URGENT Baxi 105 he fluctuating hot water

[Stevig2003]

Help required Baxi 105he. Boiler is 10 years old and has been a pain for a while now. Ch workes great no issues. Hot water however not so good. I need to turn the tap on quite fast to get the boiler to fire up. Once fired up because the water flow is so high it’s not very warm. I can slowly turn the tap down to get a decent temperature however it’s a fine line. If I slow the flow too far the boiler gets to temperature and cuts out. This is a pain in the shower as the water keeps fluctuating hot and cold
Had a new diaphragm valve fitted last year didn’t fix it and also had a new heat exchanger fitted this also did not fix it .
I understand the pressure switch tells the boiler when you get enough flow the pin
activates a micro switch activating the boiler but why can I not run my tap very slow and the boiler regulate the heat to give a constant temperature. I’m sure when new it worked fine.
is the boiler nod designed to maintain a constant temperature at all flow rate ?
Thanks

 

[oz-plumber]

I think those boilers have a 'transducer' which detects water flow.
It could be a blocked filter at the base of the transducer or a faulty transducer.
Best get someone in to change that part, if all the other parts have been changed.
Could also be too much gas on low fire at the burners.

Giving you too much info - get a boiler tech in

 

[Gary.thornton]

Theres a flow venturi on the pump block where to 2 thin pipes go across. They deteriorate and cause issues. I change lots of them, 23mm spanner needed to get at it. Just make sure you get it all out because it will be broken inside the housing.