Unvented cylinder relief valve letting by

[John.g]

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.

 

[Bwani]

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 ...

 

[John.g]

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.

 

[Bwani]

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.

 

[Best]

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.

 

[Bwani]

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.

 

[Best]

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.

 

[John.g]

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.

 

[fezster]

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.

 

[Welder]

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