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I have too high water pressure (120 psi) on the main line. If we assume the water pressure on is constant, then is it possible to reduce it by reducing the flow using a ball valve?

The reason I'd like to go this route, is that installing a pressure regulator is a lot of work, and there's not much room where the main line enters the stucco of the house. Turning the pre-existing ball valve, would be a lot easier than installing a regulator.

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    Here is a decent article on the difference between regulating pressure vs regulating flow: ctgclean.com/tech-blog/2012/03/… for more info Google water pressure vs flow – Tyson Aug 10 '16 at 0:58
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    No this will not work. Reducing flow only reduces pressure while there is flow. So when you stop the flow, the pressure goes right back up where it was. – brhans Aug 10 '16 at 1:43
  • Installing a diaphragm unit (pressure reducer, if not regulator), is relatively easy & requires only about 6-8 inches of pipe run to fit one in. – Carl Witthoft Aug 10 '16 at 14:41
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To answer your question; A ball valve only controls flow not pressure. A few seconds after water is flowing a ball valve will limit the pressure based on the flow. A pressure regulator requires no more plumbing than a ball valve. A regulator will limit the maximum pressure but not flow until the set point is reached. If you can answer the question. Why do you want to limit pressure or flow. We may be able to point you in the direction you need.

  • Reason I am doing this is the pressure on the main line is 120 PSI- too high. I already have a ball valve on the main and was hoping to reduce the flow to reduce the pressure. Does this sound reasonable? – anon Aug 10 '16 at 1:22
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    As stated by @Ed Beal and brhans, no it is not reasonable. Pressure downstream of the ball valve will only drop while there is an opening in the line downstream. As soon as flow ceases, the entire line will equalize at the pressure upstream of the ball valve. You need a regulator. – Jimmy Fix-it Aug 10 '16 at 5:58
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The relationship between resistance, pressure, and flow is complicated. But in brief, a restriction like a half-open ball valve will only reduce the downstream pressure and volume when the water is actually moving. It does nothing to alter the "static pressure", which is the pressure in the lines when no water is being used. So a half-open valve may help in some situations (like if you shower head is too powerful) but it will not help with problems caused by high static pressure, like running toilet valves and other leaks.

As @EdBeal said, the right tool for this job is a pressure regulator.

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you can go with using ball valve. ball valve is commomly used to regulate flow and it will not reduce pressure. however, installing ball valve on a branch very away from desired exit and not opening it fully will give desired output

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    As plenty of answers and comments above have noted, a ball valve will not do anything for pressure when water is not flowing - which is the case the vast majority of the time in a house. – Mark Jul 8 '17 at 12:34

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