Would you work on a boiler that has no data plate? I went to a faulty Vaillant combi today and I still have no idea what model number it was. There were no manufacturer's instructions either. On CH mode, the burner ignited on high rate for 5 seconds then went on very low rate at about 70c for a minute, then the burner went off for about 5 mins. It kept cycling like that continuously. The burner stayed on full rate for longer in HW mode but behaved similar. I've got a hunch it will be the pump but without any ID and a mulitude of pumps fitted to Vaillant, how do I know which one to risk buying?