Thermister issue?

[Daviesmith]

Hey everyone,
Just got back from a few days away and noticed my radiators were only getting lukewarm. The weird thing is, there are no warning lights on, and everything seems to be functioning normally, but the boiler is acting strange.
It only fires up for about a minute, then stops heating for roughly 5 minutes before attempting to fire again. It's doing this cycle continuously.
A heating engineer colleague at work mentioned it sounds like a thermistor problem (and yes, I know it's weird, but I don't have an external thermostat).


Temporary Fix & Question
I did a reset on the boiler, and it seems to be working correctly again for now.
My question to you all is: Does this behavior—the very short firing cycle and lukewarm heat, followed by the reset fixing it—sound like a classic thermistor fault to you experts?


Context
* Last Service: 6 months ago.
* Boiler Longevity: My boiler ( baxi 105e) guy advises against changing the boiler until something major breaks. I assume changing the boiler must be a particularly troublesome job he wants to avoid!
Any input or shared experience would be great! Thanks
in advance!

 

[Jonny brown]

Just change the thermistor it costs £3 delivered from eBay. If I can remember right it's just a 13mm spanner to screw it off and replace it . If that doesn't work phone your gasman

 

[gas112]

my 105 is 23 years old now it is one of the most bullet proof combi`s out there and have loads on books around the 20 year old mark still going strong so he is giving you good advice.
Nope its not a troublesome job it can be changed for the latest baxi purely by using 5 sockets /same flue hole and minor alteration to prv then the condense pipe .
.
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When your boiler played up did the hot water also stay luke warm or did that heat up as it should

 

[Daviesmith]

No the hot water was completely normal

 

[gas112]

No the hot water was completely normal

ITS not the thermistor sensor then as the main sensor initially controls both .
.It will be a case of waiting to see if it does it again then investigate further

 

[Daviesmith]

The heating worked initially (when it went funny) but after an hour or so the radiators where much cooler than usual

 

[gas112]

The heating worked initially (when it went funny) but after an hour or so the radiators where much cooler than usual

usual reason for luke warm rads and cycling every few minutes is a slight blockage in a gauze filter in the return isolation valve it can also be caused with a sticking divertor valve .

 

[Daviesmith]

I've got the diverter valve internals kit from eBay but not sure I could replace everything correctly. Is it a hard job fitting a refurbished diverter valve?

 

[Daviesmith]

Update the issue happend again with the cool radiator and short cycling. I turned the dial up on the boiler and it's now working like normal again ( just needs the dial turning further than before)

 

[Daviesmith]

usual reason for luke warm rads and cycling every few minutes is a slight blockage in a gauze filter in the return isolation valve it can also be caused with a sticking divertor valve .

What would you suggest trying first? .
You were correct about changing the thermistor it didn't help

 

[Jonny brown]

The return valve filter is on the right hand side pipe with a blanking nut on it . There's a video on YouTube telling you how to remove it . Considering you'll need to drain the boiler down to clean the filter might be worth looking at the pump