On 25 Jan, 19:51, Fred wrote:
Hello,

Our oil fired boiler is normally hidden in a cupboard. I've removed
the cupboard to do some decorating and it's the first time I've seen
the boiler close-up.

Out of the top of the boiler comes the flue and at the join with the
boiler there seems to be a circle of dust. I think this might be
concrete that has crumbled.

Further up there is a *take the flue through the wall and this is
totally loose. There are signs of an odd flake of concrete but it
looks as though the majority has long gone.

I'm thinking of calling someone out to take a look at it. I suppose if
it were leaking we would have been poisoned by now and I have placed a
CO meter in the kitchen and it's showing a green light,s o hopefully I
am being paranoid; but better safe than sorry and all that.

Is there a flue within the flue? Could it be the internal one is ok?

There's also a removable (if you unscrew it) plate. I've left it
alone. What's that for and what's behind it?

Thanks in advance.

The white dust is fire cement which has been fired too soon after
installing rather than letting it dry slowly before giving it full
heat. Very common and hardly a fault if the flue is clear, it is under
negative pressure (draw) when running, same negative pressure prevents
fumes escaping from the entrance to the hole in the wall above if it
gives leakage to the innner part of the flue.
The plate is to open up for internal cleaning purposes in case you get
a bird or debris down the flue from above.
I suggest you get some Flue grade silicone seal and dress the joints
with it, removing any obviously loose material where necessary.
Alternatively get some fire cement and re-caulk the joints and ALLOW
TO DRY before firing. Oil makes soot in preference to Carbon Monoxide
unless you have a vapourising burner (the majority are pressure-jet on
current boilers)