I recently added expansion tank to my hot water heater. The water heater has a recirculation loop with two check valves, and the house water inlet has a pressure regulator, so is a "closed" system. At the same time I added expansion tank, I replaced one corroded/defective check valve on cold side, and two corroded hammer suppressors on each side of very expensive water heater. Now I have excessive water hammer on every valve in house, but only when cold water is turned on, but not when turned off. Otherwise, plumbing works fine.

Are there any solutions to this water hammer problem?

but only when cold water is turned on, but not when turned off.
Then you do not have water hammer.
Water hammer occurs when a fast acting valve,such as a toilet or single lever faucet, closes and causes a momentary spike in water pressure. This spike in water pressure causes a pipe to move and bang against s structural member.

Because this noise occurs only on the cold side, I think you can eliminate the expansion tank and hammer suppressors as the source of the noise.

I too have a hot heater re-circulation line. It runs from the furthest point on the hot water line back to the cold water input line to the water heater. There is a check valve on the re-circulation line to prevent cold water from going up that line. There is also a check valve on the cold water input line, before that connection, to prevent hot water from the re-circulation line moving back up the cold water input line.

If your re-circulation line is plumbed similar to mine, I would say that you are getting valve chatter from that check valve on the cold water input line.

When you open a cold water faucet the cold water pressure is reduced relative to the pressure in the water heater and the re-circulation line. Water moving through that check valve is causing it to chatter.

You didn't state what kind of pipes you have. I am guessing copper. When soldering copper, if too much solder is used (or you have difficulty soldering a joint) the melted solder can run inside the pipe. Hot solder does not stick to a cold pipe. Due to water flow these bits of solder can later move to the next available valve.

I suggest replacing the check valve on the cold water input line.

And don't feel too badly. I had a job re-plumbing some pipes which I had great difficulty accessing, therefore difficulty soldering the joints. I have been back twice, once for bits solder in the toilet valve causing it to not close completely and once because of a leaking shower valve. I have just received a call that the shower is leaking again. I quite certain that I will again find a bit of solder in the valve. It has been six months since the original re- piping job.

Then you do not have water hammer.
Water hammer occurs when a fast acting valve,such as a toilet or single lever faucet, closes and causes a momentary spike in water pressure. This spike in water pressure causes a pipe to move and bang against s structural member.

Because this noise occurs only on the cold side, I think you can eliminate the expansion tank and hammer suppressors as the source of the noise.

I too have a hot heater re-circulation line. It runs from the furthest point on the hot water line back to the cold water input line to the water heater. There is a check valve on the re-circulation line to prevent cold water from going up that line. There is also a check valve on the cold water input line, before that connection, to prevent hot water from the re-circulation line moving back up the cold water input line.

If your re-circulation line is plumbed similar to mine, I would say that you are getting valve chatter from that check valve on the cold water input line.

When you open a cold water faucet the cold water pressure is reduced relative to the pressure in the water heater and the re-circulation line. Water moving through that check valve is causing it to chatter.

You didn't state what kind of pipes you have. I am guessing copper. When soldering copper, if too much solder is used (or you have difficulty soldering a joint) the melted solder can run inside the pipe. Hot solder does not stick to a cold pipe. Due to water flow these bits of solder can later move to the next available valve.

I suggest replacing the check valve on the cold water input line.

And don't feel too badly. I had a job re-plumbing some pipes which I had great difficulty accessing, therefore difficulty soldering the joints. I have been back twice, once for bits solder in the toilet valve causing it to not close completely and once because of a leaking shower valve. I have just received a call that the shower is leaking again. I quite certain that I will again find a bit of solder in the valve. It has been six months since the original re- piping job.

REPLY:

Sorry for delay in reply but just got a round-tuit to try fixing the problem. I agree with your comment about the momentary pressure drop on the cold water side when a cold water facet is opened. BTW, when the recirculating pump is turned off (so no hot/cold pressure differential) the water hammer goes away.

I solved the water hammer problem by installing another pressurized hammer arrestor on the cold side of the recirculating check valve. The water hammer clunk is gone. I did not replace that check valve since it had just been installed new. The explanation involves the traveling water pressure waves (at the speed of sound in water) within the pipe itself and the rascally behavior of that check valve with a pressure differential; a very interesting fluid dynamics problem left for the student to resolve. Nevertheless, having a hammer arrestor on both sides of the check valve solved the problem.