New two-port valve - water hammer on opening

[dilalio]

After all of this input.
I'd suggest ensuring the pipework is properly secured as a start.... Cheapest and easiest thing to try first.

 

[dylan_t42]

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 there anything we can do without having to drain the system? Otherwise, suggestions were to replace two-port valve that appears to make the noise and/or convert from Y-plan to S-plan by ditching the three-port?

I’ve tried to summarise where we got to at the start of November, where we isolated the issue down the DHW two-port valve opening after the CH has been running for some time. We can reliably re-create the effect by forcing open the DHW two-port valve, both manually & by adjusting the cylinder stat down to call for HW, regardless of whether the CH is currently running but *only* after the CH has been running for some hours after the DHW valve closes. I found we can alleviate the noise by powering off the system at the mains which allows the three-port to return to its rest and noiselessly seems to relieve the pressure that appears to build up when the CH runs.

The existing Y-plan system had these new components added/replaced:
* Vaillant ecoFIT pure 415 heat-only gas boiler & control unit with wireless control panel & external temp sensor (replacing a Baxi Solo PF3)
* Wilo multi-mode pump running at ~5-8 cubic meters/hour in “c1” (replacing a Grundfos)
* Spirovent de-aereator between the pump & three-port (new)
* Magnetic particle filter next to the boiler (new)
* Honeywell NC two-port safety vale on the DHW flow pipe (like-for-like replacement of 20yo part)
* Heating circuit expansion vessel in the loft (replacement)
* Hot water circuit expansion vessel also in the loft (new addition to Megaflo)

And leaving these original/older components:
* Megaflo unvented 200 litre cylinder (20yo but not showing any signs of failing)
* Honeywell three-port (replaced in 2021)
* Califei air valves by boiler flow & DHW (replaced 2021)
* Cold water feed pressure control valve (replaced early 2024)
* 14 radiators over three floors with all copper piping

Looking back at questions that were posed and we maybe didn’t answer clearly.

Madrab
>It may be easier to take it back to basics, remove the 3 port, introduce a NO on the CH side and standardise the wiring as a PDHW system.
Essentially we do currently have PDHW, as the system only does CH at 30-50 Celcius, or DHW at ~70. Just not as quietly as we would wish it.

Johntheo5
Key point you made:
> If the MPV is staying in its "last port of call", say after CH mode above, then the water will be trapped between the MPV and the 2 port so just maybe building up a bit of pressure due to heat or whatever?
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 provoke it by opening the MPV?
...and the answer is “no” it seems to quietly release the pressure, after which the two-port opening is uneventful.

>Have you any wiring diagram for this setup?
It seems pretty normal in the picture, but we haven’t deciphered all the bits

> So can you confirm (again?) that if DHW reheat was last requested and then subsequently requested again then no noise
Yes, that’s what happens, as long as the CH isn’t running for long between times

>Can you manually vent that Spirovent?
Not that we’ve found a way

dilalio
> And perhaps a sketch of the installation
See photos – can produce a schematic over the weekend if useful

>I'd suggest ensuring the pipework is properly secured as a start
Mostly the pipes are well fixed, but the system didn’t suffer this problem before the new boiler went in and the additional pipes aren’t particularly loose

 

[dilalio]

Hmmmn... Perhaps something to do with the megalfo having had an expansion vessel added, as baffle failed? Is that why it was added?

 

[dilalio]

Change it to an s plan.

 

[dylan_t42]

Hmmmn... Perhaps something to do with the megalfo having had an expansion vessel added, as baffle failed? Is that why it was added?

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 proportional to how long the CH ran meanwhile, even if the CH has stopped when the cylinder stat eventually calls for HW again.

Change it to an s plan.

…in the spring …maybe ;-?

 

[dilalio]

You could try a shock arrestor after the 2 port but anything you do needs a drain down as it seems to be a hydraulic issue.

 

[dilalio]

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 proportional to how long the CH ran meanwhile, even if the CH has stopped when the cylinder stat eventually calls for HW again.

How long after the CH has stopped and the 2 port opens?

If the CH water is still hot, then the expansion and higher pressure may be at play.

Are your controls hot water priority or could you create a HW schedule that runs when the CH is Off and has been for a longer period?

 

[dylan_t42]

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 up again to re-open the two port provokes the noise without any DHW activity.

We had already shortend the HW timing, but a test to remove the overlap is easy without turning off the CH completely :chilly:

This has set me thinking about reducing the HW cycle so it never completely heated cylinder, which is an option until it gets warmer next spring...

 

[dilalio]

though I doubt it's related to the cylinder's internal pressure.

That's not what I was referring to.
I'm thinking about the pressure differential in the CH primaries, either side of the 2 port and a shock wave being produced when it opens.

Wondering if cylinder side of primaries (downstream of 2 port) is isolated from CH EV so it doesn't equalise?

 

[Madrab]

I can't get my head around this, the system should not produce a large enough pressure differential whereby a 2 port valve, that has no issues, creates a clunk like that. Is it definitely the valve that's causing it, can the valve be held and can the clunk of the valve be felt?

Does the system pressure rise significantly whilst it's running, >0.3 bar?

I'd now be taking the 2 port right out of the system just to test behaviour, during testing there wouldn't be a safety issue.

 

[dylan_t42]

That's not what I was referring to.
I'm thinking about the pressure differential in the CH primaries, either side of the 2 port and a shock wave being produced when it opens.

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.

Wondering if cylinder side of primaries (downstream of 2 port) is isolated from CH EV so it doesn't equalise?

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 leaking through the three port into the DHW loop

I can't get my head around this, the system should not produce a large enough pressure differential whereby a 2 port valve, that has no issues, creates a clunk like that. I

Quite!

Now worked out it takes about three hours of CH operation to create the effect. Less than that there minimal noise. And longer it's left, the louder it is.

s it definitely the valve that's causing it, can the valve be held and can the clunk of the valve be felt?

Yes - definitely triggered by the two-port opening & mainly felt through the hot side of the CH pipework

Does the system pressure rise significantly whilst it's running, >0.3 bar?

Nope, pretty much static at 1.2 bar

I'd now be taking the 2 port right out of the system just to test behaviour, during testing there wouldn't be a safety issue.

Possible & I'll be happy to try, just the prospect of two DIY drain-downs before Christmas is putting us off.

We'll see how it goes tweaking the timings to alleviate the issue, at least until the spring, unless we can get the original fitter back in, or someone else more capable who can spend one or two half-days re-jigging it.

 

[Madrab]

I think this is where I get to the stage where thinking over all my experiences with systems like this, there is nothing I can think of that can create a system pressure imbalance that can create a 2 port valve, that doesn't have any issues, to clunk

When thinking hydraulically about the way a sealed CH system works with it's circulator and what pressure is/could be created around the system, the only thing that could cause a 2 port valve to slam shut or bang open is a significant pressure differential from one side of the valve to the other. Looking at a system, the only thing that could cause that would be a restriction/or an accumulator that doesn't balance itself out, that then creates that differential.

As far as draining, if the system is sealed and the 3 port in CH mode then the HW circuit will only need depressurised and then a quick swap over to a replacement piece of pre-made pipe, with a tray and towels handy, then it should be relatively easy without draining down. Only trouble I can see with that is the nut from the right hand side of the 2 port. For some reason that I've never ever been able to fathom, the threads are always different on the valve nut from standard compression coupler nuts.

 

[dilalio]

the threads are always different on the valve nut from standard compression coupler nuts.

Honeywell threads... I always harvest the nuts when removing old ones!

 

[Johntheo5]

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.

Is the above EV in addition to the Megaflo air bubble?? so has nothing to do with a (red) EV installed if a heat only boiler.

Looks like it, so should have been installed downstream of the expansion valve on the cold feed to the Megaflo.

 

[dilalio]

so should have been installed downstream of the expansion valve on the cold feed to the Megaflo.

Perhaps it is?
Can only assume that's where it's been teed in... As it should be.