Audi c5a6 heater which is the air inlet pipe?

The one below is the water inlet pipe. There is a water tank in the gantry in front of the Audi c5a6 (the mechanic calls it the water tank). There are two pipes, one connected from the lower left corner or the lower right corner, and the other in the upper left corner or the upper right corner. There is a cover on the pipe body. Below is the water inlet pipe, and above is the water outlet pipe.