I would increase the flow temp to 75C then go to CP2 and write down the boiler deltaT and the pump flow,head & watts.
Then ditto for CP1, PP3 & PP2.
Remember though that if the head is too low then some rads will run colder than others and require balancing, for the moment don't bother with balancing.
I run my boiler at 75C with TRVs on 8 of the 10 rads and I've often seen 40C return temps, I have never balanced the rads, I run on PP3 which, with my system, gives ~ 12LPM @ 3.4M head. I would expect your system to run at
[email protected] on CP1, 8.5LPM@3M on CP2,
[email protected] on CP3. 9
[email protected] on PP3, &
[email protected] on PP2.
Q1. If you mean boiler heat recovery = efficiency then the lower the return temp the greater the efficiency due to enhanced condensing effect.
Q2. I have used a factor of X2 on any replacement rads to allow for weather compensation etc if my present non condensing boiler ever gives up the ghost.
Q3. A lot of boilers now use DHW priority where, when DHW is required, the boiler switches to HW heating and automatically increases the boiler SP temp to 80C which gives very fast heating times but little or no condensing benefit and then switches back to CH after the 30/45 mins or so required.
But if you are not too worried about the length of time required to heat the cylinder then you can install a balancing gate valve on the cylinder return and by throttling it in to say 1/4 turn open you can easily get 50C for a lot of the heating period even with a flow temp of 75C.
Q4. As above.
The effect of return temps on condensing benefit is clearly shown below, not any huge benefits at 50/55C but below 50C, not bad at all.
Happy testing.