When is an auxiliary drain pan required on a split-system air conditioner?

An auxiliary drain pan is required on a split-system air conditioner primarily to protect the building's interior finishes from potential damage caused by leakage from the primary drain pan. When the primary pan fails or overflows, it can lead to water dripping onto ceilings, walls, and flooring, resulting in significant water damage and costly repairs. The auxiliary pan acts as a backup to catch any excess water that may escape the primary system.

While other factors, such as the inability to drain condensate by gravity or the lack of a trap in the condensate line, can influence the design of a drainage system, the primary concern driving the need for an auxiliary drain pan is the protection against possible leaks that can compromise the integrity of the home's interior. Keeping the living space dry and preventing water damage is a critical consideration in HVAC system design and installation.