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Ahh, yes - that's exactly what' I think is happening. As mentioned, it's a 2021 vintage three port that behaved fine before the new boiler went in. Potentially... Though after tracing the pipework, I am more inclined to think the pressure builds between the three & two ports as a result of CH...

We've had the noise two hours after CH stops running, though I doubt it's related to the cylinder's internal pressure. After drawing HW, we can still provoke the issue by adjusting the cylinder stat down to the minimum then allowing the CH to run for a while. After enough time, turning the stat...

The HW EV was added as a future fail-safe, and there wasn’t a problem in the summer months when we were running DHW-only. The noise only occurs if the CH runs for at least a couple of hours after DHW fully heated & two-port closes. The loudness of the bang on DHW valve re-opening appears...

Hi again – here’s an update after a month spent trying to get our plumber to look at resolving the intermittent, but at least daily, bangs/thuds/dunks from our unvented central heating system. ...this time with photos after dissembling the airing cupboard to get better access! In short, is...

I'm happy with where we've got to with all the input & advice. We now have something that we can demonstrate on demand & show our working, before we start draining down & swapping/rearranging components. Thanks @dilalio, @madrab, & @johntheo5 - your help is much appreciated.

It is & we had ;-} It does - which is good to know. I'd agree - but it did bang while latching the DHW two-port open to do the flow test :-} The DHW activation sequence opens the two-port before the MPV moves between A/B...so the pressure is arising between the valves. Wonder if we can...

Yep - SpiroTech RV2 Deaerator - fitted between the Wilo pump & the three-port/MPV. Thanks for the schematics John - will compare & advise. I’d note we can reproduce the noise both with & without the pump running, so there must be a static pressure difference across the DHW two-port that’s being...

Thanks for all the input - will be out today, so going to leave these here & pickup sometime next week… Happens with CH both on & off - our current guess is that pressure is built up against the DHW valve in the de-aerator ^ released as the DHW zone valve opens. Yes - rewired through new...

Yes John, we’ve been looking at this: Honeywell V4073A1039 Sundial Y Plan “Three position diverter valve” - with a cute little diagram: Heating/A, Inlet A/B, DHW/B - which matches the casting letters & the functions of the respective pipes, which is nice. >Also if you latch open the 2 port...

Yes to all that, other than the EV being connected to return of the primary circuit, not the pump inlet. No noise or flow issues drawing HW from the Megaflow but it was needing more frequent recharge so believe the plumber added an EV to alleviate that - whether it was necessary, in the absence...

Morning - here’s a brief response. Appreciate all the questions & input, and although I can’t now update my original post, will look to collate the info to give a clearer picture…and some actual pictures if that’s useful? >What is the make/model of boiler… Valiant ecoFit Pure 415 heat-only...

Yes! But it still bang/thump/rattles when manually opening the DHW two-port, almost flow. What’s odd is how pressure could be held across the valve, even in the absence of any pumped flow? Yes - again! To help with diagnosis CH & DHW are now both set to run 06:00-22:00 as it was a bit...

Responses in slightly random order: July, when we had the new boiler fitted - there was also an expansion vessel added for … reasons. TBF herself specced the original system a couple of decades back & did the leg-work on working out a suitable replacement boiler, so it’d more like her showing...

Morning, There’s definitely some loose clips on the wall & it’s possible that there’s unsecured pipe work under the floor, but the previous setup never made this noise. The bang also occurs without the pump running - both during the day & out of hours. Not sure - the label says 25/1-6 130...

>Madrab said: >is it an unveiled Yes, unvented, with a pressure vessel indicating 1.2 bar Presume the original installer didn’t go S-plan as it’s always been unvented. (Should have checked back over lunch!) Yes & that’s how the MPV/three-port valve sets itself by default Yes, definitely hot...

Yep - we checked the arrows cast into the brass body

Thanks Rob - seems reasonable, though the new two-port was a direct replacement for a similar Honeywell unit.

We had a new gas boiler fitted to an existing Y-plan system back in July but, after several months on DHW only, we turned the CH on and now get a loud water hammer from the new DHW two-port valve OPENING when the pressurised hot water tank calls for heat. We can provoke the bang by manually...