boiler flue dilemma

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)