ravenheat LS white star no hot water

[nicumuc]

I have a raven heat LS 80-LS80T-LS100-LS100T no hot water the heating is fine. When I turn the hot water tap on the boiler lights for a few seconds but goes of before the water gets hot and keeps turning on and of but the water remain could.
The d.h.w sensor and potentiometer have been replaced and also the shunt valve still no difference.
Could you tell me what else can be cause because I can get no other help from raven heat manuals.
please help!!!!

 

[gasmick2]

check out the plate heat exchanger it sounds like its blocked up with sludge and corrosion replace it or clean it out but it needs to come off

 

[SortedPlumbing]

had similar fault previous and it was the micro switch.

 

[nicumuc]

check out the plate heat exchanger it sounds like its blocked up with sludge and corrosion replace it or clean it out but it needs to come off

 

[nicumuc]

had similar fault previous and it was the micro switch.