How can I stop Rainwater ingress into my boilers horizontal vent which causes water to leak into my boiler?
I understand rain water ingress to be an issue with older vaillant flues. They have developed a new flue terminal however it is not compatible with pre-ErP boilers. Although the concentric flues pipes are similar Vaillant have changed the terminal design. In summary you would need to replace a section of your old flue with that of the new design.