Why is it not safe for me to do this

[Lower]

Just to be clear, i don't intend to do this and have already engaged a plumber to do this work. But i am interested as to if there are any real risk associated with it or is it just because it falls within the bounds of unvented cylinders.

I have a megaflow with an internal air gap. It loses its bubble about 3-6 months and i have to regenerate it. This always works perfectly, but it irritates me that i have to do it that much. I notice when it needs doing, not because i notice water flowing through the tundish, but rather because when you turn a hot tap on i get an initial pulse of water before the flow rate settles down. At some point the relief valve is going to get scaled up (we're in a hard water area) so i'd rather fix the problem once and for all.

Consequently, i'm having an external expansion vessel fitted. I'm reasonably competent when it comes to plumbing so the mechanics of sourcing and fitting an expansion vessel are well within my capabilities, however i know that i'm not allowed to as i don't have the appropriate ticket to work on unvented hot water systems.

However, for the application i have, is there any real risk? I would be adding an expansion vessel to a system that already has an expansion vessel so in reality, what is the risk?

 

[Nozzle]

I think technically it is fine, provided it is sized correctly, is unisolatable and the vent path is compliant.. However, I can't comment on the legalities/regulatory requirements of taking that option - and whether it effects your house insurance if it subsequently explodes.

Nozzle

 

[Lower]

That's a good point i hadn't considered. The design of the extra install needs to be right and i assume that being appropriately qualified means that you would have the knowledge to know the above.

 

[heatservice]

How would you fit an external EV, you cant isolate the internal one which is only a trapped bubble of air after all, its no big deal to regenerate it.

 

[Hot&Cold]

It loses its bubble about 3-6 months and i have to regenerate it.

Could be other defects on the installation . When was it last serviced ?

But i am interested as to if there are any real risk associated with it or is it just because it falls within the bounds of unvented cylinders.

Risk is significantly reduced when work is carried out by a competent person,to be classed as a G3 unvented qualified operative the operative has to convince an assessor they are competent by means of practical and theory assessments.
The cost to achieve G3 is around £250 which includes half day training and then the assessment (same day).Then you can do your own unvented repairs/servicing if you want to.

 

[Nozzle]

If it's just an air bubble without a membrane, then the gas will eventually disolve in to the water and the air bubble will always disapear, that's not an installation fault, just $hit design.

Nozzle

 

[Lower]

How would you fit an external EV, you cant isolate the internal one which is only a trapped bubble of air after all, its no big deal to regenerate it.

There is no need to isolate the internal one. The plumber is going to fit an external expansion vessel in the same way as he would on a unvented tank that doesn't have an internal EV. The internal expansion bubble will just deteriorate down to nothing and the external EV will provide the required expansion.

Regenerating the bubble isn't a big deal. However, it normally disappears over around 3 months and the relief valve will start venting before i notice the increase in pressure. The relief valve is getting pretty scaled up, and i don't want to start having problems with it.

 

[heatservice]

There is no need to isolate the internal one. The plumber is going to fit an external expansion vessel in the same way as he would on a unvented tank that doesn't have an internal EV. The internal expansion bubble will just deteriorate down to nothing and the external EV will provide the required expansion.

Regenerating the bubble isn't a big deal. However, it normally disappears over around 3 months and the relief valve will start venting before i notice the increase in pressure. The relief valve is getting pretty scaled up, and i don't want to start having problems with it.

Doe that mean your cylinder HW volumn will increase after all the air has been absorbed into the water?

 

[Dan Robinson]

Doe that mean your cylinder HW volumn will increase after all the air has been absorbed into the water?

An engineer qualified to work on these units will know the answer to that and what the ramifications are.

 

[heatservice]

An engineer qualified to work on these units will know the answer to that and what the ramifications are.

No doubt some smart a$$ with 1/2 a days training & a G3 ticket will condem it because the actual HW volumn does not match the volumn specified on the data plate

 

[Dan Robinson]

Then hopefully the OP will have engaged an engineer with an experienced arse as well as a smart one.

Although someone with that little experience probably wouldn't even think of the relevance.

 

[John D v2.0]

is unisolatable

...which would be an improvement on the current system where it randomly isolates itself every few months!

 

[Lower]

Doe that mean your cylinder HW volumn will increase after all the air has been absorbed into the water?

We've not discussed that. Is the relevance of that purely that a bigger expansion vessel is required to accommodate the expansion of a larger volume of water?