Why might electromechanical type thermostats need connections to both the hot and common sides of the transformer?

The need for connections to both the hot and common sides of the transformer in electromechanical type thermostats primarily relates to the powering of display lights. Many electromechanical thermostats include display lights that indicate the current status or set temperature. These lights require a complete circuit to operate, which can necessitate connections to both sides of the transformer—one side providing the hot power and the other acting as the return or common.

When powered from both sides, the thermostat can ensure that these display functionalities operate effectively, enhancing the user experience and providing necessary visual feedback. This setup also allows for proper voltage levels to be achieved, ensuring that components are adequately energized.

The other options, while potentially relevant in different contexts, do not directly address the fundamental need for display lights in electromechanical thermostats.