I would leave d.00 at 6kw, the boiler will only go to whatever its set minimum output is anyhow, it might be a little lower than 12kw so let it at 6kw.
Do everything at 65C + 30 min anticycle time.
UFH flow at, 2X4 + 2X3, 14LPM is fine, leave as is.
Note d.40&d.41 while in anticycle and especially at the end of the anticycle time, note them again after refiring until (if you have the patience) recycle again and time taken.
So yesterday evening i tested the system at 6kw with the 4 x UFH circuits + 4 rads + Towel rails >> the boiler got to about 45c and then was going up and down so to me this suggest that the d.00 needs to be higher for all 4 rads and ufh ? I waited about 45minbefore just stopping the test.
Then i tested the above with 12kw and the boiler never hit the 65c flow but it kinda stayed around 50-55c constant flow and it never went into the anticycle mode, however it did drop to around 50c and just stop for like 30 seconds before firing again and it did this twice, during this time the Rads were warm i would say little less then the day before but rooms did get to their desired temp. in terms of time the boiler was running for about 1 hour 10min before i stopped the test.
With the above test i saw the flow and return temps were the same.
This morning i tested again with 12kw - this time it was 3 x UFH circuits and 2 Rads, with 65C flow and 30min anticycle timer >> I found that the boiler would get to about 68c and go into anticycle (pump is running) and then we it got to about 43C it stopped completely (pump is not running) with the timer sign flashing away and it would sit on this screen for about 4min before firing again.
With the above test i saw that when the system stopped complete for that 4min the flow was obv 43c and the return was 41c. Generally during the anticycle i noticed the return temp was always about 2-3c less the flow.
Generally i never had all my 4 Rads and UFH on together as usually only 2 rooms are used mainly.
UFH normally kicks in around 4am > mainly the kitchen circuits and 1 in living room and then the CH will come on around 6am to 7am for one bed room and then 6.30am to 7.30am for the second bedroom so between 6 and 7.30 is when the the output would be at its max
I am just happy to keep the boiler at 12kw and just see how it goes, i guess lowering may put strain on the boiler if and when the UFH and all 4 rads run together.
When someone says boiler should be running in condensing mode, when does this actually happen in a normal operation of a boiler> im still trying to get my head around this..