Cavitation, not calibration?

You think there maybe an airpocket in the OP valve?
That's possible I suppose, since the check valve now works the OP isn't being opened any more when the thermosyphon loop heats up.
I've gone ahead and ordered a new pump.
I reasoned that at 9 years old it'll need replacing at some point so even though it might not fix the problem it'll mitigate any new ones in the coming years. Not sure when, or even if, I'll get the pump as shipping is a mess thanks to covid. I've got a list of things I've bought that I'm waiting patiently to be delivered...

Here's a photo of the bottom of my machine. I've marked where the OP valve is.
Not sure how to de-gas it if the problem is cavitation.

do you mean one way valve when you call it the OP valve, it's unusual to have an over pressure valve at the bottom of a boiler?

Cavitation in a rotary pump is either a restriction on water flow to the inlet, or sometimes a damaged vane...nothing to do with the outlet side of things and anything beyond that.

DaveC wrote:do you mean one way valve when you call it the OP valve, it's unusual to have an over pressure valve at the bottom of a boiler?

Yes Dave, unusual but in that machine the OPV is in that position, the invisible right side of the photo should shown a silicone drain pipe.

wow, the designers must love their customers.....

DaveC wrote:do you mean one way valve when you call it the OP valve, it's unusual to have an over pressure valve at the bottom of a boiler?

No that's the OP valve. The one way valve (aka check valve) is higher up and not in that picture.
Where would you normally see the OP vavle?

DaveC wrote:Cavitation in a rotary pump is either a restriction on water flow to the inlet, or sometimes a damaged vane...nothing to do with the outlet side of things and anything beyond that.

Water flow is fine and unrestricted and the damaged vane is the theory I think best fits the symptoms.

id usually expect the expansion valve to be in an easily accessible place (for adjustment), close to the side/top of the case. What would be fantastic is if all manufacturers additionally only installed the 1 way valve after placing the pump and boilers in the machine...would ensure they install them in such a way that pump head removal isn't required, or worse to maintain/replace them..

If you look more closely at those pictures you'll see that the expansion valve (OP valve) is in an easy to access place because there's a metal plate that can be removed to access it.
Refer to the first photo I posted in this thread which was taken from the bottom with that cover removed.
In the last 3 photos I posted I've taken the sides off, which is much harder, but that plate is on.

Is there a technical reason the OP valve shouldn't be where it is on mine?
To me it looks like the perfect place for it.

Also it isn't a valve that I would recommend anyone adjusting.
I'd critique my machine for having the pressure nut on the pump in a tricky spot to access unlike some other machines which have it poking out of the bottom of the chassis.
The OP should be calibrated on fitting and left well alone. Here's a perfect example of what happens when owner muck around with this valve.

Considering that your machine has a rotary pump with bypass, i don't see any reason why the OPV should be in a different place, at least on that machine.
Anyway apart the OPV matter, after some test you did what is the actual issue, check valve rattling, inconstant pressure at the gauge, or what else?

Check valve rattling and the pressure manometer judders too so I think my original suspicion about the rotary vane pump needing replaced is probably correct. I appreciate those who've made other suggestions for me to check.
Once I have the new pump I'll post a follow up here to let you all know if that fixed the problem or not.
Hopefully nothing else gets damaged or worn out while I await the delivery of the new pump!