Water Flow Vs. Pressure

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It more than likely has happened or will happen to everyone at some point--you turn on the water, and nothing comes out but a weak, tiny excuse for a trickle. You definitely have a plumbing problem, but is it the water flow or is it the water pressure? Knowing the difference will help you solve this home maintenance puzzler.


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Water flow refers to the amount of water coming out of a hose, faucet or other pipe fixture in a certain amount of time. Water pressure refers to the amount of force that is put on the water to make it move from one place to another, or to the amount of force the water exerts when coming out of the pipe. Water pressure often is caused by gravitational pull.



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Water flow and water pressure have separate units of measurement. Water flow is measured in liters per second, since it is a measure of how much liquid is being dispensed. Water pressure is measured in kilopascals (kPa). It is a measurement of how how much stress, or force, is put on the water as it moves through the pipe or other container.



adjusting sink
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Because water flow and water pressure are two very different things, they are adjusted in different ways. Water flow is changed by adjusting the opening to the pipe, such as the shower head you use. Water pressure is changed by altering the diameter or texture of the pipe, using a different pump/regulator or pump/regulator setting, or changing the amount of water that is elevated above the water coming through the line (the weight of the water creates pressure on the water below).



water pipes
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Water flow and pressure both are related to friction. As water moves through a pipe, friction will slow it to a certain degree, depending on the texture and diameter of the pipe. The smoother the pipe, the less friction there is and the faster water can move through the pipe, provided that the water pressure is sufficient. With good water pressure, the friction in smaller pipes can be overcome so that the water flow remains high.


Pipe Size

man adjusting water pipes
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In general, the larger a pipe, the higher the water flow. However, the water pressure level always has to be considered. If water pressure is too low, then even the largest, smoothest pipes will not have good water flow because they don't have enough pressure to overcome the force of friction.



running sink water
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If you think you may have a problem with your water pressure or flow but aren't sure where to start, think about whether the water flows more vigorously at certain points of the day or when many lines are open at once. If the water flow slows during peak water usage hours or if it changes only when you turn on multiple faucets, then this signals that pressure is not high enough to keep water moving through all the lines at one time. If, however, water flow is reduced to a trickle at all times, or if water runs well through one line and not another, then the problem probably is a blockage resulting in poor water flow out of the pipe.


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Wanda Thibodeaux

Wanda Thibodeaux is a freelance writer and editor based in Eagan, Minn. She has been published in both print and Web publications and has written on everything from fly fishing to parenting. She currently works through her business website, Takingdictation.com, which functions globally and welcomes new clients.