A diverter valve is a moving part that opens and closes to regulate the flow of heat. Since it is a moving part, it is bound to develop a fault at some point. Most diverter faults have to do with the valve sticking. Some of the indicators of a faulty diverter valve include lukewarm water from the taps and showers and having to leave the heating on. It is not advisable to try to fix this by yourself, instead hire the services of a qualified engineer who will run a fault check to see that all the controls are working properly as they should and to make sure that the problem is actually a faulty diverter valve. In some cases, the engineer might have to replace the entire diverter unit. In other cases, it might only be the diaphragm that needs to be replaced.