A little more detail of the troubleshooting involved in diagnosing exactly what is wrong with your heater would be helpful. From your description of the problem I am assuming that you did not self-diagnose the problem with your heater, but were told the description of the problem by a technician that you called in to fix/repair your heater. To start with, the high limit switches are attached to the underside of the water header. It is highly unlikely that a water leak at the attachment to or from the water header itself would spray water that would cause a short to or directly effect either the PCB or Fenwal as they are both mounted higher and have a different orientation than the water header. The transformer secondary is fused to protect the transformer and 24 Volt circuit against excessive amp draw ( a short). The high limit switches appear to be self contained externally mounted-temperature operated switches that would require an active leak at the header and a break in the casing in order to short out. If there is an active leak at the water header, it should be addressed first so that any parts (if actually needed) do not fail in the same way as the previous parts. I do not have a lot of faith in the diagnosis as it was presented to you by the technician. It seems more likely that you may have an overtemp problem that has caused the high limits to fail from over-cycling as they are meant to be a safety control rather than an operating control. If you do not feel safe/comfortable troubleshooting yourself, I would get a second/third etc. opinion from a "qualified" company/technician (possibly NATE certified) before throwing parts at the heater in an attempt to repair. I will be happy to assist you more if you can give me any further information.
