When are AM-14 tank heaters energized?

Just finished electrical installation for AM-14 for a customer, who purchased the dishwasher at an auction. It didn't come with a booster heater or gas heater. I've downloaded the operating instructions, but haven't found an answer to this question: the tank heaters are energized during the wash cycle (and maybe during the rinse cycle too--didn't check) but at no other time. Is this the normal operation? It would seem that if this is correct, then there wouldn't be much of an opportunity to raise the water temperature since the cycles are so short.
I would have expected the tank heaters to be energized as long as the tank water level was correct and the water temperature was below the setting. I'd be grateful for any thoughts, and thanks!...........Lee