Megaflo UVC - Very Loud Noise When Hot Tap Opened

[techweb]

Hi,

I have noticed that when a hot tap is opened for the first time after a heating cycle (in this case a bath tap), the water flows for a couple of seconds and then there is a very loud noise as can be heard in the video.

Any ideas what it may be? I couldn't deduce the source. The EV was recently checked and pressure increased from 1 to 3 bar per instructions but notably it did this before. I just at that time didn't know the trigger. It hasn't always done this however, probably the last couple of months or so.

Thank you.

 

[techweb]

Did you get much water from the tap when you opened it?.
If the air bubble is still intact, (60 years ago, we had very large cold water UVCs on board ships with a air bubble, NO diaphragm, but had a gauge glass and the level wouldn't change in 3 months or more), and assuming 2bar pressure when you shut the feed, 35L of water would/could flow with the pressure falling from 2bar to 0bar or 26L in falling from 2bar to 1bar, due to the air bubble expanding.

Oh yes plenty came out and only after a while did the pressure start the drop.

 

[John.g]

You might consider repeating that exercise and measure the EV with a tyre pressure gauge before and after. If the precharge pressure has fallen to 1.0bar then the available vol would still only be 9.0L even if the UVC pressure was 3.0bar on shutting the feed valve, so looks like your bubble is hale and hearty.

 

[techweb]

You might consider repeating that exercise and measure the EV with a tyre pressure gauge before and after. If the precharge pressure has fallen to 1.0bar then the available vol would still only be 9.0L even if the UVC pressure was 3.0bar on shutting the feed valve, so looks like your bubble is hale and hearty.

Since running the test I carried out yesterday with the inlet valve shut, I cannot reproduce the noise once again. I imagine it will rear it's head again in time, but isn't that strange! I can't see how the 10-15 liters of water that got drained out of this whilst testing made any difference. The only thing I wonder is if something is adrift in the tank and moving around until a point it gets stuck / lodged. Pure speculation of course.

 

[ShaunCorbs]

Baffle ?? As 15l would be the top bit of the cylinder where the baffle was / is

 

[techweb]

Baffle ?? As 15l would be the top bit of the cylinder where the baffle was / is

The baffle on mine has started to break up anyway and I have an external EV.

 

[ShaunCorbs]

The baffle on mine has started to break up anyway and I have an external EV.

Yes as stated before this will be your issue

 

[John.g]

Since running the test I carried out yesterday with the inlet valve shut, I cannot reproduce the noise once again. I imagine it will rear it's head again in time, but isn't that strange! I can't see how the 10-15 liters of water that got drained out of this whilst testing mabarde any difference. The only thing I wonder is if something is adrift in the tank and moving around until a point it gets stuck / lodged. Pure speculation of course.

The EV is (presumably) set to 3.0bar and because your dynamic feed pressure is ~ 2.0bar then the EV has no effect since the bubble must still be existing to some degree, it would seem that the bubble is almost completely intact (if charged as per instructions) because it will emit ~ 17L of water (fairly close to what you observed) in falling from 2.0bar to 0bar, if the UVC was refilled without opening any hot tap or the T&PRV then, if this bubble was fully intact it should emit ~ 50L in falling from 2.0bar to 0bar and has now lost ~ 75% of its bubble, it doesn't matter really as your test above does mean that any plastic bits will fall down to the dip pipe end and possibly cause this noise but you don't really care as long as there is no noise when operating "normally".
IMO, if properly set up the UVC PRV should be set to 1.8bar (dynamic pressure is 2.0bar) and the EV precharge pressure set to 1.8bar, Baxi in their wisdom supplied a unadjustable PRV set to 3.0bar so the final pressure after a full reheat with the PRV set to 3.0bar & the EV precharged to 1.8bar will only be 2.76bar after a full reheat (assuming the bubble has finally expired) and still only 4.5bar if the mains pressure increased to 3.0bar or higher, this (4.5bar) is almost exactly the same as the final pressure if the Megaflo was operating as per design conditions.
Baxi also don't seem to place much importance in using a balanced cold since they don't provide one off their PRV, yet the so called (in their view) incorrect combination valve set(s) do, they then install a NRV on the hot outlet which isn't required if the balanced cold is taken from the UVC.

However, it is what it is, but as recommended by most, mains pressure only should be supplied to the UVC PRV., the EV precharge pressure should be checked and IMO reduced to 1.8bar. (only IMO), ideally, the balanced cold should then be teed off downstream of the UVC PRV but before the UVC expansion valve which contains a NRV.

 

[techweb]

Thanks for all the perspective on the pressure and expansion. However, what I can’t get my head around is why after shutting the inlet off and draining 10-15 litres of water whilst reproducing the noise and turning the inlet back on, the noise is mysteriously gone (for now). Of course a part explanation might be that this small / partial drain off recharged the bubble to some extent (if it still exists).

On the subject of the bubble / baffle with mixer strainers getting clogged with the debris from this, I can’t see how it is still working. I am lucky that only my kitchen tap has a strainer and it’s easy to remove. If you look online, people have showers and all sorts that get blocked with this and then it requires a callout from a plumber.

 

[John.g]

You may be better off with no bubble as any debris, if floating, will be up high against the cylinder top, the HW drawoff which is from the end of the dip tube is ~ 25% further down the cylinder but you can remake the bubble if you wish, it will/may not long as last as with a intact baffle but you can certainly remake it.

 

[techweb]

@John.g I've noticed the noise has returned but I am satisfied to have ruled out expansion.

When this video was taken a significant amount of hot water (half a bath) had been used since the last heating cycle and testament to this fact is the EV was cold to the touch as was the pipe between that and the cylinder.

I am leaning towards thinking either a dodgy NRV (single check at the top of the cylinder) or something within the cylinder. Interestingly, this time the noise was much louder and echo's. Any feedback welcomed. I've decided the next course of action is the NRV.

 

[techweb]

@John.g I've noticed the noise has returned but I am satisfied to have ruled out expansion.

When this video was taken a significant amount of hot water (half a bath) had been used since the last heating cycle and testament to this fact is the EV was cold to the touch as was the pipe between that and the cylinder.

I am leaning towards thinking either a dodgy NRV (single check at the top of the cylinder) or something within the cylinder. Interestingly, this time the noise was much louder and echo's. Any feedback welcomed. I've decided the next course of action is the NRV.

Interestingly, if you watch the video zoomed in, you can see the tundish and pipe near it move / vibrate with this noise. I am even curious if this could be the PRV but I can't think of any reason why it would do this. No water residue in tundish.

 

[techweb]

I’ve decided that I’m going to remove the NRV on the top of the cylinder and replace the T&P valve whilst I have the plumber out.


After that the only part not replaced will be the EV.

 

[John.g]

Did that noise occur while drawing off HW and stop when the HW draw off stopped?.
Get the plumber to recheck the EV precharge pressure, it was down to 1.0bar previously? so if not set to that initially then must be losing pressure, it could possibly be at 0bar in which case there will be no movement of water in the pipe, hot or otherwise., and the expansion is just been taken up by the bubble which was recently remade, of course that was the original design, removing that NRV is probably no harm.
I am a bit surprised that baxi didn't make a song and dance about not having the balanced cold supplied from between the PRV and the expansion valve, maybe thats why they install a NRV on the hot outlet.
What about reclipping the pipework?.

 

[techweb]

Did that noise occur while drawing off HW and stop when the HW draw off stopped?.
Get the plumber to recheck the EV precharge pressure, it was down to 1.0bar previously? so if not set to that initially then must be losing pressure, it could possibly be at 0bar in which case there will be no movement of water in the pipe, hot or otherwise., and the expansion is just been taken up by the bubble which was recently remade, of course that was the original design, removing that NRV is probably no harm.
I am a bit surprised that baxi didn't make a song and dance about not having the balanced cold supplied from between the PRV and the expansion valve, maybe thats why they install a NRV on the hot outlet.
What about reclipping the pipework?.

Thanks for your continued input…

The sound this time (not sure if you heard the latest video) happens a few seconds after starting to draw hot water. Probably around 4-6 seconds.

I will ask plumber to check EV pressure again but remember this happened when it was at 1 bar and also at 3bar.

Baxi were fine with the setup. The balanced cold comes by virtue of the PRV set at 3bar on the incoming cold water main and the NRV on hot side as belt and braces.

I do worry that this vibration / hammer may cause something to fail / burst, although my plumber assures me it won’t, but concedes this needs to be resolved. In terms of clipping pipes, everything is well clipped apart from the pipes that run vertically to the cylinder. Not much that can be done with those.

 

[techweb]

@John.g
OK, so my plumber came today and for the first time I reproduced it with him in the loft next to the cylinder.

I had him out to replace the NRV, T&P and EV. Figured I'd make sure everything had been done but after hearing the noise he told me it is definitely coming from inside the cylinder. On another group a plumber suggested that the breakup of the baffle can cause this noise as it catches / releases. Anyway, I've contacted Baxi to get a replacement.

 

[ShaunCorbs]

Yep seen it many times

 

[techweb]

Yep seen it many times

I wasn’t aware the baffle making this noise was common prior.

 

[ShaunCorbs]

I wasn’t aware the baffle making this noise was common prior.

Guess you missed post 3

 

[techweb]

Guess you missed post 3

Very good point. I just wasn’t aware that you meant it made the noise. Thinking about it now. I feel a bit silly.

 

[John.g]

Probably influenced by the ceiling having to come down to install new one, post #8.

 

[techweb]

Probably influenced by the ceiling having to come down to install new one, post #8.

Interestingly, my plumber did some measurements whilst he was here and it can come out but the hatch will need to be removed so not too much damage / disruption.

 

[John.g]

As a aside, I see OSO say you can increase their UV cylinder capacity by removing the dip tube and installing a EV, assuming that these too have a baffle, then I wonder why this isn't causing noise if floating about, albeit, in one piece, maybe because the cylinder is now full to the top, no air bubble. If the megaflow could be confirmed full, air bubble completely depleted, would the pieces of plastic float and be hard up against the top of the cylinder with no noise??. also no bits of plastic might flow out as the dip tube end is a 1/4 way further down the cylinder.

Also I see that the OSO effectively charges the bubble with a empty cylinder which should give a 5 fold increase in the air bubble to ~ 50L vs 10L in a 210L cylinder after a full reheat, maybe they don't have a baffle.

 

[techweb]

As a aside, I see OSO say you can increase their UV cylinder capacity by removing the dip tube and installing a EV, assuming that these too have a baffle, then I wonder why this isn't causing noise if floating about, albeit, in one piece, maybe because the cylinder is now full to the top, no air bubble. If the megaflow could be confirmed full, air bubble completely depleted, would the pieces of plastic float and be hard up against the top of the cylinder with no noise??. also no bits of plastic might flow out as the dip tube end is a 1/4 way further down the cylinder.

Also I see that the OSO effectively charges the bubble with a empty cylinder which should give a 5 fold increase in the air bubble to ~ 50L vs 10L in a 210L cylinder after a full reheat, maybe they don't have a baffle.

As far as I am aware, the Megaflo is not something that has this feature or can be dismantled. For example they won’t replace the bubble / baffle. This would make sense to be serviceable in the field.

 

[John.g]

As a aside, I see OSO say you can increase their UV cylinder capacity by removing the dip tube and installing a EV, assuming that these too have a baffle, then I wonder why this isn't causing noise if floating about, albeit, in one piece, maybe because the cylinder is now full to the top, no air bubble. If the megaflow could be confirmed full, air bubble completely depleted, would the pieces of plastic float and be hard up against the top of the cylinder with no noise??. also no bits of plastic might flow out as the dip tube end is a 1/4 way further down the cylinder.

Also I see that the OSO effectively charges the bubble with a empty cylinder which should give a 5 fold increase in the air bubble to ~ 50L vs 10L in a 210L cylinder after a full reheat, maybe they don't have a baffle.

For interest,
Oso technical inform that there is no baffle in their cylinders, so it seems that repressurizing a almost empty cylinder without a baffle will give, presumably, the same times between recharging as repressurizing 25% of the vol of a baffled cylinder.