On Friday, 13 January 2017 22:37:44 UTC, John Rumm wrote:
On 13/01/2017 19:16, wrote:

That pretty much leaves the pressure reducing valve on the cold water
inlet side, and/or the 8-bar/90c safety valve.

I believe that safety valve also contains a non-return valve.

The one on my Vaillant does not appear to have any additional non return
on the side mounted over temp / pressure valve - it relies on the
tundish for that. Its basically a rubber faced piston held against a
flat brass mating surface by spring tension.

Could the water hammer be some sort of interaction between the
air-pocket in the cylinder and a defective non-return valve?

Have you checked there is no let by into the tundish? I noticed with
mine that if one turns taps off abruptly, it can shock a tiny amount of
water past the side mounted over temp/pressure valve. Over time that
built up some scale on it and then it started letting by continuously. I
had to descale it and polish the brace facing part to get it to resel
properly.


I'm getting to the point where "replace every component near the
tank" is sounding a sensible course of action.

You may find some of the controls rather pricey alas!

New ideas/theories very welcome.

Change in the volume of the air pocket? (Is it in a bladder on the
megaflow?[1]) - you could try adding a bit more air charge to it and see
if that makes a difference.

[1] The unistor has a separate expansion vessel externally.

--
Cheers,

John.

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Ah - sorry, I was being unclear.

According to the Megaflow documentation, the non-return valve is inline with the cold water feed into the cylinder,
i.e. pressure-reducing-valve - safety-valve (with non-return on inlet side) - tank-inlet.

I'm guessing the intent here is that as the pressure rises inside the cylinder, it can't create back-pressure on the pressure reducing valve.

My (current) theory is that this non-return valve isn't effective, and the water-hammer is some sort of resonance between the cylinder air-pocket and back into the cold supply.

The tundish here had been wet recently, but now seems to be completely dry since I regenerated the air-pocket in the cylinder as per the instructions on the megaflow.

(the air pocket AIUI is simply the top portion of the cylinder, with the hot outlet pipe dipping sufficiently deep into the cylinder that the air has no way out.This would tally with their instructions to turn off the cold supply, open the farthest hot tap, and then hold open the safety valve - until it stops gurgling - to regenerate the air pocket)