We just had a home inspection prior to selling our house. We have a gas water heater & gas furnace tied together for the exhaust venting. The inspector noticed that there is noticeable backdraft into the gas water vent when the gas furnace is running. We took apart the metal vent ducts & no blockage was present. There is also a sizable ice build up on the chimney vent. What else could be the problem? The inspector assumed that something was blocking the vent but we found nothing on the inside. Could something be blocking at the cap? Due to the weather, we have not yet trekked onto the roof to check.