I don't think you need a UDF. This is my understanding to your case .. correct me if I am wrong. nYou have a small cylinder heater heating a larger tank.nYou have a heat generation inside the heater only.nAll the system starts from the room temperaturenYou want to turn the heat generation off, when the heater wall temperature reaches 90 CnnIf this is the case, you don't need UDFnIf the volume does not exist, this means you are not simulating solids ... Can you show the geometry and a screen shot of the boundary conditions?.n