Why Would Water Pressure Drop?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by nipandgab, Apr 23, 2006.

  1. nipandgab

    nipandgab New Member

    Apr 23, 2006
    We are so glad to find this forum. Our son noticed a few nights ago that the cold water from our bathroom faucet starts at normal pressure and then, in less than a minute, the pressure drops to about half of what it was. If we turn the water off and then back on again, the pattern is repeated--the pressure starts out fine and then drops. Can anyone tell us what might cause this? Thanks!
  2. breplum

    breplum Member

    Oct 2, 2004
    Plumbing and heating contractor
    San Francisco Bay Area
    This type of symtom is usually due to a constriction somewhere in the water supply. Make sure the angle stop is fully open, they sometimes magicaly close due to beings in another dimension crossing over and providing work for us plumbers.

    You can get this same effect with a water hose which has a spray nozzle; if the hose bibb is only cracked open, you benefit from the supply pressure only as long as the pressure has a chance to build up.
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  4. Cass

    Cass Plumber

    Nov 12, 2005
    I have seen when the faucet is closed the pressure will keep a kink in the supply tube open but when you turn it on, the pressure drop will cause a kink in the supply tube to pinch and slow the water flow.
  5. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    You probably have a pressure reducing valve on your main line. It is failing, but will restore the pressure when the faucet is closed momentarily.
  6. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    The pressue regulating valve is probalby "gunked up" , meaning the pistons and springs that should move in response to changing water demand are stuck form corrosion, mineral build-up, etc. When no water is running, the downstream static pressure does build up to proper leve. But when water is demanded, the valve cannot open to allow increased flow. This is often compounded by a pinhole leak in the diaphragm, so static pressure rises all the way to street pressure. Dynamically, the pressure quickly drops to a very low level.

    A simple an inexpensive pressure gauge will allow you to confirm the diagnosis
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