Vaillant Ecotec plus 418 - why constant F23 fault?

20 Aug 2020
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United Kingdom
Vaillant Ecotec plus 418 - why constant F23 fault?

I had this boiler installed two and a half years ago. It was working fine until last Sunday when it all shut down, no HW or CH and displayed the F23 “System shut down Temperature spread too large” fault. Plumber came and replaced the pump. All OK except the main downstairs rad was cold. Plumber returned and flushed the radiator and rebalanced rads I think. He said the system was clean and there was no air. All fine for 24-hours then when the boiler came on this afternoon it only stayed on for 10 minutes, then everything went quiet and it is now displaying the same F23 fault code. Does anybody know why this has happened a second time, given the pump was replaced?
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Does the pump have a number of speed settings and if so which is it set to.
I have to run my own Vaillant on the highest setting in order to avoid similar problems.

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