The friction of the water moving through pipes also causes resistance to flow. A small diameter pipe causes more friction than the same length of a larger pipe, and the resistance increases as the flow rate increases. Because the resistance due to friction is not constant at all flow rates, it is called dynamic head.
If you plan to include additional equipment in your circulation system that might further restrict the flow of water (such as a UV water clarifier) or requires a certain pressure to operate (such as a pressurized sand filter), you'll need to take these into account . Every such addition will effectively increase the work load on your pump (the dynamic head), and decrease the rate of flow, everything else being equal. If you know the additional head created by the equipment at your target flow rate, then simply add that to the last input below. If not, you can either contact the manufacturer of the equipment for further information, or increase the computed head by a safety factor when actually choosing a pump.
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Please note that the images used in this section are intended to help you to identify the fittings and parts. The actual products may vary in color.