A “high efficiency” furnace is a furnace that is 90% or more efficient. The trick to getting this efficiency is a second heat exchanger that condenses the moisture contained in the combustion exhaust and thereby capturing much more of the heat that would be lost up the chimney with mid-efficiency (80-85%) or conventional furnaces (50-79%).
The problem is that now the furnace produces condensate, the acidic liquid that goes up the chimney in less efficient furnaces. The condensate goes through a variety of hoses and drains, and then, if everything works right, it ends up in a drain or laundry tub.
But if one of the tubes or pipes leaks, the condensate will end up somewhere in the furnace compartment and start rusting–it’s quite corrosive.
Much of the time, these condensate leaks are repaired and the furnace continues to function as normal. But if the leak is not coming from a hose, and instead from, for instance a screw for the access panel that holds the secondary heat exchanger, then there may be a larger problem–a failed heat exchanger. If the heat exchanger is damaged, you need a new furnace.