Your calcs, above, are quite correct.
There are a few questions re the rated output of this boiler, the MIs that I looked at shows a gas rate of 3.46m3/hr ~ 38.06kW, IF the boiler is 36kw output then its efficiency is 36/38.06, 94.6% (a bit on the high side), if the output is 32.7kW then the efficiency is 32.7/38.06, 85.9% ( way too low expectation for a condensing boiler.
Then it states that the "36" will give a DHW flowrate of 14.5LPM at a dT of 35C which means a heat input/boiler output of 14.5*60*35/860, 35.4kW (close to a "36kW boiler), it also states that the lower value input (MIs & post #2) is 32.7kW, equals and if true then the DHW output is only 13.39LPM at a dT of 35C.
Your installation data shows a (measured? gas rate of 3.34m3/hr which is "only" 3.34/3.46, 96.53% of its design gas rate, so ~ 36.74kWh or a boiler output of 36.74*0.88, 32.33kW so one way or the other, if you want the maximum possible boiler output you need to get somebody to check/recalibrate the gas rate (as suggested above), if not then just accept that its max output is, just about 29.0kW.
The reason Mira specify a minimum DHW temp of 55C is probably because the mixed max shower output is ~ 45C and 10C is a recommended dT between the HW and the , mixed flow outlet emperature, this shouldn't, IMO, cause the problem you have, but what you can do while awaiting your spares is to set the DHW temp to 65C and then (assuming a max boiler output of 28kW) control the max shower flow to 11.47LPM or less which means a mixture of 7.31LPM at 65C + 4.16LPM at 10C to give 11.47LPM at 45C.
There are a few questions re the rated output of this boiler, the MIs that I looked at shows a gas rate of 3.46m3/hr ~ 38.06kW, IF the boiler is 36kw output then its efficiency is 36/38.06, 94.6% (a bit on the high side), if the output is 32.7kW then the efficiency is 32.7/38.06, 85.9% ( way too low expectation for a condensing boiler.
Then it states that the "36" will give a DHW flowrate of 14.5LPM at a dT of 35C which means a heat input/boiler output of 14.5*60*35/860, 35.4kW (close to a "36kW boiler), it also states that the lower value input (MIs & post #2) is 32.7kW, equals and if true then the DHW output is only 13.39LPM at a dT of 35C.
Your installation data shows a (measured? gas rate of 3.34m3/hr which is "only" 3.34/3.46, 96.53% of its design gas rate, so ~ 36.74kWh or a boiler output of 36.74*0.88, 32.33kW so one way or the other, if you want the maximum possible boiler output you need to get somebody to check/recalibrate the gas rate (as suggested above), if not then just accept that its max output is, just about 29.0kW.
The reason Mira specify a minimum DHW temp of 55C is probably because the mixed max shower output is ~ 45C and 10C is a recommended dT between the HW and the , mixed flow outlet emperature, this shouldn't, IMO, cause the problem you have, but what you can do while awaiting your spares is to set the DHW temp to 65C and then (assuming a max boiler output of 28kW) control the max shower flow to 11.47LPM or less which means a mixture of 7.31LPM at 65C + 4.16LPM at 10C to give 11.47LPM at 45C.
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