Which of the following defines a heating system's supply temperatures?

The correct answer identifies that a heating system's supply temperatures are typically less than 250°F. In most commercial and residential heating systems, particularly those that utilize water or steam to transfer heat, supply temperatures generally do not exceed this threshold to ensure safety and efficiency. Operating at temperatures lower than this standard helps to minimize the risk of burns and system failures while also enhancing energy efficiency.

In design and operational guidelines, such as those provided by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), systems are engineered to work effectively within this temperature range. Maintaining lower supply temperatures can contribute to longer equipment life and reduced maintenance needs, as high temperatures can lead to rapid wear and tear on system components.

Other options suggest higher limits, but these extremes do not align with typical operational practices for heating systems, which favor safety and efficiency. Thus, the defining threshold for supply temperatures in heating systems is appropriately captured by the choice indicating temperatures lower than 250°F.