There are a couple of scenarios that could lead to water leaks from a furnace during a/c operation: If the water leak appears after rain, then there may be a leak in the furnace venting system on the roof.   Condensing (high-efficiency) furnaces may have leaky hoses or clogged drain parts that need to be repaired or cleaned. The primary condensate drain may be backing up and causing the evaporator drain pan to overflow. The evaporator drain pan may have failed. The evaporator may be icing causing the leak. Poor maintenance – Dirty filter  All above conditions require professional service. Call us to schedule your service appointment!