potterton puma 80E DHW problem

[polypro]

Hi all, When I turn a tap on, the boiler fires up ok for about 10 seconds and then turns off again, the water is then very very hot and flows slowly for a while, if I leave the tap on, it starts the sequence again, I have replaced the flow sensor, the hot water temp switch has resistance of 10k ohms at room temp. Central heating is working fine, if anyone has any ideas, thanks

 

[taydo]

could be a scaled up exchanger

 

[polypro]

aahh! how do I go about getting it de scaled?

 

[tariq]

it might sound obvious is the main pump working.sorry you said the central heating is fine . so my first idea does not hold .