Unvented Hot Water G3 discharge

[philtheplumber]

I see daily now more and more installs approved by NHBC where the discharge goes to plastic stipulated by NHBC to be polypropylene and then to a standard plastic soil stack.
Sadly I have now seen on numerous occasions when the T&P reliefs at 90-95 degrees the Hepvo trap and polypropylene pipe are fine but the plastic stack has softened/missshaped and even broken.
NHBC and G3 really need to investigate this as the last one I went to was in a block of flats and the stacks had to be replaced.
Ludicrous in my opinion to change it from being Metal all the way to outside.

 

[ollski]

Pretty unlucky, I've never seen a t&p fail for temperature.

 

[philtheplumber]

I feel NHBC obviously have the same attitude, it shouldn't go but it's there in case it's needed. So therefore should do the job it's intended for.
I solely work on Hot water cylinders and for a cylinder company so unfortunately it does happen.

 

[Roger928]

I see daily now more and more installs approved by NHBC where the discharge goes to plastic stipulated by NHBC to be polypropylene and then to a standard plastic soil stack.
Sadly I have now seen on numerous occasions when the T&P reliefs at 90-95 degrees the Hepvo trap and polypropylene pipe are fine but the plastic stack has softened/missshaped and even broken.
NHBC and G3 really need to investigate this as the last one I went to was in a block of flats and the stacks had to be replaced.
Ludicrous in my opinion to change it from being Metal all the way to outside.

Did you identify the cause of the t&p discharging in these cases where the soil pipe is deformed?

 

[philtheplumber]

It turned out to be stats overheating on the immersions. So yes problem solved.
If this happened before 2008 then the discharge would of been metal and not an issue. As it's after and approved then sadly it's a problem that is going to happen more and more .
Stats fail I'm afraid and being side entry at bottom of cylinder the T&P generally goes before the overstat trips at 85ish

 

[Agile]

Can you clarify that?

The immersion has a separate o/h stat at about 85 C.

So why has that not limited the cyl temp to 85 C as expected?

Tony

 

[philtheplumber]

Because being side entry at the bottom of the cylinder the T&P is at the top.
Top of cylinder will get hotter because of Stratisication/heat rising.

 

[Agile]

Does that mean that the immersion o/h stat really should be at a lower temp?

So what is your maker doing about it?

 

[philtheplumber]

Let's not get away from the issue. The discharge should handle the T&P discharge.
The stats currently used as I can only guess the reason being that they are a stat and the overstat/safety is built in. The older immersions that were used and are still available with some manufacturers have a separate overheat button which only reads the temperature of the immersion boss and is not taking a reading within the cylinder itself.
That in my opinion is neither here nor there, the Discharge to a standard plastic stack going up so many floors will not take a constant running high temperature of water.
Rented property tends to be the worse as the cylinders don't tend to be serviced and can discharge for long periods before anything is done.

 

[Roger928]

It turned out to be stats overheating on the immersions

Don't they have to be wired through the cylinders stat and thermal cut out first? Isn't that the first line of defence?

 

[philtheplumber]

That's all part of the immersion.
The issue here is the temperature of water going to a standard plastic stack. It should take the discharge without an issue like metal pipework. In my experience it doesn't hold up in the few cases this has happened.
My point is it's not able to take a constant flow of 90 degrees water without softening and causing damage like a metal pipe so therefore should be stipulated that the stack should be of material that can take 110degrees like the polypropylene pipe.

 

[Steelmasons]

Can you clarify that?

The immersion has a separate o/h stat at about 85 C.

So why has that not limited the cyl temp to 85 C as expected?

Tony

Bottom immersion heater set to 60c will yield a temperature in excess of 90c @ TPR due to stratification.
Had this a few weeks ago when the boilers were shut down with immersion heaters being used as back up.
Bottom stat temp was lowered in order to compensate , had these calorifiers been fitted with De-Strat circulators then it wouldn't have been an issue (school)..
Your above post being a school boy error...

 

[Steelmasons]

Dual immersion heaters should have both stats wired in series.

 

[Dan Robinson]

Does that mean that the immersion o/h stat really should be at a lower temp?

So what is your maker doing about it?

Physics is a basted like that Tony.

They should be linked through a stat at the top of the cylinder.

I've said for years that G3 is a joke. Look at the fuss manufacturers make over two port valves and boilers, yet their factory fitted immersion are total warnk.

 

[Agile]

The older immersions that were used and are still available with some manufacturers have a separate overheat button which only reads the temperature of the immersion boss and is not taking a reading within the cylinder itself.

That is really the crux of the problem that the o/h device is reading the temp of the immersion boss and not the temp of the water leaving the element.

Even the rod type is not going to be reading the highest water temp leaving the element.