hot water starts then stops

[jmg1676]

hi , i have a hot water system with a flow sensor the pump is a wsp gold star pump which turns on and off ok with the sensor seems to work ok spins ok when tested gently with a small screwdriver ,when you turn on the hot tap you only get hot water 4 about 20 seconds then gos cold boilers working ok
??????? thanks

 

[Phil Stenning]

what boiler are we talking about here?

 

[jmg1676]

potterton prima

 

[wazzer]

probably a thermistor

 

[jmg1676]

where and how can i test the thermistor ? thanks

 

[wazzer]

You need a multimeter that reads ohms it will have different values at different temps you can buy them cheap of e bay worth a try before you start thinking pcb