Lol it's just an illustration, I happened to be replacing a sensor on one while reading the thread.
With regard to what it's showing it's purely good combustion and clear flow through the hx same as every other boiler. No emissions test with the fire back on as the fires are not designed to burn efficiently necessarily.
Not putting down anyone who doesn't bother but it's been my way for 20 years and I can't see any customer benefit for not doing it.
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