Promax 28 boiling when hot water is used?????????

[Richie_77]

Hi all
Went to a promax yesterday and the thing doesn't seem to be modulating down on the hot water causing it to overheat and boil.
I'm guessing its the hot water thermistor that sits on the brass body in front of the plate heat ex has become dirty and needs cleaning or am i missing something????

 

[Ermintrude]

Is DHW hot? Or does it go cold? Does boiler over pressurise as it's boiling it's **** off ? If so it sounds more like the plate. Easy was to tell is if temp sky rockets when initially fired in DHW.

 

[arran197]

There's several things that could be the issue here. Firstly, the ntc could be relaying the wrong resistance to the pcb. Easy was to tell is, get your multimeter and test the resistance. When cold it should be about 12 kohms. Usually you get a fault code of E50 when the sensor is faulty, but I have had exactly this when the primary ntc was faulty. The temp was saying 8 degrees, but rads were very hot. This would cause the overheat on both ch and dhw though.


Another fault could be the plate heat exchanger, may be blocked or scaled causing boiler to overheat.

Thirdly, the diverter valve passing will cause the boiler to continually try to get the water hot enough. When the ch is off and cold, feel the flow pipe whilst running the dhw and see if it gets hot.

 

[Richie_77]

Well as it turns out plate to plate was blocked solid and the change over valve too bathed the plate to plate in hydrochloric acid for 15 mins cleaned out the change over and done very happy customer
thanks for all your comments