Baxi 105e thermistor

[jime17]

Hi all,

Wondering if someone can help...

DHW on my ageing baxi 105e (not HE) is cycling.

Gets nice and hot, then after a few minutes the temp lights go out, the 70 degree fault light flashes once indicating a DHW temperature fault, the pump and burner (if it's lit at that particular time) stop momentarily and then everything restarts as per normal. However this causes a drop in DHW temp whilst everything gets back up and running which is annoying if you're in the shower. (Mixer type not thermostatic shower)

The pump sensing pin drops momentarily which confirms the pump shuts down.

CH appears fine.


My first port of call was going to be the DHW thermistor followed by the plate heat exchanger.

I don't want to disturb the plate heat exchanger during winter whilst ch is working ok as if I can't re-seal it that would be a problem.

Any ideas? Am I barking up the wrong tree?

I'm in the NW so water is soft with no limescale by the way.

Thanks folks

 

[CountryFan]

Test the thermistor using a multimeter

 

[jime17]

My guess is that it would go open circuit momentarily when it gets to a certain temperature, causing the boiler to think it's overheated and shut down for a second. Have I got that right?

 

[gas112]

if the 70 degree light flashes then yes its usually the dhw thermistor which sits in a wet pocket so turn the cold water off before removing it
Part number 5108265 for sensor 248069 for the washer that comes with it . But if you look on ebay using sensor part number some will come with the washer

 

[jime17]

Thanks. Yes it flashes just for a second nothing more as is it shuts down, then the 70 light stops flashing and the ignition sequence starts and the boiler fires up. Then as the temperature of the DHW rises, it reaches a certain point and it shuts off/restarts again

if the 70 degree light flashes then yes its usually the dhw thermistor which sits in a wet pocket so turn the cold water off before removing it
Part number 5108265 for sensor 248069 for the washer that comes with it . But if you look on ebay using sensor part number some will come with the washer

 

[gas112]

If it doesnt solve it then you are looking at a blocked plate dont buy a new one just remove it and fill the side that is black with brick acid /patio cleaner from wickes cost about 12 quid let it work for about 20 mins then flush it out pref under a garden tap then fill it again and leave for about ten mins before washing it out again .
The seals for the plate are usually 99.9% ok and dont need changing .

 

[jime17]

Is it the boiler circuit side of the plate that clogs or the DHW?

 

[gas112]

Is it the boiler circuit side of the plate that clogs or the DHW?

boiler side