Boiler has a mind of its own

[Quidam]

I have a 6 year old Ariston Micro Genus thats decided to do its own thing...

BG have been round and replaced a thermistor, air pressure sensor and the PCB (twice) in the last few weeks to stop the boiler firing up and cutting out almost immediately, and in the regard they have been successful.

However, now it appears to have really lost it... The heating runs fine, and when the timeclock turns it off the burner cuts out as expected and the pump continues to run for a short while as it has always done.

However instead of finally switching off, the burner fires up again and the temperature LEDS all light up, then the yellow flue gas LED starts flashing. The pump continues to run and the radiator (without TRV) becomes so hot you can't touch it. There's also bubbling noise coming from within the unit. The only way to stop this is to unplug the thing at the mains socket!

Has anyone any experience of this sort of thing? I'd appreciate any suggestions as to the likely cause before BG get back out, I've e-mailed Ariston who say they think its the heating temperature sensor but I can't see why that would keep it running after the timeclock should have shut it down.

I know Aristons dont have a great name but aside from the last couple of months mine has been generally reliable.

Thanks guys

Q

 

[Billy Bob Bob]

it is the heating thermister,this is how they fail on the microgenus

 

[Quidam]

MArk,

Thanks for your reply, this will be the second thermistor in 6 weeks to fail... (although whether the original was faulty or was replaced as a shot in the dark is another question lol!)

 

[Billy Bob Bob]

MArk,

Thanks for your reply, this will be the second thermistor in 6 weeks to fail... (although whether the original was faulty or was replaced as a shot in the dark is another question lol!)

have seen this fault many times your boiler has 2 thermisters 1 for ch 1 for dhw the ch one will have failed sometimes if they leak they can do this

 

[Quidam]

I see! I never thought a bust thermistor would cause the boiler to effectively over-ride the timer clock...

Out of curiosity, is the thermistor on the "gas side" of the boiler or is it something I could swap myself?

 

[Billy Bob Bob]

no its not on the gas side but you will need to drain the system,its on the diverter valve just above the low water switch

 

[Quidam]

Think I'll leave it to someone who knows what they're doing lol!

Cheers Mark

no its not on the gas side but you will need to drain the system,its on the diverter valve just above the low water switch

Mark,

Have been giving this some thought... is there any way this part can be replaced without draining the system? The reason I ask is BG claimed to have replaced the CH thermistor 6 weeks ago but I know for a fact that the engineer never went near the drain valve outside.

I'm beginning to wonder if it was replaced at all!

Cheers