The objective of this paper is to discuss the effect of reducing the stack to flue flow (S/F ratio) of draft hoods on the seasonal efficiency of spare and water heating systems. The recommendations presented resulted from laboratory studies with four typical gas-fired appliances. A procedure was developed to simulate actual effects of a 25 foot chimney in the laboratory. Minimizing the S/F ratio of the draft hoods resulted in energy savings as much as 143 THERMS per year or $89.56 per year. The procedures studied included:

-the use of an aerodynamic valved design incorporated into the draft hood, and
-retrofit methods for reducing the draft hood dilution air ratio or S/F ratio.

Finally, recommendations are presented to incorporate draft hood S/F ratios into the appropriate DOE measurement of the Annual Fuel Utilization Efficiency (AFUE) of gas heating systems.