runs load calculations on every install — Seattle's varied housing stock (pre-war to 2000s suburban) makes square-footage guessing especially unreliable here.
Where this decision happens
Nearby in the glossary
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