Pressure Tank

A pressure tank is a sealed vessel plumbed into a water line after the pump. Part of it holds water and part holds air, and because air can be squeezed while water cannot, the trapped air works as a spring: the pump compresses it while filling the tank, and it pushes the water back out whenever a tap is opened. The point is pressure rather than storage. Without one, every tap runs the pump for as long as the water flows, and that starting and stopping is what wears a pump out.

A switch watching the pressure does the work. The pump runs until the air is compressed to a set figure, commonly about 2.8 bar (40 psi), then stops; use bleeds the pressure back to about 1.4 bar (20 psi) before the switch starts it again. Only the water held between those two pressures comes out between runs - roughly a fifth of the tank's rated volume - so it looks far larger than the buffer it gives. A bigger tank is not the answer to a source that cannot keep up: it buys a slightly longer run of water, and a supply that falls short at the busiest hour needs a separate store filled slowly over the day.

The characteristic failure is waterlogging: air and water are no longer kept apart and almost no cushion is left. A perforated bladder - the flexible membrane separating the two in most household tanks - a low air charge, or a tank too small for its pump all produce one symptom: the pump starting and stopping every few seconds, so a change in cycling rhythm is the signal to inspect the tank rather than the pump. Upkeep is checking the air charge against the pressure at which the switch turns the pump on. A bladder tank averages five to seven years before the membrane fails, while a plain air-over-water steel tank has nothing to perforate and lasts far longer. Anything downstream that needs steady pressure depends on it, including treatment equipment with a minimum feed pressure.