Biasi 24s overheat switch tripping

If, as usual, I want to check the heat transfer of the secondary HE, I run it for a minute at the 7-8 li/min and 43° to see what secondary temperature is reached. Thats the test where I dont expect to see more than about 60-65° C

This statement was like a bolt of lightening to which I reacted to write previous reply. In fact due to massive smokescreen, above statement did not make sense until I heard the clap of thunder, hence the remark in previous post for which I am sorry.

Thats the test where I dont expect to see more than about 60-65° C

To expect something and see it for real are two different things. So, if the stat was at max, flow rate, not reduced to adjust to a personal setting, but at max, burners pressures correct (hasten to add both max and min would have been varified during service), primary temperature would not be far off at 70 to your 60 to 65 degrees C. At maximum setting of HW stat and stipulated flowrate of water from the hot tap, main heat exchanger is producing maximum heat for the hot water heat exchanger to produce stipulated hot water rate at stipulated temperature rise AND modulation kicks in if needed. This I have observed time and again on different makes of boiler with different types of hot water heat exchangers on boilers that are brand new to decrepit old timers limping along.

If the primary temperatures were anything else, I would be looking for the cause. Boiler I serviced couple of weeks ago, at the end of the service was showing a primary temperature of 75 to 80 degrees C. Burner was modulating but temperature rise was 30 degrees C. a sign of secondary HE with poor heat transfer.
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