VIessmann 26KW WB1B boiler short cycling due to blocked heat exchanger. This is what happens when you install a new boiler but can't be bothered to flush the system.
I had this issue and found it was air in the system following a service, I also found it didn’t like the pump speed setting at 2, when set to 3 it stopped the short cycling leading me to think that the boiler is oversized.
