Combustion control and performance of household condensing boilers

The present work presents a model that is composed of a conventional boiler model with a main counterflow gas-water heat exchanger at which a condensing heat exchanger is added. Prior to the second heat exchanger the exhaust gas are converted to equivalent wet air as the wet heat exchanger is simulated by a cooling coil used in air treatment. Six parameters are needed for calculation and these parameters are fitted with experimental data: one test in dry regime and one test in wet regime. Steady-state tests have been performed on two household boilers (a gas-fired and a fuel oil boiler), whose nominal outputs are 24 kW in order to fit the model parameters and check the model behaviour. It seems that the model gives the correct trend for the thermal efficiency prediction. Besides, combustion controls sensors (non nernstian sensors) have been tested by varying the excess of air. In the range tested (from =1.05 to =1.25), the increase in the boiler efficiency is around 1% as the excess of air decreases. However that increase is lower than the measurement uncertainty. The developed model predicts very well that trend. The pollutants emissions have also been measured and the results show that the NOx emissions decrease as the excess air increases.