Determining System Demand
The system demand is a function of water usage and location, expressed as gallon(s) per minute (gpm) and pound(s) per square inch gauge (psig), respectively. Usage or flow (gpm) can be determined using one of several methods (refer to Table IV.1.1 for typical demands):
1) Pressure required, as stated in pounds per square inch or PSI, is the pressure needed for the fixtures in the house or irrigation system to function properly. The most common pressure range is 40/60, which starts the pump at 40 PSI, and stops the pump at 60 PSI. This gives an average operating pressure of 50 PSI.
2) Pump capacity, as stated in gallons per minute or GPM, is the volume of water that the pump can produce after lifting from the depth of the pumping level, and producing the average pressure as stated above, plus all friction losses in the piping system.
The pressure required for most fixtures in a house, or for sprinklers outside the house, is about 50 PSI. Using the chart or methods below, you can figure the maximum gallons per day required, and the maximum gallons per minute (GPM) needed to supply peak demands.
According to the chart below, a house with 3 bathrooms needs a peak demand of 14 GPM. Having a well that produces 14 GPM or more makes this an easy project. Simply design a pump system that can produce at least 14 GPM at the depth and pressure required. The pump must be able to produce 14 GPM during peak demands. The pump control system must also be able to safely supply any flow rate less than 14 GPM as required, when flows needed are less than peak demands.
A pump system should be designed that can provide the peak demand required. If the well cannot supply the volume needed for peak demand, a storage tank system should be designed. See "Low Yield Wells and Storage Tanks".
The fixture method determines the system demand by totaling the number of fixtures in the home, including outside hose bibs, and multiplying this number by 1 gallon per minute (gpm). For example, 10 fixtures x 1 gpm = 10 gpm.
The peak demand method determines system demand considering that more than one fixture will be in use under peak demand. The number of fixtures being used at the same time is determined and multiplied by 3 gpm. For example, 4 fixtures x 3 gpm = 12 gpm.
An alternate method determines system demand by calculating the number of bathrooms (half baths are considered as 1) and multiplying by 4 gpm. For a home with 2 1/2 bathrooms, multiply 3 x 4 gpm = 12 gpm. Use the largest system demand determined by the above methods.
The pump capacity should be selected according to the system demand. If a pump exists, the capacity must be determined. The well capacity should be documented when the well has been declared ready for use and will often be referred to in gallons per hour (gph). If the well capacity is unknown, it should be determined by measuring the well water level. The water level must be lowered via pumping, measured, then allowed to recover to the static water level. A record of the time required to return to static water level along with the well pipe size can be used to calculate the well capacity (recovery). An alternate method of determining if the well capacity is sufficient for system demand is to draw water from the well at or above the peak demand and determine if the well can sustain the peak demand flow. A typical water system will have adequate well capacity and pump capacity to meet or exceed the system demand.