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Posting here as the unvented G3 forum does not let me post.

I am hoping someone could apply some logic and help me with a problem that two plumbers have not been able to diagnose properly.

Details :
Unvented system.

Incoming mains with prv set at 2 bar. Plumber has checked this at ktchen mains.

Cylinder has two immersions. I can hear both cut off and cpme on again so presumably no thermostat problem.

The cold water relief valve set to 3.5 bar is letting by ONLY when the immersion is on (either of them).

The G3 plumber who serviced in November found the expansion vessel pressure at 2.4 bar. He recharge to 3.5 because a sticker on it said 3.5 - presumably this is the pressure put in it in factory ?!

After he left, started letting by again - so next suspect was the valves.

I got other plumber to replace the erv and tp valves. Now still letting by cold water when immersion on. This plumber said it now can only be the incoming reducing valve that has failed.

To me both these are illogical.
Can someone explain why 3.5 bar is right pressure ? To me this is too high relative to the 2 bar incoming so that it is like not having an expansion vessel at all, so the ERV then opens.

Also if the main pressure reducing was faulty the valve would be letting by all the time because the whole system would then be above 3.5, cold or not.

I am at my wits' end because you pay good money and they seem to do the wrong thing and not be able to think through. The second plumber did not even have a pressure gauge wih him and did not say anything about the expansion vessel.

I now have new valves and am inclined to just reduce the expansion vessel pressure closer to 2 bar which i bet should solve it.

Any advice please ?


Many thanks
John
 
Has either plumber used a pressure gauge to check if the Pressure Reducing Valve is functioning?

You are right that if your system is specified to work at 2 bar pressure then pumping the expansion vessel to 3.5 bar is far too high. The relief valve (which opens at 3.5 bar) will always kick in before the EV can take up any expansion.

Having said that, when the EV was at 2.4 bar and your relief valve was still opening, indicates some other problem. Is the EV sized correctly, for example?
 
Thank you.

The kitchen tap was at 2 bar when we checked.

This was prior to change of relief valve.

It could be that the relief valve did need replacement and the new one is now letton by because of the 3.5 bar in expansion tank.

What is thr guidance on the expansuon vessel pressure ? Logically i would think too low or too high is same as not having a vessel or having a concrete ball in there. So symptom would be the same.

Is there an ideal pressure ? midway between incoming pressure and rv setting ?

With mains checked at 2 bar, immersions coming on and off and no leaks it can only be the expansion vessel pressure. Unless a new erv valvr is also faulty - but it only lets by when immerson on. So it does hold closed at system 2 bar.

I am inclined to buy a foot pump and correct the pressure in the expansion vessel. Do you need G3 even for just this ?

Thank you.
 
If by sized you mean the presssure rating, i have replace like with like. There was a 3.5 bar relief valve and since this was probably installed long ago before i bought the flat, i have no instruction manuals or log.

It is a 210 litre tribune mark 2. Could not find any specs online because superseded by new models.
 
It’s difficult to diagnose a problem like this without being there.
However, the incoming Pressure Reducing Valve is set at 2bar (normally 3bar). This can as
as your plumber says, be checked via the hot tap with immersions turned off.
As the water is heated it expands and this is taken up by the Ex Vessel. If the initial static change of the vessel is set at 3.5bar it will reduce the available volume of the vessel in turn the pressure will rise and activate the Expansion Relief Valve.
 
When system is fully cold, the pressure in the expansion vessel should equal the incoming pressure (2 bar in your case). As the water in the tank heats and expands the pressure in the expansion will rise as water is driven into it and compresses the air it contains. The size of the expansion tank and the amount of air it contains needs to be sufficient that the pressure never exceeds the value set by the PRV.

A G3 plumber should know this stuff backwards. They will also carry out additional checks to ensure that the expansion tank and valves are working correctly and will be able to calculate the correct pressure in the expansion tank when the tank is full of water at 60°C. It wouldn't surprise me if that doesn't turn out to be 2.4 bar but you need someone competent who has actually inspected the system to confirm.

In your position, I'd contact the manufacturer of the unvented cylinder and ask them to recommend a G3 plumber in your area.

What you should not be doing is trying to fix the problem yourself. They can be extremely dangerous if not installed and maintained correctly. The above information is purely 'educational' so you can understand how a correctly functioning system operates.
 
The expansion vessel pre-charge in your case should be set to 2 bar. If it is less than 2 bar, the EV will be occupied with some water under normal conditions (i.e. no water heated) and therefore will have less room for expansion when it is heated. If it is more than 2 bar, it will not allow expanded water to enter until it hits it's pre-charge pressure, which means the pressure in your system will exceed 2 bar.

"Sizing" correctly means that the volume of the EV is sufficient to take up the expansion of hot water from your system. You can check some calculators online or check the instructions for your cylinder, but the volume of your cylinder and the pressure it's designed to work at will determine this.
 
Get another G3 plumber. The plumbers you have used obviously have little idea. Pumping the e/v to 3.5 bar just because it says that as factory setting is not clever.
This in my opinion is not for open forum
 
Indeed. Of course i am not intending to touch it but tryin to understand so i can challenge the third plumber to look at this !

Anyone near london SE16 who is qualified and would like to see it ?
 
I managed to locate a label on the side of the cylinder (stupidly attached so that you need a mirror to see it).

Cylinder 210 ltrs
Ev 24 litres
Recharge pressure 2.1 bar.

Consistent with what you guys have said.

The mystery is how can it be that 2 plumbers pass their g3 , charge 130 quid an hour and are not able to think of this. Recharging to 3.5 bar was evidently stupid. Also not noticing that the 3.5 bar could be the problem was stupid , suggesting instead another royal job to replace the pressure reducer when evidence was that cold water was 2 bar as should be.

I really do not know how they keep their business or evem worse not put anyone in danger. It is as if they weere doing this the first time. Other professions would lose their licences.

Very sad because this has caused mnths of worry and i now do not trust anyone.
 
Posting here as the unvented G3 forum does not let me post.

I am hoping someone could apply some logic and help me with a problem that two plumbers have not been able to diagnose properly.

Details :
Unvented system.

Incoming mains with prv set at 2 bar. Plumber has checked this at ktchen mains.

Cylinder has two immersions. I can hear both cut off and cpme on again so presumably no thermostat problem.

The cold water relief valve set to 3.5 bar is letting by ONLY when the immersion is on (either of them).

The G3 plumber who serviced in November found the expansion vessel pressure at 2.4 bar. He recharge to 3.5 because a sticker on it said 3.5 - presumably this is the pressure put in it in factory ?!

After he left, started letting by again - so next suspect was the valves.

I got other plumber to replace the erv and tp valves. Now still letting by cold water when immersion on. This plumber said it now can only be the incoming reducing valve that has failed.

To me both these are illogical.
Can someone explain why 3.5 bar is right pressure ? To me this is too high relative to the 2 bar incoming so that it is like not having an expansion vessel at all, so the ERV then opens.

Also if the main pressure reducing was faulty the valve would be letting by all the time because the whole system would then be above 3.5, cold or not.

I am at my wits' end because you pay good money and they seem to do the wrong thing and not be able to think through. The second plumber did not even have a pressure gauge wih him and did not say anything about the expansion vessel.

I now have new valves and am inclined to just reduce the expansion vessel pressure closer to 2 bar which i bet should solve it.

Any advice please ?


Many thanks
John

You have a 210 Litre Water Cylinder, if its heated to 60C, giving it 10C deviation up to 70C then the expansion at this temperature is 2.18% so your expansion vessel must accommodate 4.6 litres of water without lifting the relief valve.
Assuming a pre pressure of 2 bar (the correct one) and a mains pressure at 2 bar here are the final pressures for a few different sized E.vessels
15 Litre: 3.3 bar (too near the limit)
20 Litre: 2.9 bar.
25 Litre : 2.7 bar.
30 Litre: 2.5 bar.
You are happy that the mains pressure isn't creeping up and I assume that the hot water temperature isn't exceeding 60/70C?

As you are probably well aware its vital that the pre pressure is not greater than the mains pressure of 2 bar, when checking it make sure that there is no pressure on the water side.
 
Thank you for this.

What you say is consistent with the spec label.

At correct pressure charge the 24 litr i have is adequate.

Thermostat has only gradings with no temp indication but it is a 3 kw and says not to exceed 65 which is then near maximum presumably. Measured at hot tap it is around 45 deg with a fever thermometer . This includes losses in the pipe and inaccuracies . So 60-70 in tank is about right.


The mistake is clearly that the plumber charged it up to 3.5 so of course leaves no space fo expansion. Prior to servicing it was 2. 8, still too high. I am stumped why instead of putting to 2 he raised to 3.5.

I believe he did shut off water and open a tap when charging up oherwise the pump would not budge I imagine.

Bottom line is :
get someone else to
Confirm mains is 2
Shut water and open tap
Bring down recharge pressure to 2.1
Refill and reheat.
I doubt new relief valve is faulty. Likely the other was not faulty either !

I am 400 quid poorer and not fixed yet. At least i deserve a g3 more than these cowboys surely ...
 
Thank you for this.

What you say is consistent with the spec label.

At correct pressure charge the 24 litr i have is adequate.

Thermostat has only gradings with no temp indication but it is a 3 kw and says not to exceed 65 which is then near maximum presumably. Measured at hot tap it is around 45 deg with a fever thermometer . This includes losses in the pipe and inaccuracies . So 60-70 in tank is about right.


The mistake is clearly that the plumber charged it up to 3.5 so of course leaves no space fo expansion. Prior to servicing it was 2. 8, still too high. I am stumped why instead of putting to 2 he raised to 3.5.

I believe he did shut off water and open a tap when charging up oherwise the pump would not budge I imagine.

Bottom line is :
get someone else to
Confirm mains is 2
Shut water and open tap
Bring down recharge pressure to 2.1
Refill and reheat.
I doubt new relief valve is faulty. Likely the other was not faulty either !

I am 400 quid poorer and not fixed yet. At least i deserve a g3 more than these cowboys surely .

Yes, the 24 Litre should give a final pressure of 2.71 bar @ 70C, a pre pressure of 2.6 bar would give a final pressure of 3.45 bar (borderline) and the 2.8 bar pre pressure that you mentioned above would give a final pressure of 3.7 bar but of course the relief valve would have lifted prior to this.
 
Many thanks.

These are exactly the parameters i need to know and expect a g3 would be able to know

It explains why even 2.8 would blow the relief valve.

Perplexing how someone can pass g3 and not realize this immediately.
 
The relief valve pressure is often quite a lot higher than normal operating pressure of stainless steel unvented units.
Very typically the relief valve can be 6 bar for example and T&P at around 9 bar (stainless steel units).
The expansion vessel is not a prevention of danger. Cylinders often have a failed vessel and work without it.
The T&P valve and relief, cylinder motorised valve, and cylinder thermostats are the critical safety components.
 
This is an old copper tank.

The tp valve is 4.5 and relief valve is 3.5

On the plus side the 3.5 is well below what the tank can withstand.

What has become clear is the pressure in the ev is high enough to make it moot.

Perplexed how 3 ppl servicing his and changing valves over the years have not been able to logically explain half as much as i have learnt toay here.

I seriously doubt what this g3 means if ppl getting it then abdicate common sense and dont apply what they learn. They give a bad name to the good engineers who i know must be out there.
 
This is an old copper tank.

The tp valve is 4.5 and relief valve is 3.5

On the plus side the 3.5 is well below what the tank can withstand.

What has become clear is the pressure in the ev is high enough to make it moot.

Perplexed how 3 ppl servicing his and changing valves over the years have not been able to logically explain half as much as i have learnt toay here.

I seriously doubt what this g3 means if ppl getting it then abdicate common sense and dont apply what they learn. They give a bad name to the good engineers who i know must be out there.

I thought yours had to be a copper unit - with vessel pressure and relief valves low.

The unvented exam can be very basic.
A bit of knowledge would help though.
I rarely see an unvented cylinder that has been installed properly.
 
The relief valve pressure is often quite a lot higher than normal operating pressure of stainless steel unvented units.
Very typically the relief valve can be 6 bar for example and T&P at around 9 bar (stainless steel units).
The expansion vessel is not a prevention of danger. Cylinders often have a failed vessel and work without it.
The T&P valve and relief, cylinder motorised valve, and cylinder thermostats are the critical safety components.

Its the cold water relief valve that is set at 3.5 bar, the Tribune cylinder relief valve may well be set at 6 bar.
Re cylinders working with a failed EV, ie full of water due to a punctured diaphragm/bladder then because water is to all intents & purposes non compressible I would venture that it would lift any relief valve irrespective of its setting on a hot water cylinder.

Edit: Just saw post re pressure setting now.
 
Yes, ask anyone with a Megaflo (or similar cylinder with an internal baffle). The relief valve is the indicator to tell you it requires recharging.
 
Perplexed how 3 ppl servicing his and changing valves over the years have not been able to logically explain half as much as i have learnt today here.

There are two or threes things about G3.
  1. people who do it and mess with your system take a legal responsibility for your system as they are designated 'competent'.
  2. doing an exam (it's open book by the way) is only the start. Any good engineer applies their knowledge & intelligence as you have. They are harder to find...
  3. many people who attend these courses have not the first idea about the systems they are then legal to work on. The people teaching often have zero idea either. Their job is to get the 'bum on the seat' out the door and recommending. Welcome to education 21st century stylie!
I've been doing UVCs for many years. I knew immediately what the issue was and yet tonight I learned even more. Thank you John.G :cool:
 
There are two or threes things about G3.
  1. people who do it and mess with your system take a legal responsibility for your system as they are designated 'competent'.
  2. doing an exam (it's open book by the way) is only the start. Any good engineer applies their knowledge & intelligence as you have. They are harder to find...
  3. many people who attend these courses have not the first idea about the systems they are then legal to work on. The people teaching often have zero idea either. Their job is to get the 'bum on the seat' out the door and recommending. Welcome to education 21st century stylie!
I've been doing UVCs for many years. I knew immediately what the issue was and yet tonight I learned even more. Thank you John.G :cool:

Thanks, I have attached a extract from a S.Sheet calculation that I did 100? years ago, I think you and others may find it interesting. John
 

Attachments

  • Expansion Vessel Calculation.zip
    9.2 KB · Views: 27
Thanks All.
John.g, there are only two relief valves -one at 3.5 and the tprv at 4.5. I presume you mean the latter by 'tribune relief valve' , or perhaps you refer to newer systems that have additional third valve ?
 
Thanks All.
John.g, there are only two relief valves -one at 3.5 and the tprv at 4.5. I presume you mean the latter by 'tribune relief valve' , or perhaps you refer to newer systems that have additional third valve ?
Yes, I'm referring to the tprv.
 
Many thanks.

These are exactly the parameters i need to know and expect a g3 would be able to know

It explains why even 2.8 would blow the relief valve.

Perplexing how someone can pass g3 and not realize this immediately.


Thank you for this.

What you say is consistent with the spec label.

At correct pressure charge the 24 litr i have is adequate.

Thermostat has only gradings with no temp indication but it is a 3 kw and says not to exceed 65 which is then near maximum presumably. Measured at hot tap it is around 45 deg with a fever thermometer . This includes losses in the pipe and inaccuracies . So 60-70 in tank is about right.


The mistake is clearly that the plumber charged it up to 3.5 so of course leaves no space fo expansion. Prior to servicing it was 2. 8, still too high. I am stumped why instead of putting to 2 he raised to 3.5.

I believe he did shut off water and open a tap when charging up oherwise the pump would not budge I imagine.

Bottom line is :
get someone else to
Confirm mains is 2
Shut water and open tap
Bring down recharge pressure to 2.1
Refill and reheat.
I doubt new relief valve is faulty. Likely the other was not faulty either !

I am 400 quid poorer and not fixed yet. At least i deserve a g3 more than these cowboys surely .

Thank you for this.

What you say is consistent with the spec label.

At correct pressure charge the 24 litr i have is adequate.

Thermostat has only gradings with no temp indication but it is a 3 kw and says not to exceed 65 which is then near maximum presumably. Measured at hot tap it is around 45 deg with a fever thermometer . This includes losses in the pipe and inaccuracies . So 60-70 in tank is about right.


The mistake is clearly that the plumber charged it up to 3.5 so of course leaves no space fo expansion. Prior to servicing it was 2. 8, still too high. I am stumped why instead of putting to 2 he raised to 3.5.

I believe he did shut off water and open a tap when charging up oherwise the pump would not budge I imagine.

Bottom line is :
get someone else to
Confirm mains is 2
Shut water and open tap
Bring down recharge pressure to 2.1
Refill and reheat.
I doubt new relief valve is faulty. Likely the other was not faulty either !

I am 400 quid poorer and not fixed yet. At least i deserve a g3 more than these cowboys surely .

Assuming cold water relief valve still lifting/feathering and
ONLY until you get some competent person in and while I am always mindful of safety requirements etc I don,t think you would be contravening any safety regulations per se by doing the following, if not already done so....with no hot water usage and ensuring "whole" cylinder heating on, and near or at cut out temperature (both, or bottom immersion on) slowly release the E.vessel air pressure until the PRV (cold water) reseats. ( I am assuming no pressure gauge fitted anywhere)
 
Last edited:

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