It is a problem in the valve itself, but without being there and doing our own troubleshooting, we cannot tell you exactly what the problem is. The "apparent" problem seems to be that even when calling for just hot water, the flow is not adequate to cause the burner to turn on. This COULD be caused by reversed connections. IF this were the case, when the valve is trying to get "hot" water but the water is cold, it shuts down the "cold" side, which would actually be hot and just gets colder. I this is the situation, then it is usually just a case of reversing the control unit.