The problem with using a diesel boiler is the control of the amount of heat it produces, it fires up and gives a massive amount of heat, when you let go of the trigger and stop the flow the burner might stop but there is still a massive amount of latent heat within the heater coil that still transmits into the solution. This causes massive heat spikes as you then pull the trigger again.
You need a bypass valve to divert the water back into the freshtank, so you set your machine at 350psi then the second uploader after the heater at 300psi so your machine pump pushes 50psi back into the water tank, so rather than water standing in the heater when you let go of the trigger it trickles through the heater so stop the massive build up of heat.
You also need to adjust the flow switch so it activates at a lower flow rate , as these machines normally operate at 10-15lt a minute