Q&A / 

Stop Water Hammer

DEAR TIM: The water pipes in my new home are driving me crazy. Every time the toilet or washing machine shuts off, there is a loud bang. The noise can happen at faucets but is reduced if I shut the water off very slowly. What is happening? Can this problem be solved? Is it expensive to fix? Missy H., Dover TN

DEAR MISSY: You are a victim of water hammer. The source of the problem is abnormally high water pressure in your water system. My guess is that your existing water pressure is 115 pounds per square inch (PSI) or higher. I'll bet you live at the bottom of a hill or near the lowest point of your municipal water system. The volume and weight of the water resting in the water mains above your house creates this high pressure. Some places in my city have main pressures that commonly exceed 150 PSI!

You can barely see this pressure-reducing valve. It is the bell-shaped object close to the ground.

You can barely see this pressure-reducing valve. It is the bell-shaped object close to the ground.

High water pressure is wonderful for hosing down driveways or powering lawn sprinklers. However, it wreaks havoc with faucets, valves and water heater pressure valves. The hammering noise within your pipes is created when high velocity water flow is stopped abruptly. The average house has about 75 pounds of water within the piping system. When you rapidly shut off water to a fixture, the weight of the water crashes against the faucet, valve or sidewalls of the pipe. This creates the vibration and noise within the pipes.

The velocity of water flow in your pipes can be slowed considerably by up-sizing your water supply lines. In other words, try to run 3/4 inch pipe as close to fixture groups as possible. High pressure combined with high fixture water demand can cause water velocity to soar through smaller sized one half inch piping.

The problem can sometimes be solved by installing a pressure reducing valve in your water system. This valve will tame the beast within your pipes. The valves are made with a handy adjustable screw that allows you to adjust the pressure on the house side of the water system. Factory settings are usually between 50 and 60 PSI. I happen to prefer a setting of 70 PSI. Settings at or near 70 PSI allow you to take vigorous showers.

Installation of these valves is not too difficult. They require simple soldering skills. However, you will need to check with your local plumbing inspector before you proceed. Some states and municipalities will not permit an unlicensed person to cut into or make alterations to the public water supply system. Your household water lines are really part of the public system. Under certain conditions (fire engine pumps & water main breaks) water can actually flow from your house into the public water supply.

The cost of installing a pressure reducing valve depends on how much extra piping work you decide to do. Remember that everything on the house side of the valve will operate at the lower pressure. If you desire to maintain the high pressure for your outdoor hose bibs, you will undoubtedly have to install additional water lines from the high pressure side of the new valve. If you are able to perform the work yourself, your cost will be minimal. If you must use a plumber, the cost can easily exceed $500 or more.

The installation of a pressure reducing valve can sometimes cause problems with a hot water heater. Certain pressure reducing valves contain a bypass that allows water to flow back into the municipal system. But these bypasses can malfunction. When cold water enters your hot water heater and is subsequently heated, the volume of the water increases. Before the pressure reducing valve was installed, this water actually was pushed back towards the street. If your new valve does not have a bypass or it malfunctions, you can once again be troubled with water hammer.

This problem can be solved by installing a simple expansion tank on top of the cold water inlet into the hot water heater. This tank contains an air chamber that is separated from the water by a rubber bladder. The extra volume of heated water simply causes the bladder to expand within the tank. This expansion tank needs to be sized correctly. Just tell the plumbing supply house the capacity of your hot water heater. They will do the rest!

Author's Notes:

I would like to acknowledge the most helpful input of Ronald L. George, CIPE of Detroit, Michigan, for accurate information that has enhanced this column and the following Builder Bulletin. Ronald provided me with excellent background information and facts concerning the truth about water hammer. My hat is off to you Ronald! Many Thanks!

We've received other questions with similar problems or questions. Here's one from Cheryll k. of Fenton, MO, regarding either the need for a water hammer arrestor or just noisy duct work.

"We heard an extremely loud "boom" last night at 4:00 a.m. I was already awake when it happened because my son had come in our room about 10 minutes prior to this because of a bad dream. He was still in our room and awake and heard the noise too. The noise woke my husband from a sound sleep.

Anyway, the noise sounded like dropping a dictionary from a second story down onto a hard floor. The weird part ... the sound appeared to come from right under our bed, right in our room. We got up, checked things and could not figure it out. We even checked our mattress and box springs. Is there any chance the noise could have been caused by something structural that happened between the second story floor and the ceiling below it? We don't see any outward signs of damage, but I am looking for an explanation for the noise. Thanks so much!"


7 Responses to Stop Water Hammer

  1. I read about water pipe making hammering noise when the faucet or valve get shut off due to the high pressure in the line. My problem is the opposite. My new water pipe line is making hammering noise when I turen on the faucet or shower of the hot water line, cold water line does not make any noise. Is this also cause by high water pressure? Thank you.

    • We have the same problem with the facet in our kitchen sink. Whenever you turn it on it makes an thumping noise if you turn the water up very high. It doesn't happen anywhere else in the house, just in the kitchen sink. It' very annoying. ๐Ÿ™


  2. My problem is I get a water hammer when when our water tank goes below 30psi it kicks on the well pump (350 ft down) and it flows into the house through a 4 inch diameter pipe and feeds into a pipe that has a 3 inch diameter and hits the first right angle and BAM, water hammer time. An old retired plumber said I need to replace the 3 inch diameter pipe with a 4 inch one to match the well pipe size. 4 licensed plumbers say that's not the problem but they haven't found a resolution yet. Been going on since June 2017. Winter is coming. HELP

  3. The problem isnt at the water holding tank (I don't have a hot water tank our furnace heats our water with no problems)
    My hammer is as soon as the water gets in from the well and before it starts to fill the water tank. There is a right turn to my water holding tank and the bang happens before it enters the tank, It happens when our input pipe from our well goes from a 4 inch circumference to a right angle 3 inch circumference, that's when we get the bang in the 3 inch pipe as soon as it enters our house before it starts to fill our cold water holding tank.

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