Flexible Air Hose Disintegrated

I have a Gulfstream A-Class OV boiler which has been working well (unusually, apperently) since it was installed about 4 years ago, apart from a recent problem with the diverter valve sticking.

Recently it had become very noisy and when I popped the cover off to have a look I noticed that the large flexible rubber hose from the flue to the fan had completely disintegrated to the point that there was just the steel reinforcing and some fragments of rubber left.

Does anyone have any clues to what may have happened?

When the boiler is running (which it seems to do fine, just a bit noisily) it is now drawing air directly into the fan rather than through the balanced flue. Is it likely to be safe to continue using it pending repairs or should I just shut it down? I have a working CO monitor in the boiler cupboard and that is not registering any CO.

Any comments would be most welcome.

Thanks.

Get it fixed