Saunier Duval - cutting in and out

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Hi there - just joined today.

Does anyone have any idea what causes this? I have had three boiler engineers round and they all say different things...

Any guidance much appreciated!

 
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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?
 
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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.
 
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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?

One said it just needed a good service; the other said it was the "pump" but didn't specify which one; and the other said it was the diverter valve.

It does it a lot when the heating is turned on from cold but once the heating has been on for a while (a few mins) it settles down. It does it all the time when I draw hot water.
 
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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.

I agree with you - I was going to get rid of it pre-COVID along with a kitchen renovation but then everything got delayed. I just want a quick fix until I get rid of it in a couple of months - I just don't want it to pack up.

The heating works fine once it is going, it just cuts in and out (and the hot water doesn't get up to temp) when I draw hot water.
 
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I'm not familiar with the boiler but wonder if a faulty flow sensor (assuming it has one) 'dithering' could cause your problem?
 
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the other said it was the "pump" but didn't specify which one
Your boiler only has one pump
the other said it was the diverter valve.
It is not the diverter valve
One said it just needed a good service
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 this
 
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Thank you guys!


It definitely has something to do with air / suction etc - if I crack open the front cover slightly, as you can see from the embedded video clip - it sucks in time with the click clacking sound.

To confirm this is me applying a small amount of pressure with my finger to "open" the lid and the suction forces it closed.

Could it be the air fan next to the flue is not strong enough or the air flow switch has an issue?

More importantly where might I find a boiler engineer who understands this and can diagnose and repair this issue?!
 
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AFAIR this model has a fancy APS on it...but the 620s had the swinging paddle air sensing.
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 though
 
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More importantly where might I find a boiler engineer who understands this and can diagnose and repair this issue?!
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 honest
 

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