Heatrae Sadia Megaflo - small popping noise, when hot water is drawn

6 Apr 2021
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United Kingdom

Im hoping someone can shed some light, on a small (not loud) popping noise thats coming from the unvented cylinder, when hot water is drawn. It doesn't happen every time, however I've noticed it happens - when the hot water has not been drawn in a while (a couple of hours).

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It might be just the stainless cylinder contracting slightly as the pressure reduces when you draw water.
thanks, @Gasguru is this an issue - should I be concerned about, inlet is balanced and flow rate is is greater than 20l..
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Assuming the cylinder is regularly serviced (air bubble re-generated etc) I wouldn't worry.
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Assuming the cylinder is regularly serviced (air bubble re-generated etc) I wouldn't worry.

The cylinder is a warranty replacement from Baxi, as the original cylinder was making a popping noise - when the boiler was heating the water, however that cylinder did not make the noise I’m experiencing now (pop noise coming from the cylinder after drawing hot water for a min or two) - The noise the cylinder is making is becoming irritating, probably due to the cylinder being located in the the eaves cupboard, in my bedroom.

before I call out Baxi to have a look at this issue - Would an air admittance valve help on this occasion? - Obviously a G3 plumber will have to fit it in.

Thoughts please?
Are you 100% sure it's noise from the cylinder?

Similar noise can be created by the hot water outlet pipe heating up and expanding, particularly where it passes through holes in wooden joists or similar.
Same applies to central heating pipes when they heat up.
An air admittance valve is either used on drainage/waste pipe runs to prevent the adverse effects of negative pressure or occasionally on cylinders to prevent collapse of the cylinder when draining down (but is rarely necessary with modern stainless cylinders).
As flameport suggests have a look at the pipework..
I've tried to see, if I can replicate the noise, by running all the taps, both hot and cold and flushing the w/c - no noise this time, however its does not happen all the time, its more prevalent at night time.

The noise is like a metallic pop noise, which sounded like the issue I had previously (before Baxi changed the cylinder) - I'm not hearing a banging, droning noise that I would expect to hear with water hammer.

Something else I have noticed ever since I've had the mdpe pipe replaced (25mm mdpe communication pipe + 25mm run of mdpe into the stop valve with 22mm copper to the cylinder) the cold inlet (into the cylinder) is very noisy like a whooshing noise as the cylinder is replenishing the water, when the hot water is drawn.
Here is pic of my setup - perhaps it may yield a clue...


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When was the bubble last regenerated?
Is there any evidence of water in the tundish...water drop on the upper part inside?
@Gasguru the new cylinder was installed a couple of months ago in September, it would have been regenerated then, however I too have performed this a few weeks back by following the instructions below.

turn off water, at the lowest point - draw the hot water, whilst holding pressure relief valve - wait till water stops flowing. - them close the PRV and tap and turn the water back on.
There is no evidence of water flowing through the tundish, apart from when performing the regeneration of the bubble.

I'm hoping that the stop tap now fully opened as recommended by @flameport to check (I was able to turn it open a couple more degrees) will have resolved the issue - will monitor further.
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Unfortunately, I am still experiencing the same random issue - I've added a link to the popping noise.

I've also reached out to Baxi, to see what they make of it.

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