We are told the boiler is kettling and that disturbs him.
Kettling can occur with no deposits in the HE but is much more prevalent if there are deposits in the waterways.
All the info we can acquire here is that the gas rate is correct and the CH flow rate probably correct.
If we decide there are deposits in the HE, then they could be lime scale or iron or copper oxides or a combination of all three.
DP says ignore the kettling. But the OP seems to want to prevent it.
Unfortunately, because of the odd mentality of many in the plumbing trades which seems to be learnt during training, there is a desire to run down and make fun of others. As a result, because there is no clear cut solution, no one else is making any positive suggestions.
Whatever I suggest, if it does not work, will result in a tirade of posts which are less than complimentary to me. If it did work they will all keep quiet and not give me any compliments. Such is the nature of this forum. It was even worse on the
Screwfix forum to the point it was virtually closed down.
Consequently, I am not wanting to present myself as a scapegoat so will not make any suggestions.
If I did, then it would be to use some Fernox DS3 with about 10-20 grams applied to the boiler with all rads isolated so they don't dilute the concentration.
If that produced a well discoloured solution after running the boiler on CH and DHW for about 10-15 min in total. Then I might drain and repeat the procedure.
Tony