1 BTU is needed to heat one pound of water by 1 degree F. If cold water at 15 degree C is raised to 50 degrees C (converted to degrees F 63), Flowrate of 11l will weigh 24.2 lbs
63x24.2x60 gives 91476 BTU.
Your boiler is rated at 95500
Therefore, main heat exchanger puts out 95500 BTU. HW heat exchanger will soak up 95500 BTU.
If you reduce the flowrate then 95500 BTU out of main heat exchanger will not be used up by HWHE. Two things could happen. 1) hot water rises by not 35 degrees but much more. If heat starts to back up as main heat exchanger is producing excess heat, something needs to happen to exercise control. The burner gets downsized (called modulation) On some boilers it was step modulation (Full flame to low flame) on others like your boiler, I think it is full modulation (like cruise control that keeps track of what temperature you are asking). So, in boiler cruise control, burner size will result in heat output at main heat exchanger that is matched to what the HWHE can ‘sink’
To heat water is difficult. To heat radiators, boiler does not even need to get out of bed as it will be able to achieve that while ‘snoozing’ at lowly 12kw instead of massive 28kw
Water that leaves your boiler on its way to the radiators, enters the radiator and starts rising to the top and starts spreading out across the top. Cold water that is displaced, leaves through the opposite valve to go to the boiler to be heated. Cycle continues.
Radiator output is directly proportional to amount of water it needs- weight of water carries the heat>> larger radiator need more water, smaller less water. Pipe length and bore have to be matched to radiator size (need for right amount of water)
Radiators are fitted between two rails (flow and return). As one would expect, the radiator closest to the pump and smaller in size may get a deluge of water thus using up pump power needlessly leaving larger radiator some distance away insufficient flow.
Way I set the radiators/ balance them is by use of dual clamp thermometer. Traditionally (cast iron boiler) were set for 11 degree differential. Low water content like yours can be set up for anything between 11 and 20 degree C. Condensing boiler- better set for 20 differential
From above, you will appreciate, each of you radiators is balanced correctly, will show a temperature difference of, let us say 15 degrees C. If the new radiator pipe was undersized, then that radiator would be starved of heat but remaining rads would all heat correctly.
If you can get someone to stick a monometer on the gas valve burner pressure should swing between high 11.7 to low 2.5mbars. If the gas valve was not set correctly, of for some other reason the burner stayed at full flame, burner would be knocked out and boiler would be forced to cool down for 3 minutes (pump keeps running to dissipate heat)
Hope above is of some help.