Has this flue gone rusty because the installer used the wrong part?

[Hornsofalligin]

The rusty bit with a hole at the top looks quite old, feels flimsy, and is a single skin 100mm x 400mm. The boiler has a 600mm new pipe rising vertically, then a new 135 degree bend (although the paint has come off as you can see), then the rusty pipe, another 135 degree bend which looks very clean and finally into the flue liner, rising up the chimney.

When the Boiler was installed (5 year ago), the installer didn’t give me a completed commissioning checklist, but did leave manuals for the
Greenstar Danesmoor 18/25 (oil) boiler
Greenstar oilfit CF adapter kit
Greenstar oilfit flexi CF Flue (which I can see has been installed into the chimney)

I also see the installer screwed each pipe connection (which the manuals don't say they should).

One engineer has quoted almost £400 for the replacement parts plus almost £400 for fitting, but that seems really steep given Worcester sell 100mm * 500mm single skin flue for around £50, and it looks like a fairly straight forward swap.

Another engineer has said the wrong parts have been used and the rusty bit should have been double skinned.

Given the rusty bit quickly ends up in a single skin flue liner, can it be single skin, or must it be double?

I know the work needs doing by a professional, but i'm trying to educate myself first, so I don't pay £800 for what might be much cheaper job?

Should it be a single piece, or much more?

Thanks in advance.

 

[exedon2]

We don't/won't fit Worcester oil boilers but I always understood that the oilfit single wall system was all stainless steel?

 

[SimonG]

We don't/won't fit Worcester oil boilers but I always understood that the oilfit single wall system was all stainless steel?

That looks like oil fit twin. Insides plastic and outside is either steel or aluminium.
But the same, wont fit worcesters anymore.