When a cooling coil is installed directly to a furnace, what must the drain pan be sized for?

When a cooling coil is installed directly to a furnace, the drain pan must be sized for the entire unit. This is crucial because both the furnace and the cooling coil can produce condensate, particularly from the cooling process. If the drain pan is only sized for the coil or the furnace alone, it may not adequately handle the total volume of water that can result from both components working together.

Sizing the drain pan for the entire unit ensures that any condensate generated by the cooling coil—especially during high humidity conditions or when the cooling load is high—is properly collected and drained away, preventing water damage and potential issues with operation. Additionally, if the coil is installed above the furnace, any overflow or leaks can lead to serious consequences if not properly managed by an appropriately sized drain pan. Thus, for both safety and functionality, the drain pan must accommodate the total condensate output from the entire assembly.