What size tank should I use? (Pump running time)
The tank is the part of the system that tells the pump when to turn on and when the system has enough pressure. The pressure switch should be located close to the tank so it can sense when it should tell the pump to turn on or off.
A pump likes to run and cools itself if left running. Most pumps are considered continuous duty. This means they could run all day, as long as they are not in an enclosed area that gets hot, have their ventilation plugged, or run out of liquid to pump. Heat is the one thing that really hurts a pump. The coils of the electric motor have a varnish coating on them and if they get too hot that varnish will melt and the wires will touch and the motor will short out. There are two ways to cool a motor.
1) Let it run. Running allows the air to pass over the coils and remove the heat. Yes the outside skin of the motor can get up to 140F which is too hot for someone to place their hand on and keep it there, but if the overload is not tripping then the motor should be just fine.
2) Leave it off. When a motor turns off it really needs to stay off for a while. Of course the time off depends on the size of the tank. Most motor manufactures would like to see their motors run for a minimum of one minute. However when the motor is larger than ¾ hp it really needs to stay off for at least 12 minutes. A ¾ hp or less needs to stay off for at least 3 minutes. This means it really should run for about 2 minutes minimum.
Remember that it is the tank which tells the pressure switch when it has enough pressure. So the volume of water that the tank holds is a determining factor as to how long the pump will run. The amount of usable water in a tank is called drawdown. The drawdown of a tank can be decided by the setting of the pressure switch. (See the information on setting my pressure switch) The biggest determining factor of drawdown in a tank is the tank size. The larger the tank the more drawdown it has. The more drawdown is has, the longer the pump will run, and since running makes a pump last longer, a larger tank will make the system last longer. Why? It reduces the amount of heat that affects the varnish on the coils. Plus, when a pump turns off it will stay off longer, thereby reducing the amount of heat the pump motor will see (because it has more time to cool off).
Either way you look at it, a larger tank is better for the life of, not only the pump, but also the system. The fewer times in my life that I have to put a system in, the more money I’ll save. So, the little bit more I have to spend on a larger tank will help me to have fewer systems. The bottom line is I save more money and that’s what it is all about.