Megaflo UVC - Very Loud Noise When Hot Tap Opened First Time After Heating Cycle

[steveoelliott]

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? 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.

[JohnMo]

What temp are you heat it up to?

[ProDave]

Check the tundish to see if there is any discharge. It sounds like one of the relief valves opening or closing.

 

The megaflow tanks have an internal "bubble" for expansion and there is a procedure to regenerate the expansion bubble which would be my first thing to try.

[steveoelliott]

57 minutes ago, JohnMo said:

What temp are you heat it up to?

Around 60, not too hot.

 

45 minutes ago, ProDave said:

Check the tundish to see if there is any discharge. It sounds like one of the relief valves opening or closing.

 

The megaflow tanks have an internal "bubble" for expansion and there is a procedure to regenerate the expansion bubble which would be my first thing to try.

The tundish is dry. I check this every time I go in the loft as a matter of course. The bubble on mine has disintegrated, hence the external EV. A while ago Baxi came out to check and confirm the bubble was disintegrated and as part of this they checked the installation. They said it was fine. They no longer swap cylinders but offer to put a strainer on the hot side to catch debris, I declined as not had much of an issue with it and a strainer on the hot would just get blocked.

 

I would be surprised if the relief valve was opening as bearing in mind this happens when a hot tap is opened after a heating cycle so pressure is effectively being relieved.
 

Edited September 8 by steveoelliott

[ProDave]

How can the bubble on a Megaglow disintegrate?  It is just a dome in the top of the cylinder above the waterline so as you fill the cylinder with water air is trapped in that bubble and compresses to take up expansion.  It can lose the air bubble over time, in which case you follow the procedure to drain the cylinder and re fill it.  No need to fit a separate EV.

 

Since you have not regenerated the internal bubble have you checked your external EV has not failed?  is still retaining air pressure in the bladder etc?

[steveoelliott]

17 minutes ago, ProDave said:

How can the bubble on a Megaglow disintegrate?  It is just a dome in the top of the cylinder above the waterline so as you fill the cylinder with water air is trapped in that bubble and compresses to take up expansion.  It can lose the air bubble over time, in which case you follow the procedure to drain the cylinder and re fill it.  No need to fit a separate EV.

 

Since you have not regenerated the internal bubble have you checked your external EV has not failed?  is still retaining air pressure in the bladder etc?

It’s a well known issues with Megaflos. If you google internal baffle failure, you’ll see it.

 

The EV was at 1 bar when checked a couple of weeks ago and pumped up to 3 bar per instructions. It hadn’t failed at that point and the noise was present prior.

[steveoelliott]

For completeness, attaching pictures of the installation. I have been told by my plumber and baxi the likely cause is the water inlet control block / PRV. I will be getting this changed shortly.

[John Carroll]

There seems to be a balanced cold teed off (incorrectly IMO) after the valve combination set, this should be taken from the PRV balanced cold port and before the check valve. If you open a (balanced) cold tap before drawing off any HW, do you get this noise?, if you dont, shut the isolating blue lever valve on the balanced cold before opening the HW after the next re heat and check for noise.

[JohnMo]

A few issues unrelated to the question

 

The pipe exiting the tundish is supposed to have a continuous fall, to comply with G3. Yours goes horizontal which does not comply. If it's 22mm you are only allowed it to be a max of 9m less 0.8m for each elbow. There are a minimum of 3x elbows visible so max length allowed is 6.6m. not sure where the pipe goes off the edge of the photo. But maybe too long?

 

Insulation has way to many gaps. This needs to be sorted, plus better corner joints. All joints should be tight. Plus if outside the heated envelope it looks like thicker insulation would help.

 

As @John Carroll says cold water feed is not correct. The combined valve has a check valve within it, the balanced cold water feed is taken upstream of the check valve. In it's current location you are likely getting a reverse flow through the cylinder when both hot and colder water are asked for.

 

3 things to sort 

Connect cold to balanced port on combination valve.

Sort tundish discharge pipe. Full instructions are in your cylinder manual.

Sort insulation.

[John Carroll]

Looking at that photo again, looks like that pipe is (correctly) the T in from the expansion vessel.

[JohnMo]

38 minutes ago, John Carroll said:

Looking at that photo again, looks like that pipe is (correctly) the T in from the expansion vessel.

If that is the case, I cannot see. You need a check valve in the hot water outlet (top of cylinder) to prevent reverse flow, as cold water pressure could be higher than hot water cylinder pressure.

[John Carroll]

I allways wonder why a PG is rarely if ever installed either on the cylinder or even the PRV, which would be very useful for trouble shooting, the EV is normally sized to 10% of the cylinder capacity so the final pressure after a full reheat to 65C should still only be 3.9bar, unlikely to cause noise while releasing the expanded volume of 3 to 5 litres while falling to 3.0bar, a leaking/passing PRV could easily increase the filling pressure to 4.2bar overnight, the final pressure will then be 5.8bar, far more likely, even though it shouldn't, to cause noise when a hot tap is opened, I wonder are these PRVs drop tight.

Edited September 15 by John Carroll

[steveoelliott]

On 15/09/2024 at 11:09, JohnMo said:

If that is the case, I cannot see. You need a check valve in the hot water outlet (top of cylinder) to prevent reverse flow, as cold water pressure could be higher than hot water cylinder pressure.

Yes I have one of these.

 

On 15/09/2024 at 09:38, JohnMo said:

A few issues unrelated to the question

 

The pipe exiting the tundish is supposed to have a continuous fall, to comply with G3. Yours goes horizontal which does not comply. If it's 22mm you are only allowed it to be a max of 9m less 0.8m for each elbow. There are a minimum of 3x elbows visible so max length allowed is 6.6m. not sure where the pipe goes off the edge of the photo. But maybe too long?

 

Insulation has way to many gaps. This needs to be sorted, plus better corner joints. All joints should be tight. Plus if outside the heated envelope it looks like thicker insulation would help.

 

As @John Carroll says cold water feed is not correct. The combined valve has a check valve within it, the balanced cold water feed is taken upstream of the check valve. In it's current location you are likely getting a reverse flow through the cylinder when both hot and colder water are asked for.

 

3 things to sort 

Connect cold to balanced port on combination valve.

Sort tundish discharge pipe. Full instructions are in your cylinder manual.

Sort insulation.

 

Thanks for your analysis...

1) Whilst the EV is not connected to the output on the Calefe combination valve, it is (to my knowledge in the right place and doing it's job). It is notably not a genuine Baxi component.

2) The insulation is on my to-do list.

3) Discharge pipe from tundish, that's interesting as I wasn't aware of this but having said that when I have gone through the process of attempting to recharge the bubble in the past, it has worked fine with no problems. I appreciate it may not be to code though. The total length of the pipe is way longer than 9m as it goes from the loft, behind a stud wall, across a side / lean to extension and then outside. Having said that, when Baxi came out themselves to check the bubble, they never commented on this. The guy even went as far to say it was a good installation.

 

The issue with the noise in the video now appears to happen EVERY time a hot tap is opened. It has got worse. I sent a video to Baxi and my plumber and both believe it to be a faulty PRV set. I am therefore pending a genuine Baxi one to arrive and my plumber will then fit it.

 

Edited September 17 by steveoelliott

[JohnMo]

1 hour ago, steveoelliott said:

The total length of the pipe is way longer than 9m as it goes from the loft, behind a stud wall, across a side / lean to extension and then outside.

Your issue is, if it was used for real, you could have the pipe backup the fluid flow, the result hot water spilling out the tundish.

 

Let us know if the valve fixes the issue.

[steveoelliott]

18 minutes ago, JohnMo said:

Your issue is, if it was used for real, you could have the pipe backup the fluid flow, the result hot water spilling out the tundish.

 

Let us know if the valve fixes the issue.

Will do... I understand the potential problem but in the past when I have discharged the relief valve manually, this hasn't been a problem, but I appreciate it could be and also appreciate you highlighting something I never considered.

[John Carroll]

On 17/09/2024 at 09:54, steveoelliott said:

Yes I have one of these.

 

 

Thanks for your analysis...1) Whilst the EV is not connected to the output on the Calefe combination valve, it is (to my knowledge in the right place and doing it's job).

 

 

Where is the EV connected into the system?, it looks to me  to be teed in to the cold feed after the combination set which is correct?.

 

[steveoelliott]

@John Carroll @JohnMo @ProDave

 

Firstly @John Carroll, the EV is the other side of the combination valve.

 

For the benefit of all that advised, this issue appears to now be resolved following the replacement of the PRV block. The one that was on the system was a Caleffi and not a genuine part (it worked fine for years) so I opted to swap with a very expensive set from Baxi which cost £204.00.

 

My plumber did the work earlier today, cylinder was drained, he replaced the PRV with the new one and refilled. So far, no noise on tap opening. It got the point it was happening every time the hot tap was opened unless done very very slowly.

 

One thing I noticed right away was how much more pressure there is when using the hot taps. It wasn't bad before, but now when opened, the pressure is much higher and it tails off after sometime. I actually remember it being like this a long time ago but hadn't notice it drop.

 

With the old PRV block out, I decided to have a look at the mesh filter. It was impossible to pry it out and I had to force it out by ramming a screwdriver down the edge to lever it out. On one side of the mesh there were some crusty deposits so it could be that this was part of the problem or contributing to it. If a cylinder is serviced annually, I guess these filters are cleared but speaking to my plumber, very few people bother.

 

I have another adjustable PRV by the main stockcock with gauge (this is also Caleffe), this provides a balanced cold. I may now with the knowledge from the other have this mesh cleaned or may just opt to swap it completely in case it leaks after being dismantled.

 

Thanks all for your feedback.

 

 

[John Carroll]

I know that the pressure is subjective (my kingdom for a pressure gauge) but IF the EVs pre charge pressure was checked/adjusted to 3.0bar gauge then the final pressure after reheating the full contents to 60C will be only be 3.93bar, hardly noticeable, one would think, when a hot tap is opened, if the EV precharge pressure wasn't checked or charged incorrectly to 1.0bar, then that final pressure will be 5.43bar, in either case the pressure will fall to 3.0bar when the  expanded vol of 3.4L has flowed from the hot tap, in the very unlikely event that the precharge pressure is only 0.6bar then the final pressure will be 6.58bar.

Re the EV pipe outlet, just to be absolutely clear, is it teed in where I show it in the photo?, if so, quite correct, if not, and teed in before the combination valve (effectively, no EV in the system) then the expansion will still be taken up by the Megaflo's bubble which will still be there if the dip pipe hasn't been removed, I reckon  that the final pressure (with no EV) will then be 4.28bar after expansion, not bad until the bubble disappears, if the EV is installed correctly and the bubble exists then there could be some shunting between the EV and the cylinder which could conceivably cause noise again eventually. Theres definitely something not quite right IMO because the final pressure with both the EV and the air bubble should only be ~ 3.4bar after expansion, certainly not  noticeable?

 

The other possibility for the high pressure is that the balanced cold isnt, it may be at a higher pressure and leaking through a mixer and pressurizing the cylinder.

Edited September 18 by John Carroll

[steveoelliott]

@John Carroll I can confirm that labeled pipe is the EV outlet. When checked ~3 weeks ago the EV pressure was 1 bar and we increased it to 3 bar. I have another PRV on the cold mains set to 3 bar but with a cold tap running it's just under 2 bar.

 

There certainly is no issue with backflow from mixer taps. The pressure generally from the hot taps is much better than it was prior and I suspect this may be due to the mesh being partly blocked / defective PRV. We know that was faulty as replacing it solved our issue.

 

The bubble on the megaflo is knackered... I had deposits of it coming through taps, caught by the mesh on taps, hence the external EV. This was also confirmed by BAXI. I wonder if there is still any benefit going through the bubble recharge process even with a knackered baffle. I suspect not.

 

 

[steveoelliott]

@John Carroll Also on the mixer taps pressuring the cylinder, there is a non return valve on the top.

[John Carroll]

Well, if the pressure is exceeding 3.9bar after a reheat which I think it is from what you are observing, the only other explanation is that the new PRV is creeping/leaking past say overnight, can you post a photo of any details from any labels etc. 

Re re doing the bubble, if the bubble formed while just filling the cylinder then I reckon the final pressure would only be 3.2bar, if re bubbked per instru tions, pressure around 4.3bar but both ev and cylinder pressure will equalize so I don't see any great benefit in re doing it but why not?

If you have a single lever mixer and leave it in mid position the PG on the cold PRV might tell something overnight or after a few hours, have you noticed what it reads any time the hot and cold havn't been used for awhile?

 

Attached is a handy calc sheet I made decades ago and added the bubble calcs a few years ago, it may be of interest.

Expansion Vessel Calculation Steve.xlsx

Edited September 18 by John Carroll
Adding File

[steveoelliott]

@John Carroll Thanks for providing the spreadsheet. Quite interesting and clearly you understand this topic well

 

The PRV was replaced yesterday afternoon.

 

In terms of the increased pressure, it isn't just on re-heating cycle or overnight. It is consistently much higher than it was before. his led me to believe the old PRV was faulty / blocked in some way. The new PRV is a genuine Baxi / Heatrae Sadia part. https://www.screwfix.com/p/baxi-95605894-inlet-control-kit-comp-3-part/187GW?gclsrc=aw.ds&gad_source=1&gclid=CjwKCAjwl6-3BhBWEiwApN6_kgqdF7Smm3DfsRkfvSYrULnx0A1jc4QSjXFsD4xwVVyVb0AqxJ-mCBoCG4AQAvD_BwE&gclsrc=aw.ds

 

When I say increased pressure, I don't mean 100% more or even 50% more pressure but it is noticeable from what was there before. As an example, around 9 months ago I thought our pressure seemed low, my other PRV was showing 1.5 bar dynamic pressure, I happen to know a local senior engineer at Anglian Water who came and replaced our water stop in the road as it was "old". Once replaced, I was getting ~2 bar dynamic pressure, that was definitely noticeable. The other PRV on the stop tap isn't probably necessary for us given that dynamic pressure never seems to be much above 2 bar but then one day it might be! 

 

In terms of your question on the gauge on the stop cock PRV, with a tap open / flowing pressure ~2bar. Once switched off anywhere between 3 and 4 bar but it's instant the moment the tap is turned off and stays there. Might drop and increase a little but that will be in part due to fluctuations in the supply I suspect.

 

In terms of the bubble / air gap. If the baffle is disintegrating, I doubt there is any value in going through the process. The external EV "should" deal with expansion from heating cycles. Assuming the bubble was operational, I wonder what opening the T&P valve does after the lowest tap has stopped? I was also curious as to why you leave a hot tap on when refilling the cylinder?

[John Carroll]

You seem happy enough now, wonder why such a exotic Baxi combination valve set is required and the Caleffi was "wrong".

 

I think the procedure for the bubble is to ensure that its not being compressed until the last ~ 25% of the cylinder vol as some schematics show the dip pipe bottom almost level with (or a little below) the TPRV, that's why I showed the 100% air bubble and the 25% air bubble, the total vol of water after reheating with the 25% bubble is 200L and is only 160L with te 100% bubble but IMO the available HW is still almost identical because once cold water rises up to the level of the dip pipe bottom then thats the effective HW available, some other makes like OSO have a removable dip pipe to give you that lost vol of HW if a EV is installed, looks to me that you could lose ~ 20/25% of the cylinder vol with the Megaflo.

Edited September 19 by John Carroll

[steveoelliott]

Thanks @John Carroll for your inputs, insight and advice.