Fast hot water then noise and normal flow after heating hot water

[WizPip]

Hello,

I'm a fairly adept DIYer but I have an issue with my plumbing and not entirely sure what's left to check.

I have an unvented indirect system with 1 hot / 1 heating zone, cylinder is a Range Tribune HE 150L pre-plumbed.

For quite a while now I've had a problem where after the hot water has been heated, when I first use the hot water it's pretty fast and then there's a creaking noise and then the flow returns to normal. This only occurs once after the water has been heated.

The hot water is not scalding and as far as I can tell is not being over-heated, which leads me to believe the cylinder thermostat is fine. This issue has persisted beyond a leaky tundish issue which I resolved some months ago by replacing the monobloc (3 bar regulator / 6 bar relief valve) and I also took the opportunity to replace the white hot water expansion vessel as it was a decade old. That's charged to 3 bar (verified) and connected to the system; I have confirmed where the nut connects there is no air to this point. The TPRV on the cylinder is 7 bar / 90C and the check valve appears functional.

The hot water exit from the cylinder is at the top and so there is no air lock here. The air valve on the coil loop has been checked and is clear. I have also removed pressure from the heating circuit, verified 1.5 bar in the heating expansion vessel, then repressurised loop to 1.5 bar, just to cover everything.

Obviously I'm getting over pressure from somewhere, and the creaking sound could well be the diaphragm in the heating vessel returning to balance (or god forbid, the cylinder deforming), but I can't work out why this is occurring given the points above. What have I missed?

I have attached a diagram I have mapped out of the system as it stands for clarity.

Thanks for reading!

 

[ShaunCorbs]

You sure it’s not pipes expanding also you shouldn’t work on an unvented cylinder unless your qualified eg g3

 

[WizPip]

You sure it’s not pipes expanding also you shouldn’t work on an unvented cylinder unless your qualified eg g3

It'd be pipes contracting as the hot tap is opened, but it's the only noise my system ever makes, and it definitely comes from the cylinder cupboard (see vid). I need to do this again and put my hands on the cylinder and EV to see which one (assuming its one of those) is making the noise. My money is on the EV. I know that water expands by 4% when it's heated, so there's potentially 6 litres going into the EV which are evacuated when the hot tap is opened, but this noise / excess pressure is a feature I've lived with for only around a year and I've lived here for a decade.

Now if you told me this was normal for an EV system and it just took a while to "bed-in" then I'd believe you. It's certainly not affecting the operation at all, so it's just one of those questions - do I waste the time of a plumber on this, is it normal, etc?

 

[ShaunCorbs]

Picture of inlet valve assembly