For sure, you are correct. Unfortunately, the OPV on the QM machines is a little different than others. All (well, at least within the past couple of years) QM machines equipped with vibe pumps have their OPV on the outlet side of the hx. When the flow of hot water goes past the OPV in just the "right" amount at just the "right" opening of the OPV the combination of the valve, the spring, the pump flow characteristics, and the mfg tolerances on the assy, together with any scale present inside exacerbate any vibration present and hence the squeal.cafeIKE wrote:An OPV should not squeal.
I can easily duplicate the effect on my laundry room faucets by reducing the flow to a small stream. Perhaps the quality of the faucet enters into the equation.
How to fix:
As dsc has done, soak the OPV valve internals in lemon juice, citric acid, vinegar, etc. It might not be a bad idea to remove the housing and soak it also but be advised that the straight threads at the OPV inlet need some form of sealant (loctite, teflon tape, copper washer, etc.).
Chris Coffee sells new inserts for the valve and anyone with a QM machine should add these to the "to buy" list along with an o-ring for the adjusting nut and food grade lubricant for same.
Reinstall the OPV and attach hardware store tubing to the OPV outlet. Plug the existing machine's OPV tubing line with a golf tee. This makes OPV adjust easy as you are not kinking and untwisting the PITA original short hose.
Run the pump with a blind filter in place and adjust the OPV with the heating element disabled. What pressure do I adjust to? Everybody has their favorite but I would suggest 9.0 bar. IIRC, this should give about 8.5 bar at the puck and a little less when the element kicks in during normal ops. If it squeals, make it 9.1 or 8.9 bar.
Reattach all original hoses, undo the disabling of the heating element and pull some real shots with the covers off. Readjust as necessary - hopefully NOT.