My modena 80e would heat up the water when on summer setting but once the CH was on it would not give priority to DHW there would be the small reserve of of water then the taps would run cold and no response was made to the demand for Hot Water. I brought in a Corgi repairer who fitted part no 39805620 this afternoon, I have the CH on now and now the demand for DHW is met instantly. Hope that is of some help.
JM
April 2008