Final answer:
The ideal guest-use water temperature at the source should not exceed 120 degrees Fahrenheit to ensure safety and prevent scalding.
Step-by-step explanation:
Guest-use water temperatures should be set no higher than 120 degrees Fahrenheit at the source to prevent scalding and to ensure safety. This temperature setting is a standard recommendation as it is hot enough for domestic use without posing burn risks. It is important to consider that different temperature scales are used in varying contexts, with Fahrenheit (water freezes at 32 °F and boils at 212 °F), Celsius (water freezes at 0 °C and boils at 100 °C), and Kelvin (water freezes at 273 K and boils at 373 K).