If the auxiliary flue has been removed, or it is necessary to better prevent the flue gas from falling back, then use the flue check valve. In general, the function of the check valve of the range hood is not obvious, but installing the check valve in the flue can protect the range hood, so that the flue gas in the flue will not flow back to the pipeline and the range hood. However, after installing the check valve, it should be cleaned regularly, otherwise it will make the check valve open and lose its function. It is best to replace the flue check valve every two years or so. Secondly, pay attention to safety issues. Generally, the end of the exhaust pipe is metal. If the hole is too small, the metal heat transfer may cause the glass to burst and hurt pedestrians, and the smoke exhaust pipe will lead out of the window, and the strong north wind blowing head-on will lead to the backflow of oil smoke. It is best to extend the pipeline and nest it downward, so as to prevent rainwater from flowing into the pipeline when it rains, and to keep the windshield out.