Installing a new central air conditioning system is a major home investment — and the choices you make before installation determine how comfortable, efficient, and cost-effective your home will be for the next 15 to 20 years. The most important factor most homeowners overlook is system sizing. An undersized unit will run constantly without ever fully cooling your home, while an oversized unit will short-cycle, leaving rooms humid and uncomfortable. Proper sizing requires a Manual J load calculation based on your home's square footage, insulation quality, window placement, and local climate d