Boiler pressure rises to over 3 bar even when off

[captain_pugwash]

Hi
I have a Glow Worm 30SI boiler that has just started failing with over pressure. The filling loop is disconnected and I drained the boiler down from the drain valve to get the pressure back to 1 bar. I have it switched off at the control to not come on but to still show the pressure and it steadily rises back to 3 bar with no heat and no way of filling from the cold supply.

I have an Ariston unvented cylinder next to it that supplies us hot water and I can only imagine that somehow the internal heating pipe inside the cylinder has developed a leak and this leak is then using mains pressure to flow into the boiler??

This is my guess as I can't see any other way of the boiler pressure continually going back up with no heat. I turned the cold inlet stopcock off to stop any more mains pressure into the cylinder but the pressure to the boiler kept rising so I guess that just means that the existing pressure in the cylinder is still enough to flow into the boiler loop?

I have a call out with my home insurance but they won't arrive until tomorrow and my general experience of the expertise has been underwhelming.

Any advice on how to isolate the problem would be appreciated. I guess I could shut off the cold supply and then drain the cylinder??

Thanks

 

[Dan Robinson]

I can only imagine that somehow the internal heating pipe inside the cylinder has developed a leak

I won't say its impossible (especially as it is an Ariston), but I have never seen an unvented cylinder coil fail in this way.

I have a call out with my home insurance but they won't arrive until tomorrow and my general experience of the expertise has been underwhelming

Yep, they're usually crap. But, you may get a good one and it will be quicker than a decent local independent I would have imagined unless you get one that has had a cancellation.

 

[oilhead]

How quickly does the pressure rise? If you isolate the cylinder, open a hot tap and leave it, if it does not raise the pressure, then you have proved where your problem is.

 

[muggles]

I won't say its impossible (especially as it is an Ariston), but I have never seen an unvented cylinder coil fail in this way.

I have, and it was an Ariston too!

 

[Doggit]

Can you release some of the pressure back to 1 bar, and then close off the flow and return pipes under the boiler. If it rises, then it'll very likely be the expansion vessel.

 

[John D v2.0]

If it still rises with flow and return and the boiler off, it'll be the flow or return valves letting by plus whatever the existing fault.
As above most be a leak from dhw into the primary loop so isolate cold water stop cock and release the pressure in the HW taps. Then it should stay constant pressure or start to drop

 

[Dan Robinson]

Can you release some of the pressure back to 1 bar, and then close off the flow and return pipes under the boiler. If it rises, then it'll very likely be the expansion vessel.

Eh?

 

[Doggit]

Okay Dan, I'll take your suggestions, and quit this site

 

[Dan Robinson]

It's a system boiler, so once isolated, pressure can only stay the same or go down.

I'm not sure why you think the expansion vessel can cause an isolated boiler's pressure can be increased by an expansion vessel.

 

[Dan Robinson]

Another possiblity, that has caught more than a few out over the years, is a second filling loop on the system somewhere that has failed.

 

[John D v2.0]

Another possiblity, such has caught more than a few out over the years, is a second filling loop on the system somewhere that has failed.

Good one! Although in this case it would have to be filling off the dhw side of the prv, not impossible.

 

[captain_pugwash]

Many thanks for the suggestions - much appreciated. I will investigate to see if there is any other filling loop although I did go through the whole system with the guy who installed it and he only showed me the braided loop point and this has two shut offs and for belt and braces I have detached the braided pipe loop itself. It only takes about 5 mins to go from 1 bar to 3.2 bar and at this point I guess the boiler PRV is then venting out the excess water - so it is just constantly sitting at 3.2. I will turn off the stopcock for cold water into the cylinder and open a hot tap and see if that stops the pressure rising - at that point if the pressure doesn't rise it proves I need a new cylinder - I suppose 14 years is not so bad as it can't have any corrosion inhibitor in being potable water? Thanks to muggles for confirming that this has been seen before.

 

[captain_pugwash]

Okay so now had the issue investigated and fixed and it turned out to be a corroded heating loop inside the cylinder that was passing the potable water from the cylinder into the heating loop and thereby kept over-pressuring the boiler to 3 bar. To prove the diagnosis my plumber disconnected the flow and return loops connections, capped them off to let the boiler operate without the cylinder in the loop and the issue went away. When reconnected we could hear the water flowing from the cylinder and watch the boiler pressure gauge rise up from 1 bar to 3 bar in just a few minutes. A very expensive repair and on the as sods law would dictate just as the weather went Siberian. Hope this update helps someone else in future.

 

[Dan Robinson]

So, what Muggles and I were discussing earlier in the thread.

 

[Dan Robinson]

Thanks for giving us the feedback