In order to purchase the right sized pond pump to obtain the waterfall or water flow rate you desire, it is necessary to first understand the term Head Pressure, or the amount of resistance that the water will encounter as it travels from the pump to the point where it is discharged. Head Pressure, also known as Head Height, has two main components. First is the effect of gravity, which is easy to calculate if you know the vertical elevation from the pump to where the water reaches its highest point. Second is frictional loss, caused by the physical contact between the water and plumbing as it travels from the pump to its destination.
Calculating and combining these components will give you a total Head Pressure value for your system, and will help you to select the proper pump.
Calculating Head Pressure
Head Pressure is calculated and represented terms of feet (ft.) In order to calculate the total Head Pressure of your application, you'll need to know the difference in elevation between where the water is being pumped from, to where the water is discharged. You will also need plan the plumbing scheme ahead, including all fittings and distances between tubes and/or pipes.
The effect of gravity on head pressure is very simple; every vertical foot of distance the pump moves water equals one foot of head pressure (1:1 ratio). The effects of friction on head pressure is a little more difficult to calculate. Every ten feet of pipe through which water will travel contributes 1 foot of head height (10:1 ratio). Every 90 degree turn in your plumbing will contribute 1 foot of head pressure (1:1 ratio). For example: if you were to install your pump 30 feet from the top of your waterfall, which was 4 feet above the pump, with two 90 degree elbows in your plumbing, your water feature’s head height would be 9 feet (plumbing distance contributes 3 ft, the vertical height contributes 4 ft, and the 90 degree turns contribute 2 feet). In this example, you would want to choose a pump that has the desired GPH rating at 9 feet of head pressure.
Tubing size is also an important factor in accounting for head pressure loss, in general you should never reduce the diameter of the tubing below what the output size of the pump is, this will drastically increase head pressure, and reduce pump performance. For maximum pump performance, using the largest tubing that is practical is the best choice. In-line filters and Ultraviolet sterilizers will also add considerable head pressure, and several feet of head pressure will need to be added to compensate.
Selecting A Pump
Once you have calculated the total Head Pressure for your pond, you will next need to determine the desired turnover rate, or Flow Rate for your water feature or pond that you a choosing a pump for. For example; If you have determined that your desired flow rate is 1000 gph, and you have calculated that your total head pressure is 10ft, the smallest pump that will meet your requirements is a pump that is rated for 1000gph @ 10 ft of head pressure, this will be a pump that has a maximum gph rating much higher than 1000 gph.
Most manufacturers have a flow chart to be referenced with their pumps, as each pump varies. These charts may also tell the shut-off pressure, which is the maximum Head Pressure the pump can handle before it will no longer be able to generate flow. Be sure to reference these charts when choosing your pump to ensure the right choice for your design.
It is good idea to select a pump that is larger than what your minimum requirements have been determined to be, to allow for other factors that could further decrease pump performance.