A happy gas appliance is one that has a good supply of air plus a proper way to exhaust the byproducts of combustion. Unfortunately, providing those basic needs is not as simple or as straightforward as it used to be. In the old days (six to ten years ago), things were easy. Those old 60%- efficient furnaces and boilers produced plenty of hot, dry flue gases which could easily force themselves up almost any chimney. Little if any "design" was necessary for the vent system. Today's gas appliances, however, have improved efficiency which means less flue gas at a lower temperature. To ensure proper venting without corrosion problems, the vent system must be designed for the specific appliance. The