[RussWhite]

The HWH control board is very very sensitive to voltage. The big contactor that has to energize to run the pump just won't pickup unless it has nearly full voltage. For some reason the HWH controls have an unacceptably high voltage drop internally, so even though you might have a good 12VDC to the controls, not enough of it will get to he contactor.

This can be solved if you wish with an interposing relay between the controls and the contactor. I had to do this when my alternator took a holiday off. Now my pump easily runs even with the engine off.