Bathtub faucets receive hot and cold service from the hot water heater and cold water main, respectively. The lines run to the faucet's intake and wait for the faucet to be turned on. Underneath each knob is a stem, which is a plastic fitting that regulates the flow of hot and cold water. In a single-handled faucet, the stem regulates both the hot and cold water flow.
From the Pipe to the Mixer
The hot and cold water lines run into a central mixing area, which blends the hot and cold water so it can be dispensed through the faucet itself. Increasing or decreasing the flow of either line will regulate the pressure and temperature of the water.
What If a Shower Is Involved?
If there is an overhead shower, another step is added. When the shower is turned on, the water, rather than coming out of the mixer, will be directed by a valve to the shower pipe. The water pressure pushes the liquid up the pipe and out of the shower head.