Have a vaillant ecotec plus 824. The water gets hot then goes cold?

Have a vaillant ecotec plus 824. Real problems!!The water gets hot for about a minute then goes cold and stays cold?...... An engineer came yesterday to stabilise the pressure as needle was going into the red and to establish why the heating wasn't working. he found that the diverter valve was faulty and that there was water in the expansion vessel. having bled the system and installed a magnoclean filter - but not yet sorted diverter valve - tomorrow! we now have this new problem!! please any advice as I think the engineer is baffled! he reckons once valve is sorted all will be ok. Could magnoclean filter be somehow causing the water to cool? thanks in advance