I have a valve that does this automatically. I think Watts makes it. It has a solenoid valve for both the hot & cold water, and you plug the washer into it. It only opens the valves when the washer is drawing power. In addition, it has a water sensor that sits on the floor so that if a flood occurs while you are doing a wash, it shuts off anyway.
They apparently had trouble with the first design & were unavailable for a while. I've had mine for two years, and it works great. The plumber had never seen one and was very impressed. He's seen a lot of floods that one would have prevented.
I set up and played with a SPICE simulation (EE circuit simulation program) for a while, using transmission lines with various impedances and mismatched load and driving-point Z's, switches to simulate timed valves, and a diode for the checkvalve. I didn't try to simulate his pressure regulator. I do see some pressure variations due to reflections in mismatched lines from the transient event of first valve opening, in some cases over 50% of supply pressure peak-to-peak that rattles for a second or two. Adding some capacitance (damping) definitely helps.
The event where one valve closes and another opens doesn't generate much of a transient. This is because I simulated those lines as having fairly well-matched load impedances, my logic being that they are sprinkler lines hence lines with distributed loss. The primary oscillations in this simulation are in the line from the meter to the T and in the house plumbing with the largest pressure excursions occuring in the house plumbing.
This simulation is far from accurate, just behavioral in nature as a matter of curiosity. Zo of a transmission line is sqrt(L/C) where L and C are series inductance and shunt capacitance per unit length. These would correspond to contained water mass and elasticity of the pipe per unit length. Voltage is analogous to pressure, current to flow rate. I included no loss term corresponding to pressure drop per unit length as fn of flow rate.
I would think that the pressure regulator would keep pressure variations from entering the house, but it may not respond quickly enough to deal with transients and oscillations. It being a mechanical closed-loop feedback control system, it could actually exacerbate them at certain frequencies. I strongly doubt that the water department tests transient response or frequency response of pressure regulators. Root locus plot? "Uh, yeah, they work fine if there ain't any roots locusated so they clog the pipes."
Without any regulator I do see the largest excursions in the house plumbing, probably because it is like an open line having infinite termination impedance. The largest pressure excursions are, not surprisingly, at the far end. If that's near the bedrooms and there are some bends in that region with pipes that aren't rigidly secured, I could imagine that it could get rather noisy.
Possible remedies:
whut you said -- restriction i.e. damping. That may not be desirable because of loss of irrigaton volume flowrate.
whut others have said: capacitance, as a standpipe or small bladder tank.
A first valve that opens slowly -- perhaps a motor-driven valve rather than a solenoid valve. You're a machinist, he's an EE, might be an interesting little project using surplus parts. I'd put this in series with a solenoid valve that could be shut abruptly when second valve abruptly opens. That event doesn't seem to create much of a transient either in my sim or his experience.
FYI. Your EE neighbor may find this interesting, if not particularly informative or helpful.
Possibly unrelated to water hammering I have always been curious why hot water "sounds" different as it comes out of the tap. The pitch changes slightly and you can hear it coming. Is it because it is less dense?
It's because the valve's geometry changes as it gets hotter. Hotter water is also slightly 'wetter' than cool water. The change in surface tension affects the noises it makes going by sharp profiles in the valve.
RCM'ers sure know how to beat a subject to death. Plumbing codes here only require air chambers at locations with automatic (solenoid) valves, for obvious reasons. Ask your neighbor. ;-) The spring loaded ones work fine,will never fill with water and cost $6-$8 @ HD. They have M-F hose threads so they can be hooked up to your washer after the fact. Put one in the system between the pressure source and the solenoid valve and sleep better all night.
Brought to you by the BRS Institute for Better Sleep. Tom
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