We recently replaced our 75 gallon water heater with a 50 gallon water heater. We have a circulation/instant hot water pump on the system. The check valve that is in the circulation line was also replaced.
Now for the first time (we've been in the house 16 years) when the re-circulation pump is off and if one turns on any hot water faucet anywhere in the house there is a loud, one-time clanging similar to the sound of a water hammer. This does not happen if the cold water is turned on. It does not happen if the re-circulation pump is on.
The plumber who installed the water heater claims he has never seen this sort of problem before. We have consulted four other knowledgeable people about plumbing and home repairs and they are also at a loss to explain what is happening.
Has anyone heard of such a problem and or have any suggestions as to what might be happening?
Thanks in advance for any kind assistance from people smarter than me.