Baxi 105e erratic heating

[Bink]

I would be grateful for some advice please.

I have a Baxi 105e (DHW and CH) installed approx 9 years ago, it has provided good fault free service over that time-period.

The boiler is installed correctly. The configuration has a boiler time-clock (set to continuously on) and a heatmiser programmable room stat which controls both time and temperature of the ctral heating, This set-up has worked correctly over the past 5 years (when the heatmiser was fitted).

The domestic hot water system continues to work correctly.

Recently (within the past month) the Central heating has begun to come on erratically and independently of the heatmiser, I have checked the system and even changed the heatmiser to ensure that the fault is not with the programmer (I am confident that this is now not the case). The heating comes on even though the programmer is not demanding heat.

Additionally, I have tried turning the boiler timer onto continuously off – no effect it continues to come on erratically, I’ve turned the setting knob from CH and DHW to DHW only and the system continues to come on erratically, and I’ve turned the CH temp knob to Low, and the heating still comes on erratically at odd times.....

Due to the above I am convinced that it is the boiler electronics or mechanicals that seem to have developed a fault?..........

Can anyone advise if this is something that they have seem before, and if so, an suggestions of the rectification?

Thanks in anticipation.

 

[Joe4542]

The most likely problem would be a sticking diverter valve causing the rads to heat whenever you're using hot water, hence why still getting heating when CH turned off.

Check left most pipe under boiler when running tap, if it heats up - there's your problem.

Next could be sticking/faulty flow switch assy.

Or lastly faulty temp sensor making boiler think it's too cold and triggering frost protection.

 

[jeff the gasman]

The boiler has a domestic hot water pre heat facility.
If the diverter valve is faulty the boiler will try to heat the water section but the whole heating system will be heated up at the same time. A new diverter valve will cure this.
Jeff

 

[DP]

Jeff, only true if this were Baxi 105e Instant.

OP get the boiler serviced- that should fix the problem

 

[Bink]

Thanks for the responses, from the symptoms (and suggestions) I think a thermistor failure is favourite, I'll start there.

 

[45yearsagasman]

Thanks for the responses, from the symptoms (and suggestions) I think a thermistor failure is favourite, I'll start there.

Have you done all the suggested checks? If the LH pipe is getting hot on dhw operation then a thermistor is not the cause.

 

[Bink]

Yes it was the CH thermistor..... £3.98 to replace, job done, even no draining down as the design provides a "dry" pocket for the item.
Thanks to you all
Regards

 

[Bink]

One last question..... What is the white contact paste inside the pocket? luckily I managed to scrape enough off the old thermistor to coat the new one and make reasonable contact....... is it a PTFE paste?
Thanks

 

[gigz]

Its thermal contact paste. Helps to transfer heat.