TO-2733 -- Woodstove Efficiency Determination Using Stack-Loss Methods: Sensitivity to Measurement Errors and Assumed Elemental Analysis PDF

Inaccuracies in the stack-loss method of efficiency measurement come from two sources: incorrect assumptions made in developing a computational algorithm and the errors that occur when measurements such as composition, temperature, or flow are performed. The error in computed efficiency resulting from a specified error in measured gas composition or assumed elemental analysis of the fuel depends on what assumptions were made in developing the computational algorithm. This paper examines the algorithm being considered for use in the proposed ASHRAE Standard 106P ("Method of Testing for Performance Rating of Wood burning Appliances") to determine how sensitive its computed efficiencies are to errors in measured gas compositions and assumed elemental analysis of the fuel. Calculations show the algorithm is relatively insensitive to gas composition measurement errors during high-fire operation early in the burn cycle. Late in the cycle, measuring 15.2% O₂ instead of 15.4% O₂ would make the difference between computed efficiencies of 68.5 or 58.2%, indicating that the algorithm is unduly sensitive to small errors in measured gas concentrations. Small differences in assumed fuel elemental analysis are shown to have significant effects on computed efficiencies. Simultaneous stack-loss and calorimeter room tests on generically different types of stoves are recommended to provide a data base that can be used to evaluate the accuracy of l06P or any other stack-loss method.