Why do I have to have a pressure tank? Why can't I have a pump and a pressure switch? If I get a strong enough pump and a good pressure switch, why won't that work? I have a 1550 gallon water tank, that I am mounting right by the outlet of the tank. Why won't that work.

What pressure would the pressure switch sense? It is a pressure tank that allows water to be kept under pressure. Since the pump can generally pump more water than you can use, then every time you turn on a faucet, the pump would have to cut off/on rather rapidly trying to keep pressure between, say, 20# and 40# (depending on the switch you use).