An apparatus for heating a medium includes a combustion chamber; a burner within the combustion chamber and operable to create products of combustion for heating the medium to be heated; a conduit for the exhaust of the products of combustion; and an airflow apparatus capable of creating airflow in the absence of any opposition to the airflow, the airflow having a pressure, the airflow apparatus communicating with the conduit and operable such that the pressure of the airflow resists standby convection flow of gases out of the conduit when the burner is not operating, and wherein the airflow apparatus is adjustable to vary the magnitude of the airflow to substantially equalize the airflow and the standby convection flow of gases to create a substantially stagnant state within the conduit when the burner is not operating.