Vaillant ecoTec strange behaviour

[hellohi]

Got a Vaillant ecoTec behaving strangely and I, as an "enthusiastic DIYer", can't pinpoint the problem. Boiler installed at the tail end of 2019, so still within warranty, so I've got a Vaillant warranty callout pencilled in for Thursday if needed...

In short, boiler is taking 6-7 minutes to fire up fully when there's a call for heating from the thermostat.

Hot water is OK.
Boiler fires up ok in chimney sweep mode.

When I turn up the thermostat, the radiator picture shows up on the screen almost immediately and the boiler fires up. But only to about 20% of the full flame bar that's on the left of the screen. Live Monitoring saying it's in S.04 (burner ignited), but doesn't fully fire up.

It then sits like this for about 6-7 minutes and only then creeps up to a "full flame" over the course of about 30 seconds.

This happens every time, with timing being around the same.

Some other bits of info that might be helpful;

Target flow temp is 75C
DHW temperature is 60C
Comfort mode is off
Heating partial load is on Auto

I also tried swapping the thermostat for a new one (just in case the thermostat was playing up, but the issues are still there, unchanged.

Does this look like a warranty issue?

 

[Mottie]

I’m sure I read somewhere that the Auto partial heating load is not very effective. I had the same problem as you when I tried mine on Auto. Why not try setting it at a certain KW output?

 

[hellohi]

Thanks - will try setting that to maximum and will report back!

 

[hellohi]

Set it to the max (24) and the issue is still there.

It's not that the boiler isn't working, it's that it's not behaving correctly... This is a little annoying!