While attending to a heating service call, we found that the high efficiency furnace exhaust venting was NOT installed on a slope back to the furnace. We drilled a hole in the exhaust vent piping to show how much water accumulates in the system - in this case it was 3.4 litres (0.9 US gallons). The venting was replaced on the proper slope to allow for draining of the furnace exhaust condensate.