This photo shows why it’s not a good idea to locate a heating system exhaust vent under a roof soffit. The heat from the exhaust pipe melts the snow at the bottom area of the roof. When the heating system cycles off, ice forms. Subsequent cycling of the heating system allows the ice to build up. When snow higher up on the roof melts, the water flow is blocked by the ice at the bottom of the roof. This water is then forced back up under the roof shingles and finds its way down into the exterior wall of the house.