it was water pressure... and the problem was that one of the black wires leading to the switch had a badly made connection with a standard twistcap connector. I was just checking all the connections after determining proper voltage here and there... and suddenly I had 28v on one side of this connector and zip on the other. fixed it... low pressure indicator goes out, heater works.
Occam's razor once again...
Occam's razor once again...
