Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr..
Mains water cold 4.5 - 6.0 Barr, hot water 3.0 - 4.5 Barr.
Have checked that the 8 Barr temp/pressure discharge valve, and 10 Barr safety valve both manually operate, and discharge to tundish.
Have regenerated air pocket as per instructions on the Megaflow cylinder.



I haven't yet tried turning off the cold water inlet to the cylinder, and letting the boiler do a how-water warm-up cycle with the Megaflow depressurised - to attempt to prove the problem is definitely CH side, and not mains-water side (incidentally the quarter-turn valve just before pressure-reducing valve is frozen in the fully-on position, so have to use the under-sink main cold-water stopcock to isolate the hot-water system).



Could it be pump overun at the boiler, and the pressure-operated CH-circuit bypass valve not operating correctly? My impression (but not certainty) is the that the 2-port valve to the cylinder is still open when water hammer occurs.