Manual J and Heat Pump Selection Best Practice

About this table

Correct heat pump sizing starts with Manual J, which calculates a home's actual heating and cooling loads using verified field details (like infiltration testing and window characteristics) rather than guesswork, since any errors here poison every decision that follows. A separate but often-overlooked step is checking existing duct capacity, since retrofits usually reuse old ductwork that may not handle the higher airflow heat pumps typically require. Once loads and duct capacity are confirmed, Manual S selects equipment sized to avoid both oversizing (which causes short cycling, temperature swings, and mold risk in humid climates) and undersizing, using specific capacity ratios as guardrails. We will discuss the risks of mis-sized heat pumps and how to size them correctly.