Equipment Selection

Step 1's Manual J calculation produced a 2.5-ton (30,000 BTU/h) cooling load. This step turns that number into an actual equipment selection — and the choice made here (heat pump with electric backup) shapes every remaining step in the series.

This project specifies an air-source heat pump with electric resistance backup heat, sized to the 2.5-ton cooling load from Step 1. A heat pump provides both heating and cooling from one system, but its heating capacity falls as outdoor temperature drops — the point at which the heat pump alone can no longer meet the house's heating load and the electric backup has to engage is called the balance point. For a properly sized heat pump in Climate Zone 4, the balance point typically falls somewhere in the 25-35°F range, meaning the backup heat only runs during the coldest days of the year, not routinely.

Equipment selection isn't just picking a tonnage off a spec sheet — it means checking the specific unit's published capacity at this project's actual design conditions (both the summer cooling design temperature and the winter heating design temperature for this climate zone), not just its nominal rated capacity at AHRI standard test conditions, which can overstate real-world performance at temperature extremes. A unit that's nominally "2.5 tons" can deliver meaningfully less usable capacity at a 95°F cooling design day or a 20°F heating design night, which is exactly why the manufacturer's extended performance data table — not just the nameplate — is the real selection tool.

The selected equipment's rated airflow (CFM) and external static pressure capability are the two numbers Step 3's duct design has to be built around — a duct system that creates more static pressure than the equipment's blower can handle against will starve the system of airflow regardless of how correctly the tonnage was calculated. This is the thread that runs through the whole series: Step 1 calculates the load, Step 2 selects equipment that meets that load at real design conditions, and every step after this one is built around this specific unit's actual capacity and airflow characteristics, not a generic assumption.