Baxi 105e combi boiler; switches on by itself

twigtv

Anyone know what could be causing the above problem please?

The boiler fires up by itself randomly several times a day for around 15 mins then switches off again. This even happens with the timer and thermostat disconnected and in C/H or DHW mode.
The only lights lit on the control panel when the boiler comes on are the burner on & power on light. (No DHW or C/H lights)
This causes the radiators to warm up (even in summer) and must be adversly affecting my gas bill. Its been happening since last summer.
If I turn the power off & on again or turn a hot tap on and off again the boiler will shut down for a period until the same thing repeats again.

The boiler works perfectly in all other respects giving me a warm home and hot water when needed (but also when not needed:(). I have replaced the main pcb as I thought the frost sensor may have been the problem but there was no change.

I noticed that someone else had posted the same fault on another forum but no cure had been found. The boiler is around 8 yrs old so if no cure can be found it looks like a replacement boiler is on the cards.

I was going to drown my sorrows but unfortunately they've learnt to swim.

macman

My programmer switches on the boiler every day at mid-day to stop the pump sticking over the summer. But only for a few seconds-not 15 mins.
The frost 'stat should only activate if the flow temp drops to 5C.

twigtv

Finally sorted the problem of my boiler switching on by itself.

The C/H temperature sensor was the cause of the problem.
(Located on the vertical LH pipe - no need to drain anything to change it)

Firstly I swapped the 2 sensors over (C/H & DHW) expecting a different fault to happen if one of them was faulty. However, the boiler now behaved perfectly! As DHW sensor looked corroded I replaced it with a new one (after putting back the C/H one). Back to fault conditions! So then replaced the good looking C/H sensor and all now ok. Don't know what was up with it as comparing the resistance with the 2 good ones proved inconclusive.