How Does a Well Tank Work? There are various designs of well tanks, but the basic principle is quite simple: there is pressurized air on the top of the tank and a reservoir of water on the bottom. As the water pump fills the reservoir with water, it compresses the air on the top. When the pressure gets to around 50psi, a switch turns the pump off. Then, as the water is used, the compressed air pushes the water out of the tank. When the pressure drops to around 30psi, the pump switches back on and the cycle repeats itself.