So I'm probably going to look daft when someone posts the answer, but... We have an IDEAL LOGIC 15 boiler, it's been in about 5 years, couple of things replaced on it so far. (We have a service contract, thank goodness). But what it did today is new on me. Went into the room where it's located, and there's a light on inside the flue, quite bright, and lighting up orange on a circular thingy on the bend of the flue. I texted our amazingly helpful plumber friend, and he said he wasn't sure what it might be - a 'trace heat' that only comes on in cold weather? (Really not sure what that would be anyway). I called the service contract people, and the person on the phone (to be fair only the person who books the engineers, not a technical person) didn't know either. And I've read the manual that came with the boiler cover to cover, and though the circular orange thingy is shown on the pictures, it's not labelled with anything (it's above the 'flue gas test point', I think), and none of the text says anything about a light, or a warning, or the flue... The boiler didn't seem to be having a problem and the heating's working.
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