high pressure, water hammer in long straight pipe run

Old 11-03-16, 02:30 PM
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high pressure, water hammer in long straight pipe run

So, I have this bathroom...
Supply lines to the bathroom go through long (~25 ft) straight pipes in the crawlspace, through an opening in the foundation, and up into a finished porch that serves as the entryway/second bathroom. The pressure in this bathroom is really high, and the bathtub (right at the end of those long, straight pipes) causes a machine-gun like water hammer if the water is turned on too low. Also, the hot water flow to the tub seems to be really high at first and then taper off.
We're getting ready to completely remodel this bathroom, get the supply lines into the wall so they don't extend out of the house, re-position the door, replace some really old fixtures, etc.
What would you recommend we do about the pressure/hammer?
Old 11-03-16, 03:01 PM
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The pressure in a bathroom can't be higher than it is in the house. Are you on city water ?

You may need to install a PRV (pressure regulator valve) for the entire house.

It would be a good idea to purchase an inexpensive gauge to measure your water pressure.
3/4 in. Plastic Water Pressure Test Gauge-DP IWTG - The Home Depot
Old 11-03-16, 04:15 PM
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The rat-atat-tat is more likely the result of a vibrating valve disc, what most people call a faucet washer, than it is true water hammer. Higher than necessary water pressure is often a contributing factor as PJ indicated.

There is no need to have more than about 60 psi water pressure in a residence unless (a) it is a high-rise building of more than three floors or (b) the piping system is way too small for the volume carried. Having 40 psi at the fixture is quite sufficient provided the supply pipes are large enough to carry the volume necessary to maintain that pressure.

Incoming water service from the utility's meter should be one inch. Main feeders serving multiple faucets should generally be 3/4 inch (one inch if serving a large number of branches) and 1/2 inch branches serving individual faucets. There actually ARE formulas for calculating the recommended sizes based on flow rate and length of pipe, but these figures work well for most single-family residences.
Old 11-04-16, 05:56 AM
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Water pressure/flow that starts off good and tapers down is an indication that there is a restriction somewhere. It could be a corroded, pinched or under sized pipe or a valve that partially closed.

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