This occurs when the heating is running and you turn on the hot water for a considerable tim..ie bath or shower... it is just an overdemand on the system...it doesn't really mean anything!!
Taffyplumb, April 2011
its not a fault code d2 means delay after ch has got to temp ,same as d1 comes on after hw tap has been used.but if you have no htg and d2 appears within 5 to 10 mins of putting it on ,it will be a sticking diverter valve
