Experimental Study on the Emission Characteristics of Gas Instantaneous Water Heaters

The emissions from gas instantaneous water heaters, such as CH4, NOX, and CO, can cause significant negative impacts on the atmospheric environment and human health. Therefore, a comprehensive understanding of the emission characteristics is critical to effectively control the emitted gases. In this study, the gas emissions from 16 gas instantaneous water heaters (including 4 different models) were regularly examined to investigate the relationship between the emissions of CH4, NOX, and CO, the service time, and the heat load. The main conclusions are as follows: The CH4 emission process can be divided into the following stages: initiation, steady operation, and shutdown. The CH4 emission concentrations reached their peaks during ignition at approximately 50 s, and then decreased rapidly. Upon entering the steady operation stage, the emission concentrations became low and demonstrated no obvious variations. After the shutdown stage, the CH4 emission concentrations rapidly reached their peaks (after 150 s) and then started to decline; these peaks were generally lower than those during ignition. Longer service time can affect the ignition and fire-spreading performance of gas instantaneous water heaters, leading to increased CH4 emission concentrations. The NOX emissions were mainly generated in the combustion stage. The CO emissions were mainly produced in the combustion stage. During the steady operation stage of the gas instantaneous water heaters, the CH4 emission concentrations were positively correlated with the NOX and CO emission concentrations; in contrast, the CO emission concentrations were negatively correlated with the NOX emission concentrations. Finally, it was found that the average emission rate of CH4 was 0.14% for the tested gas instantaneous water heaters, where CH4 emission rates will be lower for new gas instantaneous water heaters or those within their service life. This study contributes to a better understanding of the emission characteristics of CH4, NOX, CO when using gas instantaneous water heaters.