Glow worm Flexicom - no fault codes, but burner not running with demand

[hartlandiy]

I have inherited a house with a Flexicom 30hx boiler (heat only). With a cold house and plenty of demand, it operates rather strangely, where it will run at full tilt for 30 seconds or so (up to ~80c flow) but then you can hear it stop, and it cools down for a long while, before then having a brief spike of burning again. There are never any F fault codes, so presumably it thinks it's fine, but it's taking all day to warm the house up (tepid rads) and no hot water for showers, so I don't think it's fine! When it's in that non-running mode, I can see from the diagnostics that it does see demand (d8 = 1) but state is 3 = ignition, rather than 4 = burner. And ionisation current = 1. What does this strange combination mean? I can present any other diagnostics or info as required of course. I can hear it, and assumed it was running in some low-burn mode, but maybe it's just the flue fan that I hear.

I should point out that it is quite capable of running at full tilt for many hours straight, as sometimes it does exactly this. After about 10 hours of poor running as described above, a few days ago, it then decided to run properly for hours.

Pump seems to be OK, though not 100% sure. Flow and return temps seems normal - e.g. 55/46 when it's idling as above, and 78/58 when running well.

Intermittent issues are a pain of course, but I really need to get it running reliably before properly cold weather sets in.

 

[cross thread]

Sounds like a circulation issue or the boiler is set on to high a output,

 

[hartlandiy]

Circulation issue is one of the things I'm contemplating, but then why would it run just fine a lot of the time? It seems as if runs badly in the morning, and reasonably well in the late afternoon/evening, though I haven't confirmed this.

If it was a circulation issue, would we expect it to be in "ignition" state for long periods? Would we expect a fault code?

Alternatives I have wondered about include a genuine ignition issue, such as low gas pressure, which is perhaps worse in the morning locally and then gets better in the afternoon (massive stretch of imagination, I know). Or could ambient temperature someone be affecting it, and it runs worse when it's cold (it's in the garage) for some reason? Or some faulty sensor or electrical issue?