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Poiuytr

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25
Good day - apologies if similar questions have been asked before but I have no real knowledge of plumbing and heating so thought I would try here to see if members might be able to help me.

I have an almost new Gledhill unvented cylinder that was fitted at the start of the year. I have no issues with hot water (or heating) generally - however I have noticed that when the HW cylinder is on that there is a (sometimes louder than others) 'bang/thunk' type noise - the noise tends to happen only once and is usually after about 25-35 mins of the HW being switched on. I pointed this out to BG during their annual check and they checked the motorised valve which they said was in order and said there was nothing obvious - the MV does switch on and off as and when the timer for the HW kicks in. Its hard to catch the noise as timing is random but doesnt sound like its coming from the cylinder itself - but not 100%

After the noise the boiler light suggests that the boiler is no longer firing the HW up - I think, given that it tends to happen 'mid timer' that there's a valve just switching off when the cylinder reaches temperature and making the noise - there is no noise from CH other than usual radiator noises?

Unfortunately (and I feel even dafter for admitting this) I don't know when this might have started - we've had water on and off periodically to have some work done on taps/showers etc (these all work fine) and I did mess around with the timer timings so cant say if this is a new thing or whether its being going on (at a different time where I didnt notice it) since install.

I've had a look and cant see any arrows etc on pipework or valve to check if the MV is the right way round as I have read this can sometimes make a noise - there is a separate MV for HW and CH - however they don't both face the same way.

Again, apologies for asking what might be a stupid question and thanks in advance for any suggestions and help - the noise is annoying I think rather than concerning - but once you hear something its hard to unhear it.

Many thanks...
 

Poiuytr

Messages
25
Hi there - there's no gauge on the cylinder unfortunately. I did think that I would get a service after a year (due Jan/Feb) and this would be something that would be checked. BG have generally been good to me so I'm not having a pop at them, but their annual check is basically just looking at the boiler. I presume a decent G3 qualified plumber would be able to help me look at the pressure? There is a expansion vessel (just out of shot I think) above the cylinder
 

John.g

Messages
2,960
A gauge would be handy but not usually fitted I think, useful to see what the pressure is after final heating in case the expansion vessel is losing its charge. Is there any sign of water around the tundish where the cylinder and expansion PRVs (safety valves) drain into. Your system should look like the schematic.
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Poiuytr

Messages
25
Hi thanks for coming back - the set up pretty much looks like that and the illustrations shown in the Gledhill manual. There are no signs of water anywhere which made me think it might be something just closing, albeit noisily, when the temp is met. I have measured the temperature of the water coming from the taps and its not above the temperature set on the cylinder.
 

Poiuytr

Messages
25
I thought I would post again on this as its been a while and the issues with our indirect unvented hot water cylinder are still there to some extent. As an update to the above I discovered that the cylinder was not reaching anywhere near its target temperature of 60 when it was heating up - there is a manual bypass and this was fully opened so when water was circulating the water was heading down the bypass mostly rather than into the cylinder. I think this issue was causing the loud banging - at the cylinders first annual service I discussed this with local G3 engineer who closed this bypass off significantly and it settled down a bit after this in terms of repetition and also in loudness but did not disappear altogether. After this change the cylinder however was able to reach its target temperature and click itself off as and when, which was good. The local G3 engineer heard the loud banging as the cylinder heated up but after some discussion about a total system flush suggested contacting Gledhill about it.

After this change there also appeared to be a 'flushing' type noise which occurred mostly after the cylinder was up to temperature or just after it had switched off (this wasn't there before) - as a result of this, and the continued, but reduced banging I called out BG to have a look, they adjusted the bypass a bit more, to open it up a bit as they reckoned it was now too closed. They also recommended that I keep the HW on constant most of the day (just as a possible energy saver) with the idea that small amounts of water would be topped up as and when used and the temp of the tank dropped enough for it to kick in. I did this for a few days, but was nervous about this given the previous behaviour of the cylinder - there wasn't any noise after this that I detected, but its not easy to monitor once you have moved from a timed environment.

With this in mind I switched back to a timer (twice a day for 30 mins which is enough for us). Generally speaking things remain quieter, however on a heating cycle after the hot water had been well drawn down due to usage (with related temperature fall in the cylinder) the banging came back (again, less loud than previously) - it stopped (apart from the odd tick and click which I believe is fairly normal?) I wonder if the banging is being caused by some kind of expansion issue within the cylinder as it heats up - particularly as it was working harder to bring the temp up on this occasion. I am aware that the water will expand as it heats by a reasonable percentage. I have an 18L Zilmet expansion vessel as part of the install - but previous engineers haven't been able to check this much as when it was installed a year or so again it was set so high in the cupboard that they cant get into it properly (which isnt great). I wonder if anyone has any thoughts on this - I dont know how these vessels work - do they take water or air or both and where does the water go after if it moves into the vessel on heating? The expansion vessel just seems to make a ping noise wherever I tap it on the outside so I don't have much to go on here.

I know there are some negative reviews on Gledhill cylinders out there anyway but they can't all be bad - I don't even know if this one is or not? The banging type noise from the cylinder seems to be a mix of plastic and metal when it occurs which would work back to some extent to there being an issue inside it.

No issues in terms of piping of the cylinder etc were mentioned last couple of visits (or at all for that matter) and as noted above the install matches John's previous schematic and the layout as shown in the Gledhill installation plan.

If anyone had any ideas that would be great. It's a bit less scary than it was - it feels like there has been a bit of progress as things have settled a bit but would dearly love to sort this out.

Am happy to get BG (or Dyno) back if we think we can figure out potential issues as the cylinder is covered under their Homecare plan, (I think they would be reluctant to replace it thought as there is no water or leakage and its just over a year old) - this would seem to be a better idea than speaking to Gledhill and overall its hard to replicate the noise which is random, though it may occur more regularly when the cylinder is having to make a greater leap in termperature - clearly, no-one but a G3 will be touching any of this (I check this when I speak to BG - and I certainly won't be doing anything myself) so am not looking for ideas as to how to fix this on my own - just a gathering some thoughts ahead of getting an engineer back to look at it. Please be gentle with me here - I am not really able to provide much more than the above which has developed over time and I am not great with any technicalities of the plumbing and heating world.

Thank you very much in advance.
 

John.g

Messages
2,960
If I was trouble shooting this I would install a pressure gauge to monitor the pressures under changing conditions as a sudden change in pressure on opening a hot tap could cause problems with a cylinder that has 0.5mm?? wall thickness.
Assume a 200 litre cylinder/18litre EV that has a cold water pressure of 2.5 bar via a PRV. The EV would/should have been pre charged to 2.3 bar, heating a stone cold cylinder to 65C will result in a final pressure of 3.46bar, when a hot tap is opened the pressure will fall to 2.5 bar, this shouldn't happen too suddenly as the EV air pressure (like a spring) should supply a few litres ( 4) of water to the system while the pressure is falling and if the pipe from the EV to the system is adequately sized then the pressure drop should be cushioned and not rapid. If the cylinder is being reheated after 100 litres is drawn off then the final pressure is 2.92bar after reheat and only 2.7bar if reheated after only 50 litres draw off. One wouldn,t expect any of the above to cause problems with any cylinder but maybe these cylinders because of their wall thickness are more sensitive to pressure changes?. The reverse happens on re heating but the pressure rise is far slower so wouldn' expect the same problems.
Whoever is checking this out should ensure that the EV pre charge pressure is 0.2 bar lower than the cold water pressure but how does he do this if he hasn't a pressure gauge mounted somewhere even temporarily?. It can also be calculated once the water side is pressurized but most would not do this.
 
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Poiuytr

Messages
25
Hi John - thanks again for coming back on this. I would guess that the thickness of the steel on the cylinder is pretty thin. Its a 150L cylinder with an 18L EV so the sizing is right according to Gledhill if nothing else.

I think you are right re things not being monitored pressure wise when they have come out previously. BG installed it all so if they cant get into the EV to check he pressure then they should be willing to rectify it by dropping it a few cm if nothing else.

In super laymans terms for me - if the EV pressure is way below that of the cold main (this is limited by a PRV under the sink to 3 which matches a setting on the combination valve on the cylinder) what affect might that have - would that get in the way of the expansion of water into the EV? This was my basic thinking, therefore the pressure rises as the water heats up, it has less space to go and we get a bang from the cylinder in response. (or is is the reverse - if the EV is set way below the cold then might this cause issues? We had some problems with the incoming cold pressures due to a blockage - this blockage was removed and a new a new 3bar PRV installed under the sink at this time) Am I right in thinking that the cylinders PRV is there ensure that if the pressure rises too far then this will kick in the safety functions - I think there also cut outs for thermostat on the tank and on an valve?

Its hard to tell how much water was used when it was run down a bit - I do know that the cylinder thermostat ran down closer to 40 from its usual starting point around 60. My guess is about half of the 150L? - thats a guesstimate though)

What I am trying to do now is monitor those days where the cylinder had been been drawn down a bit more to see if there is a link between this and the noise - I did try and measure this before, but it was not conclusive as the noise occured anyway (lets say 25% of the HW heating runs) I presume due to the bypass issue and temp never being reached. I think if I can somehow manage to ascertain that this is the case then it might give Dyno/BG something more to work with.

Thank you once more, sorry for my clumsy and most likely very basic questions here - am pretty sure that they will need to come back - its becoming a bit of a long running saga.
 

John.g

Messages
2,960
A 150/18 lire HWCyl/EV with precharge/coldwater pressures of 2.8/3.0bar will have final pressures of 3.76/3.35/3.17bar after drawoffs/reheating of 0/75/38 litres.
If precharge pressure is 0.5bar then final pressure is 5.74bar, 4.76bar at 1.0bar prepressure and 4.02bar at 2.0bar prepressure. (CW pressure 3.0bar)

Glendhill replaced some cylinders I think where the insulation curing period was too short leading to this noise, don't know if the replacements cured the problem or not.
 

Poiuytr

Messages
25
It might be worth my while contacting Gledhill as I run through the process. I have also heard that there were some issues with Gledhill cylinders and the time my cylinder was installed I believe was around the time of this problem. Thanks again for taking a look at my questions.
 

Poiuytr

Messages
25
Hi John. one last quick Q on pressure etc, just trying to think of things for when I get someone round to look at it. There is a combination valve on the cylinder, but the balanced cold is blanked off. The combination valve operates at 3 bar, I also have a PRV on under my sink where the cold mains comes it - this is also 3 bar. There are a few mixer taps and and a mixer shower in the house. The noise is generally when the tank is heating (though it appears there may be a noise after a shower is run and the tank is cool which may go back to expansion/contraction etc inside the cylinder. Would having these mixers (tap / shower) cause any possible issues? I presume that if I were to run a bath or a shower then no water could flow back (from hot or cold supplies back into the cylinder? (cant see how as the cylinder will always be filling itself up?) - apologies again for a dumb sounding question.
 

John.g

Messages
2,960
Don't think that should be a problem, only problem I read on here was that someone had connected the EV to this unused balanced supply resulting in the expansion valve lifting as the EV was on the wrong side of the NR valve in the combination valve.
 

Poiuytr

Messages
25
Cheers... couldnt see myself how water could be getting back into the cylinder etc - taps and shower work fine too so nothing obvious there at all that could influence the banging coming from insider the cylinder.

If appears that the more the cylinder has cooled down, and then has to get up to speed, the more banging I am getting - so sounds like I have a cylinder that works well - unless I ever have to use it...

Have contacted Gledhill in the interim - either way, something clearly not right here.

Thx again
 

John.g

Messages
2,960
The heating coil in some of these cylinders is corrugated and flexible, (like a wire rope) and is suspended/supported internally so just maybe this is making the weird noises?.
You could rule this out to a degree by heating the cylinder with the electric immersion, if installed, and see what happens.
 

Poiuytr

Messages
25
Hi there... looking back at the schematic above it shows that there is meant to be a balanced cold connection in the combination valve, shown in-between the PRV and NRV. Mine appears to be blanked out though - Gledhill install manual talks about the cold water supply to any mixer taps / showers must be taken from the cold water tapping of this (combination) valve to ensure balanced hot and cold water pressures. I dont quite get this is simplistic terms but as this is blanked out the system doesnt appear to be set up this way. My house has always had mixer taps and showers - the taps have separate handles or just one that moves from side to side.. the shower thermostatic on a bar just now, previous to this it was a single old mira round plastic mixer type thing. I presume that all of these are mixer type products. Does this mean that BG have therefore incorrectly chosen the Gledhill cylinder or effectively incorrectly fitted as it would appear that any cold to taps / shower would come off the main?

I dont see how this would explain why it was banging away every time it heated up regardless of whether someone had had a shower or not, though of course taps are used all the time in the household.

Super daft question - presume that if the water to a tap or shower comes out of one opening (supplied by a separate hot and cold supply pipe into one body) that it is effectively a mixer? I have some taps where there is one handle (monobloc?) and some where there is a single tap unit, but with a separate handle for hot and cold). Naturally the shower is just a single handle that is turned. Surely a lot of (most) people have mixer taps / showers?
 
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John.g

Messages
2,960
I think most thermostatic mixers require the hot/cold pressures to be within something like 0.5 to 1 bar of each other to allow proper temperature control.
There have been a number of posts on here where Glendhill havn't raised any objections to a separate PRV, in a ideal world/new install it probably is taken from the combination valve but in older/retrofitted systems this may involve major pipework alterations,

A twin flow mixer has a diffuser at the exit from the spout so non return valves are not required.
 

Poiuytr

Messages
25
Thanks - so basically sounds like regardless of whether my taps and shower are two handle / one handle or whatever this shouldn't make any difference.

I can understand why the cold and hot pressures should be the same - but as mentioned there is a PRV limiting to 3 bar under the sink, and likewise a PRV as part of the combination bloc at the same level so cant see why there would be any significant difference. I assumed that the pressure of the water coming out of the cylinder would be that of the cold anyway given its mains pressure? Cant see how even on a single handle tap / shower (or whatever) how this would be compromised surely they are effectively just turning on the two (underlying) supplies) but just delivering through one tap.

I'll wait to see what Gledhill say when they have evaluated things in terms of the sound. I have sent them some pictures of set up too. So either the cylinder is a duffer, or BG havent installed right / thought through what shower / taps are in the house... either way, someones needing to fix this
 

John.g

Messages
2,960
As a matter of interest where is the E.vessel connected to the system?, is it at the expansion valve?.
 

Poiuytr

Messages
25
Hi there.

The expansion vessel is attached to one outlet on the combination bloc - I have attached a picture. The cold comes into this bloc - the pipe marked with a C then working its way round - this pipe then goes into the cold inlet on the cylinder. As you can see blanked off for balanced cold
 

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