How big is the Elysee heating pipe?

The inner diameters are 16mm, 18mm, 20mm, 22mm, 25mm, 32mm and 35mm respectively. The warm air pipe is a pipe that brings the heat of the engine to the car through the "water" in the car water tank to keep the car warm. Usually one in and one out. If installed incorrectly, the water flow may be too smooth. If it is broken, it will cause the coolant of the car to leak, the engine will not dissipate heat well or can not, and the water temperature will rise, which will do great harm to the car.