Water Hammer issue

jshi99July 7, 2011

Hi All,

Before I start, I want to thank all for your advice and help! Please bare with me, it's going to be a little long. Also I've no experience in plumbing at all.

House Age: 10

Water Hammer: No water hammer issue before the remodeling

Waster Pressure: between 70-75


Expansion Tank: No

Water Heater: A.O. Smith 50gl

Water arrestors: Two at the washing machine

It's all started few months back when we remodeled our bathrooms. We remodeled three bathrooms. Two at upstairs and one at downstairs. We remodeled one bathroom at a time. Here's the order: Upstairs hall way bathroom first, then downstairs bathroom, and finally the master bathroom.

The water hammer issue was discovered during downstairs bathroom remodeling. We noticed that the hot water came out really slow and weak (no pressure), it's barely enough to take a quick shower. The issue was resolved after the contractor fully opened up the cold water to the water heater.

After the remodeling done, we can hear the banging noise at every bathroom when you turn off either hot or cold tabs, showers, and toilets. The banging noises were located some where else, not directly next to the tabs, shower heads, and toilets. For tabs, if you close slowly then there's no noise. So I started searching and reading the forums in regards to water hammer issue. Here the things I have tried:

1. Bleed the air out of the system (tried many times but no success)

2. Replaced the Water Heater (No more water hammer from hot water tabs)

Here's my questions:

1. Should I install an expansion tank first and try to see if that will resolve the problem?

2. If step 1 didn't resolve the problem then should I install PRV and try to reduce the pressure to 60's range?

3. If above steps didn't work then should I install arrestors on all tabs and toilets?

I appointments setup already for getting the quote to install expansion tank and prv for the next couple of days.


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Did you remove existing ares tors during the remodel?

They may have been as simple as a vertical section of pipe above the faucet take off from the line.

If you have single handle valves installing point of use water hammer arrestors, and a PRV for the whole house would be a good idea.

Higher than reacquired pressure can lead to increased flow speed making water hammer worse.

    Bookmark   July 8, 2011 at 9:34AM
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thanks for the reply.

What's "ares tors"?

Also more updates:
1. There's no water hammer issue for the hall bathroom.
2. Still have water hammer issue on both hot and cold water for Master and downstairs bathrooms.
3. Had city inspector came out and did the pressure test, it's normal (70-75psi).

Is there any issues if I install the expansion tank without the PRV?

If I install the expansion tank only then what pressure should I set it to?

    Bookmark   July 8, 2011 at 1:20PM
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arrestors with a typo.

Most PRVs require an expansion tank also.

The PRV makes the system 'closed' and does not allow water to flow back into the system. Water expands enough when heated (even in a water heater) to require a way to release the pressure that would otherwise build up and can cause damage.

Just an expansion tank is not going to reduce you excess pressure, and may not have any effect on pipes hammering other than the one it is attached to.

Hammer arrestors are not very large and need to be near the valve that causes the hammer to be effective.
Sioux is a common brand.

    Bookmark   July 8, 2011 at 4:57PM
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Since this all started with the remodel, and only two of the three bathrooms are affected, I would suspect a pipe that was not secured. Hopefully you know the layout of the pipes in the walls. Have one person turn the faucet on and off while the other person listens to the wall along the path of the pipes. Although the sound will travel, it should be noticeably louder at the point where the loose pipe is banging against the framing member.

    Bookmark   July 8, 2011 at 7:56PM
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Pressure is a bit high but not excessive. Make sure all piping is properly secured. Install one full size 3/4" hammer arrestor (gas filled shock absorber for water distribution) at water heater location on cold side. If original air chambers were removed when shower valves were replaced 1/2" hammer arrestors should have been installed at shower valves. Purchase hammer arrestor with 3/4" male thread, most plumbers overheat sweat type arrestors and destroy O-rings sealing gas filled chamber rendering arrestor useless.

    Bookmark   July 9, 2011 at 8:35AM
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I was thinkging about going to install arrestors at valves which causes hammer noise but since the water hammer noise is not near the valves, it's somewhere inside the house and I can't locate the exact place. Installing arrestors might not be effective right? Or I should just go ahead and install one arrstor and give it a try?

As for installing a full size 3/4" hammer arrestor at WH location on cold side, can someone elaborate little more on why you need it since there's no hammer noise from or near the WH. No offense to anyone, I just want to have a better understanding on things because when comes to the actual installation, I have to have a plumber do that job and I don't want to shell out unnecessary money.

As for the original air chambers, I really don't know if we had any or not. I'll have to call up my general contrator to find it out.

But I do remember one thing when they changed the bathtub faucet, they had to remove/cut off some plastic part and replaced it with a copper thing to cure the water leak.

All advice is good, really broaden my knowledge on plumbing and I really appreciated!

    Bookmark   July 11, 2011 at 6:24PM
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"Installing arrestors might not be effective right? Or I should just go ahead and install one arrstor and give it a try? "

The arrestors may work just fine near the valves that originate the water hammer.

They are designed to absorb the pressure wave created when the valve is closed quickly, instead of allowing to travel down the pipe and cause the water hammer.

They are normally installed near the valve for most effectiveness.

    Bookmark   July 12, 2011 at 9:57AM
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Here are some links that may help



There are many variables, proper pipe strapping and support is very important. Are the new shower valves anti-scald with integral pressure balancing spools, might one of the balancing spools be faulty? Is the system on a dual check backflow preventer? The code requires hammer arrestors on supply to quick closing solenoid valves such as dishwasher, clothes washing machine, refridgerator ice maker. These are 1/2" mini arrestors. A 3/4" arrestor is full system protection.

    Bookmark   July 13, 2011 at 8:35AM
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