Water Hammer After Powers MM430 Tempering Valve Install

Hello, I installed a MM430 to temper the water stored in a Triangle Tube Smart 80 gallon tank. The previous tank and tempering valve did not have any noticeable noise. I don't have a sense of the magnitude of the water hammer, but it's not opening any emergency pressure release valves, at least. The expansion tank on the cold input is pressurized correctly. Then the cold splits and feeds two check valves. Check1 for the tank and Check2 for the input to the MM430. The MM430 has the "checkstops" too, in addition to Check1 and Check2. My impression was that the checkstops are just isolation valves. Although, it seems they might be check valves, too. I can't tell from the documentation. The water hammer sound is a "ping" type sound. It's unclear if the sound is water hammer or the MM430 valve body slamming around, if that is a possibility. I'm pretty sure the water hammer wasn't a problem before, so I don't think there is anything special about our fixtures. The building is a 12 unit condo building with toilets and washing machines, although like I said, I would have heard something if it was there. I spent a lot of time tuning the system. Any help is greatly appreciated. I think I'm making a fundamental mistake. For what it's worth, the water hammer seems like it's more pronounced on the cold side piping.

I checked the cold supply pressure and it's at 45psi. I did some more exploring and I noticed the expansion tank is a light in terms of weight. The expansion tank is upside down (nipple down) and elevated over the DHW tank. The pipe to the expansion tank is about 4 feet long and there is no way to vent the air from the pipe. Does anyone know if this setup could cause or contribute to the water hammer I am experiencing?

FWIW, there should be no check valve between the water heater and the expansion tank.

Air in the line to the expansion tank should not contribute to water hammer. The air will eventually be absorbed into the water stream.

Update

Hi Jim, I confirmed that the expansion tank connection is between the cold input check valve and the cold supply to the DHW tank. I was really hoping this was going to be the solution.

I experimented with the system a bit last night. There are hose fittings at various locations:

1. on the expansion tank pipe T near the DHW cold inlet
2. on the recirculation return port on the DHW tank
3. on the DHW hot output/MM430 hot input

I hooked up a hose to all three hose fittings and opened the valves quickly, slowly, etc. I was able to create a "hammer" situation at all three locations. Even when I closed the valve at a rate I would consider slow, the water hammer would occur. It seems like I just need a water hammer damper? What is messing with me is I'm sure the previous mix valve/tank did not have this problem. It almost seems like the valve actuator in the MM430 is slamming around like there isn't enough preload on a spring or something - pure speculation here.

Any ideas, guys and girls?

Just to be clear, water hammer occurs when a fast operating valve closes...the moving water has kinetic energy, and when it hits that 'brick wall', it acts like a hammer...often, moving the piping and banging it into something. That, technically, is water hammer. A loose washer in a valve can chatter, starting and stopping flow and make a nasty racket, and water initially filling an empty pipe and fixtures can make some weird sounds. So, without being there, it is hard to say if it truly is water hammer, or some other problem in the system. FWIW, turning a multi-turn valve on or off is very hard to create a water hammer situation, but a bad washer in it could create one as could some obstruction (piece of junk) that is bouncing around starting and stopping flow.

Check valves that are not fully closing with pressure imbalances across them can make some weird noises, too.

I've had some more time to examine the mixing system. It does indeed seem to be water hammer on the cold line, which is induced by fast closing of sink or other fixtures that utilize the hot water line. The water hammer in the cold line occurs right near the boiler room, which is on the roof, so am pretty confident the the water hammer is not induced closer to the city field. Plus, the water hammer began after the mixing valve installation. I'll try to post some pictures.