Single Thud sound from cold water pipe by the boiler after flusing

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Scranton, PA
Hi guys,

Thank you in advance for your response and the previous forums on this site. I have read through almost all the water hammer problems that were posted on these threads and I think my problem is slight bit different.

I am the first owner of this newly built house in 2007. It has been over ten years and never had any problems. All of a sudden this month we get this single loud thud sound by the water heater after we flush the toilets in the house. It's a two story house with a basement (half bath on first floor and two full baths on second floor). The hot water heater is in the basement. The sound only comes when we flush the toilets and I have narrowed the noise down to the cold pipe from the hot water heater. Everything is normal when we use the sinks and the showers. I have replaced all three fill valves to Fluidmaster 400a. I have also tried shutting down the main water pipe coming in from the city and draining out water from all the pipes to remove any type of air buildup. I have also checked the thermal expansion tank and it has air pressure of 50psi (seems to be working fine with no water built up in it). Water pressure in the home is roughly about the same.

I can't figure out what is causing this and why all of a sudden after ten years. I hope the pictures attached helps (also link below).



  • Hot and Cold Disbursement.jpg
    Hot and Cold Disbursement.jpg
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  • Hot Water Boiler 1.jpg
    Hot Water Boiler 1.jpg
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  • Hot Water Boiler 2.jpg
    Hot Water Boiler 2.jpg
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