A good question, can you measure the flowrates separately with both taps fully open, then with say the hot only open to give you around say 3LPM with the boiler set to 55C, measure the flow temp at the tap, then open the cold tap to give 6LPM and retake the tap te.perature
Then it is definately a flow rate problem with the new tap simply set the temp at the boiler at what temp you want a shower and only open the hot tap and it will work fine , this is the problem with large output boilers, not the boilers problem, the manu tells you the flow rate it needs to work properly boilers boilers
Yes but you can do a workaround with what you have, it is just balancing what you actually want out of the shower / bath tap, the boiler has no clue what it is supplying, it just knows it needs to come on, if it cant get the heat away it will modulate down and sometimes go off, 5L/min from a 40KW boiler will never work, you can sort it by just adjusting the boiler
to prove I am correct, open the hot basin tap in the bathroom and leave it running and have a shower using your hot and cold supply and the temp will be constant, it is simply a flow rate problem you have, if you changed the shower for a much higher flow rate unit , you would get a great shower, the one you have is 10L/min, that is between hot and cold combined, so lets say 5L/min each , but your boiler is capable of 16.4 L/min @35 degree rise, up to you but as you dont have a thermostatic tap setup, I would be going for the highest flow rate possible, you could have a great shower
It doesn't mention a DHW thermistor (sensor) in the manual so possibly a derived DHW temperature even though its settable and may be more sensitive to low flow/high temperature, also, may be worth cleaning the DHW filter which should also be carried out with a boiler service.