It depends. Normally, hot water re-heat is between 6 and 9.30 am. If I have a shower then the cylinder temperature will drop but stay within the 5C temp range. If my wife has a bath, then the cylinder will normally re-heat a second time and pass 9.30 at 60C. Last night, I checked the HW temperature before going to bed and it was 48C. This morning it again took 46 minutes before the BDR clicked off. Your hypothesis about what I am seeing makes a lot of sense.

Interestingly, a bath was run this morning at about 9am and the cylinder didnít get up to 60C. It only reached 58C before re-heating ceased. I checked just before 9.30 am and the boiler flow temperature hadnít reached the TMax set temperature of 70C: I would say that the boiler had been re-heating for about 20 minutes. Clearly, under OT control, the boiler does more than just crank up to TMax.