Lots of water hammer when washers are running

If I have it correct, the only zone downstream of a check valve and/or a backflow preventer, is the auto boiler fill loop. And the only connection between the boiler system and the domestic water system is that auto fill line.
So to start that clattering and shock waves simply by flowing H&C water to fixtures is somewhat different than your typical bouncing off check valves. I think!!!
Although that momentary higher pressure elsewhere in the house, due to flow and no flow changes could, I suppose, push through into auto fill system, causing the start of the clattering and bouncing of pressure waves. Don't mind me, just thinking while I'm typing.
I've been assuming there was an expansion tank already on the heating loop but don't recall it being confirmed. And of course, as you pointed out, the pressure setting would be important.

Jamesplumbing06 said:

Start back at square one with expansion tanks. Did you calibrate each tank to each system? Each one needs 2psi less than the pressure for that zone of plumbing. Every check valve creates a zone. So you have accurately check each side before tank can be considered to operate if correctly.The water bounces off check then tries to absorb off expansion tank. If that tank pressure is harder than the incoming pressure then your bouncing off that. Or in reverse. Bounce off check and then not enough pressure in expansion. Then tank slowly absorbs the shock just to create even shock wave to open check then you got a chatter box. Check the instructions for calibrating expansion tank to each zone. The boiler side will be less. Or should be. It’s just moving water not pushing like a freight train. I think your experiencing check valve chatter on steroids . Never had it happen on a combi unit but I never installed one. sounds like a well pump that isn’t calibrated with tank and switch.

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The boiler side is fine. Already tested and haven't had any issues with it beside it taking forever to purge lol. The expansions are 70 psi right now while the pressure is 60 psi. A lot of people said to go 10-15 psi above the working pressure. Not sure of the reason except maybe to give it a cushion when it does go up in pressure but doesn't give way under normal pressure.

There currently are no zones. The boiler is also only a one zone system

DO YOU HAVE AN EXPANSION TANK ON YOUR ONE ZONE HEATING SYSTEM. Which by the way would of course be a low pressure system.

Yes it has an expansion tank. The heating part of the system is all but isolated from the rest of the system. It cannot go into or out of the system without several components breaking at once. There is no way for excess pressure to go in or out. The line in question is strictly the DHW.

mdk0420 said:

Yes it has an expansion tank. The heating part of the system is all but isolated from the rest of the system. It cannot go into or out of the system without several components breaking at once. There is no way for excess pressure to go in or out. The line in question is strictly the DHW.

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Yes, that's why I had said the only connection to the potable water is through the auto makeup system.
And the pressures you just mentioned do not apply to that system or expansion tank.

Yeah I didn't consider the heating side at all because of it being isolated to its own system. The DHW had no expansion tank up until yesterday when I put one in. Jamesplumber06 mentioned that putting an expansion tank after a check valve would put it in its own zone, which I sort of agree (Although high pressure can still come into that zone, it's just a one way street). I haven't done the check valves yet because I'm still not sure the best way to put them. The expansion tank seemed to have solved the issue but it was still a very weird violent shaking that's much more then just water hammer since while it continued to flow it continued shaking until the valves were closed or the water was turned off. Flow restriction makes sense except that the main line hasn't been changed. The volume of water changed dramatically from probably around 95 gallons in the system to around 45. Still don't know if that really matters when it comes to water hammer. Would more water absorb more energy? Can it compress that much? I just want to make sure that it doesn't happen again incase the tenant takes a 30 minute shower while I'm not home to fix the violent shaking and the pipes destroy themselves lol.

It certainly is, what appears to be, a very unusual reaction.
Lets not for get that the PRV at the incoming service, although not a check valve, does tend to close when downstream pressures increase. Not sure if they act fast enough to impact the conditions encountered.

Yeah regulator can chatter. But when I say chatter. You have to remember I deal with little old ladies that don’t like the word or words like “violent”. So to me check valve chatter is like driving 80 then suddenly a tire blows out. So yeah violent shaking. I dunno why other people say to increase tank pressure above static pressure? I just assuming they were plumbers and close buddies? Best answer is in directions. Google that expansion model number and air pressure. But I do always say “if it’s working don’t look at it”.

Jamesplumbing06 said:

Yeah regulator can chatter. But when I say chatter. You have to remember I deal with little old ladies that don’t like the word or words like “violent”. So to me check valve chatter is like driving 80 then suddenly a tire blows out. So yeah violent shaking. I dunno why other people say to increase tank pressure above static pressure? I just assuming they were plumbers and close buddies? Best answer is in directions. Google that expansion model number and air pressure. But I do always say “if it’s working don’t look at it”.

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Was some plumbers from another forum with a similar question that I found. After I put the expansion tank in it hasn't done it since. Not sure if its the right fix, but its working. I also notice that I no longer drip out of the BFP, not even a little bit. Before it used to drip here and there when using different faucets and such.