Alarm18 means that the supply air temperature is too high and then the heating element is on constantly and probably a relay is broken on the circuit board.
In the picture below you can see which relay controls the heating element and you can easily replace this yourself, just move the steel bracket that holds the relay to the side and lift out the relay. There are 2 similar relays on the other side of the board that you can use if nothing is connected to the red marked terminals.
If nothing is connected to the terminals, remove one of them and install it where the broken relay was.
