Dear Plumbing Experts:

Over 3 months ago, a plumbing company installed a 40 gallon AO Smith gas hot water tank, which replaced a similar 13 year old tank in our basement storage room. An expansion tank was added since we previously did not have one. Immediately afterwards, we intermittingly heard a “thumping” noise from both our toilets. Noise occurs a few seconds after flushing the toilet and happens around 65% of the time.

Plumbers came back to the house and confirmed noise. Said expansion tank PSI was not set at install; therefore they set both expansion tank and pressure relief valve (PRV) to 60 PSI. Noise returned, so they recommended changing the PRV at an additional cost. Current Wilkins PRV was installed 8 years ago.

Before we spent additional money, we decided to wait for the city inspection. We purchased a water pressure gauge and checked multiple times near the main and outside. Pressure is consistently between 55 – 58 PSI at various times of the morning, day, night, and overnight. Passed city inspection per hot water tank install but inspector said noise is not acceptable. He verified 55 – 58 PSI; felt air trapped in the lines; detected “water hammer” noise from kitchen sink (hot water only) in addition to toilet noise. We think we’re hearing “water hammer” noise also from washing machine and dishwasher, but the toilet thumping noise bothers us the most.

Plumbers returned and tightened pipes above hot water tank and in laundry room ceiling; Installed 2 air hammer arrestors (company made pipes – not the arrestor bought at a store) at main. Bled system of air; installed new toilet fill valves. Still have thumping and water hammer noise. Plumbing supervisor came to house and confirmed noise. Adjusted toilet pull chains. Suggested new toilets. Said pipe straps may have broken in our walls. We shared how one of the company plumbers said that he would re-plumb this differently if he had initially installed the tanks and that we agreed to pay for arrestors, with the understanding that everything would be re-plumbed the way the plumber recommended if arrestors did not work. Supervisor said he would check with Plumbing company owner. Supervisor reported back that “we’re done. There is nothing wrong with the install.” Said won’t troubleshoot the expansion tank, re-plumb anything, or work with the manufacturer, A.O. Smith.

We called another plumbing company. This plumber said he’s not sure. He’s leaning towards the PRV, even though it’s not giving telltale signs of not working (it appears to be holding the correct PSI, he doesn’t feel any vibration from the PRV, the water pressure seems consistent in the house.)

We called the city water and sewer departments; they aren’t aware of anything that would be causing this. We never had these noises until the new hot water tank and expansion tank were installed and we’re so frustrated.

It’s been suggested to remove and temporarily plug the expansion tank with a ¾ inch plug, to troubleshoot; neither plumbing company wanted to try this.

It seems to us that the only different items are the new hot water tank and the new expansion tank. Nothing else was changed.

We’d really appreciate some guidance on this. Thank you!



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It is a good idea to bleed all air out of the lines by opening all valves in the home including hose bibs outside. This is a two person chore and you should start with the lowest valves in the home and end with the highest valve in the home. Take your time and let the water run for at least 1/2 minute at each valve. Also your PRV may need to be rebuilt with new rubber diaphram and other included parts in the kit. Also those home made air hammer arrestors eventually fill with water and are rendered non effective.



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You said the noise happens after flushing a toilet. I know this sounds weird, but I would check the shut off valves to the toilets. If they are the washer type, the handle will turn multiple times. A worn out washer can flutter, causing a "hammering" noise. Good luck! I have never heard of an expansion tank causing noise. They are installed to eliminate noise. The expansion tank is required in a closed system. Your PRV makes yours a closed system, meaning there is no where for the expanding water to expand to.