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]

It then has to pass through the cylinder PRV which effectively is doing nothing as its wide open with a dynamic pressure of 2bar, this IMO should be looked at, the mains should be available right up to the cylinder PRV which is set to 3.0bar, its after this PRV that the balanced cold should be taken, the pressure in the cylinder and the balanced cold will then be truly balanced, Alternatively, if piping too costly/difficult to change, the cold PRV should be relocated on a T taken off the mains, there is then the best possibility of the cylinder PRV actually being in control since its upstream pressure will be at the max available.
I think the EV precharge pressure should be set to 2.0bar, at least this should get the diaphragm to "float" and the final pressure (with PRV still set to 3.0bar) will still be 4.35bar max after a full reheat.

The PRV on the mains doesn’t actually do anything. The dynamic pressure never goes beyond much above 2 bar and I have to wind it down to actually start restricting the flow. Even if we took mains to the cylinder that one on the inlet would still do nothing… But, of course one day mains pressure could exceed 3 bar and then all of this would have a job to do.

I am not sure if the static pressure ever makes the PRV on the cylinder work albeit momentarily.

A while ago before I even posted here on this issue, the EV pressure was just 1 bar. It was increased to 3 bar when my plumber was doing some other work and I asked him to check. It made no difference to the noise and the EV states charge to be 3 bar.

 

[John.g]

The PRV on the mains doesn’t actually do anything. The dynamic pressure never goes beyond much above 2 bar and I have to wind it down to actually start restricting the flow. Even if we took mains to the cylinder that one on the inlet would still do nothing… But, of course one day mains pressure could exceed 3 bar and then all of this would have a job to do.

I am not sure if the static pressure ever makes the PRV on the cylinder work albeit momentarily.

A while ago before I even posted here on this issue, the EV pressure was just 1 bar. It was increased to 3 bar when my plumber was doing some other work and I asked him to check. It made no difference to the noise and the EV states charge to be 3 bar.

A few points, its relatively easy to see what the static pressure is even if the filter is (and it quite possibly is) partially blocked on the EV, just ensure no hot or cold water draw off, the adjustment screw should be turned clockwise, DOWN, until the press stops rising, once thats established just open a cold tap that gives a good flowrate and even measure it and note the pressure again, you can then get a good idea of the filter condition etc., close the cold tap and open a high flow hot tap and again note the pressure.
The EV precharge pressure of 3bar is not set in stone, its a recommendation based on a assumption that 3 bar can be maintained at any given HW draw off, again, it will just go wide open and the EV will just run at whatever dynamic pressure thats available, say 2.0bar, the EV diaphgram though will be bottomed out and when the HW is shut off then the pressure (if available can rise very suddenly and just perhaps cause some problems. Far better off IMO to establish the dynamic pressure and at least set the EV prepressure say 0.2bar below this but not lower than 1.8bar, even though not essential I would aso set the cylinder PRV dynamically (with HW draw off) by just turning it anticlockwise UP, until the pressure as shown on the cold water PRV just rises a fraction, both the EV and the PRV are then in control and will give the smoothest cylinder operation.
However, IMO, the cylinder should not be fed with two PRVs in series.

 

[techweb]

A few points, its relatively easy to see what the static pressure is even if the filter is (and it quite possibly is) partially blocked on the EV, just ensure no hot or cold water draw off, the adjustment screw should be turned clockwise, DOWN, until the press stops rising, once thats established just open a cold tap that gives a good flowrate and even measure it and note the pressure again, you can then get a good idea of the filter condition etc., close the cold tap and open a high flow hot tap and again note the pressure.
The EV precharge pressure of 3bar is not set in stone, its a recommendation based on a assumption that 3 bar can be maintained at any given HW draw off, again, it will just go wide open and the EV will just run at whatever dynamic pressure thats available, say 2.0bar, the EV diaphgram though will be bottomed out and when the HW is shut off then the pressure (if available can rise very suddenly and just perhaps cause some problems. Far better off IMO to establish the dynamic pressure and at least set the EV prepressure say 0.2bar below this but not lower than 1.8bar, even though not essential I would aso set the cylinder PRV dynamically (with HW draw off) by just turning it anticlockwise UP, until the pressure as shown on the cold water PRV just rises a fraction, both the EV and the PRV are then in control and will give the smoothest cylinder operation.
However, IMO, the cylinder should not be fed with two PRVs in series.

On the cylinder, the genuine Baxi PRV is not adjustable. It is set at 3 bar permanently. Not sure on the PRV is series piece, I can’t see any harm in it and also prior to the mains water getting to our house, the water company will also no doubt have PRVs.

 

[John.g]

Ok, I'll leave it at that as I can't offer any more advice, good or bad, please let us know how you get on.

 

[techweb]

Ok, I'll leave it at that as I can't offer any more advice, good or bad, please let us know how you get on.

Thanks so much for all your input. It’s truly appreciated. If you were local to Northamptonshire, I’d buy you a drink

 

[techweb]

@John.g I had a thought. I am going to try and test to see if I can reproduce the noise with the inlet valve closed. Now what this would prove, I don’t know but it would rule out anything related to inlet pressures etc.

I am wondering if the NRV on the hot outlet on the top of the cylinder could be causing this.

I’ll give this a go either tomorrow or Friday.

 

[John.g]

@John.g I had a thought. I am going to try and test to see if I can reproduce the noise with the inlet valve closed. Now what this would prove, I don’t know but it would rule out anything related to inlet pressures etc.

I am wondering if the NRV on the hot outlet on the top of the cylinder could be causing this.

I’ll give this a go either tomorrow or Friday.

I would get a pressure gauge installed, you can then monitor what's going on, the NRV could cause a problem but again the supply to the UVC should not be passing through two PRVs IMO.

 

[John.g]

Even though shouldn't make a difference, have you considered remaking the air bubble?, it might smoothen out any pressure spiking, in effect you will be back to the original bubble only with a EV setting of 3.0bar and a cold water pressure of (if) 2.0bar dynbamic.

 

[techweb]

@John.g Well, I can certainly try that, it was done 4 weeks ago when the cylinder was drained to allow replacement of the inlet valve / reducing valve etc. However, remember my bubble is probably useless / non existent as it is slated to have disintegrated and we see pieces of it on the tap mesh in the kitchen.

This afternoon, I shut off the incoming feed into the cylinder and was able to still reproduce the problem. This in my opinion removed the possibility of anything on the inlet side.

 

[John.g]

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.

 

[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.