Which specific Thelia 23?
What does your system consist of (vented, sealed, hot water cylinder, tanks, thermostats, motorised valves etc)?
Under what conditions does the problem arise (hot water on/off, heating on/off)?
Which possible causes have your three engineers suggested and which have they rejected?
It's a hateful combi from the late 90s based on an even earlier design, very few engs. will have any experience of these models.
Could be several things on that boiler but you'll struggle to find anyone knowledgable as they've more sense to look look at such a problematic
model especially when many parts are likely to be obsolete or very expensive.
Your boiler only has one pumpthe other said it was the "pump" but didn't specify which one
It is not the diverter valvethe other said it was the diverter valve.
Yep probably correct, your boiler has a thing called an air flow switch, this is basically a small aluminium flap the moves when the fan runs and if they get a build up of crap the boiler behaves like thisOne said it just needed a good service
indeed depending on the serial number there were different switches used, if I remember correctly there was one APS that was adjustable, had a small dial on it been a long time since I seen one thoughAFAIR this model has a fancy APS on it...but the 620s had the swinging paddle air sensing.
if you say where you are in the UK someone might be able to recommend someone for you, it wont be the fan, it might need cleaning, but as already said differnt models used different air movement sensors, not a huge deal to fix to be honestMore importantly where might I find a boiler engineer who understands this and can diagnose and repair this issue?!
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