When you open a spigot in your home, you expect water to flow at a certain speed. Water pressure in the line causes that water to flow. If your home is on city utilities, water pressure is controlled (for the most part) by the city. If your home is on a well, you need something to build that pressure and this is the purpose of the pressure tank.
In a nutshell: The pressure tank in a water well system creates water pressure by using compressed air to bear down on the water. Because of this pressure, when a valve is opened water is pushed out of the tank through the pipes in your home. When the water in the pressures tank drops below the required amount, a pressure switch is activated turning on the pump. The pump then refills the pressure tank. The combination of the pressure tank, pressure switch and the pump is what allows water to flow through your home.
Pressure tanks, which also act as reservoirs for the home, are generally set to keep pressure within 20 pounds per square inch (PSI) of your starting and stopping parameters. For example, most homes have pressure of 40 PSI to 60 PSI. If the pressure drops below 40 PSI, the pump kicks on and brings the pressure back to 60 PSI before cutting off again.
Because the tank acts as a reservoir, it allows water to be drawn from the tank without the pump cycling on and off each time the water is turned on. This ultimately protects and prolongs the lifespan of the pump. Pressure tanks also reduce instances of water hammer as they reduce air in the pipes and control the pressure of the water.
When choosing a pressure tank you will need to know the gallons per minute (GPM) – or if they ask for gallons per hour (GPH) just multiple that number by 60 – your pump pushes water through your pipes. You’ll also need to count the number of plumbing fixtures, including outside spigots, the system needs to service.
Most manufacturers provide a chart to help you size your pressure tank. Like everything, if in doubt, it’s always better to go with larger pressure tanks and have them installed by a professional.