A representative system includes: an inner stack defining a flow path to direct exhaust gasses from the exhaust air outlet of a heating appliance to an exhaust; an outer stack disposed about an exterior of the inner stack and being spaced therefrom to form a combustion air passage in a heat transfer relationship with the exterior of the inner stack; a combustion air opening disposed at the intake end of the outer stack configured to receive combustion air and provide the combustion air to the combustion air passage; an air dam positioned to terminate the combustion air passage; a combustion air conduit configured to direct the combustion air from the combustion air passage to the combustion air inlet of the heating appliance; wherein, in operation, heat is transferred from the exhaust gasses to the combustion air directed through the combustion air passage.