If a tank fills once but will not refill until power is cycled, what is likely the issue?

The situation described, where a tank fills once but does not refill until the power is cycled, suggests that there is likely a malfunction within the system's float switch mechanism. The float switch is responsible for detecting the water level in the tank; when the water level drops to a certain point, it signals the system to refill. If the long probe circuit, which is part of the float switch, becomes shorted, the float switch may not be able to accurately detect the water level after it's initially filled. Consequently, the system fails to activate for refilling, requiring a power cycle to reset it.

This reasoning illustrates why the issue is tied to the long probe circuit rather than other components. A faulty power supply would generally lead to broader operational failures, affecting multiple functions of the equipment. A broken float switch might prevent the tank from filling at all, not just after the first fill. Lastly, a clogged intake valve would typically hinder the flow of water, resulting in an inability to fill the tank initially rather than the described behavior of filling once and stopping. Thus, the nature of the problem aligns well with a shorted long probe circuit in the system.