Vaillant EcoTec Stops Firing Before Set Temperature

[FishBed]

Hi all,

Preface; checked and can't find this exact issue on here, but similar to this one where someone suggested 'plate heat exchanger needs cleaned /replaced'. Unlike them, however, the boiler was looked at pretty recently.

Setup:
-Vaillant Ecofit Pure 825
-Radiators all bled and flow adjusted
-Heating water temp set to 65C
-Power set to 13kW

Situation:
Turn up thermostat in the morning when I start work, boiler fires. Sometimes it reaches the set 65C once, but often it doesn't get there and instead fires to around 50C, then switches off and sits there for *ages* pumping water round that rapidly drops to around 40C. And it pumps... and pumps... and pumps... and then eventually fires up again back to 50C before repeating. The thermostat never reaches temp so it just keeps going forever. (I'm all but certain it's my imagination, but it seems to be less of an issue in the evening. Mentioned in case I'm not imagining it)
Losing my mind trying to find if there's some setting I could change to fix it. I've not long had a new radiator installed (2 months or so) and the plumber checked everything then and said it was fine. All help gratefully appreciated!

 

[Deleted member 267285]

Are you using Vaillant controls? I think there’s something in the setup that allows 50 degrees max using ebus.

 

[FishBed]

Hi, the thermostat isn't Vaillant, if that's what you mean (it's Honeywell). Could I see this on the boiler settings somewhere?

 

[Deleted member 281264]

So is it going into anticycling mode? where you see the timer flashing on the boiler and the raidator symbol stays on

try going into the installer level settings by pressing +- at the same times and putting in the code 17

honeywell is crap for vaillant, just turns the boiler on and off. 65c is too high anyway.

 

[FishBed]

Yeh that's the one: timer flashing, 40˚C just for ages and ages. Doing it again today. What do I do once I'm in the installer settings? I've dipped into it before.
I had wanted to turn down the flow temp but I'd rather sort out this mess first If I buy another controller will that likely fix it?