Gas Fitter Test 2026 – 400 Free Practice Questions to Pass the Exam

The correct choice is based on the functionality of the thermostat controlling the heating and cooling systems. A thermostat with a single mercury bulb that manages both a single stage heat and a single stage cool unit operates through a switch that can direct current flow between two paths. This is characteristic of a single pole double throw (SPDT) switch.

An SPDT switch is designed to have one input and can connect that input to one of two possible outputs. In the case of the thermostat, it connects the heating circuit at one setting and the cooling circuit at another, depending on the temperature settings and the operation of the mercury bulb. This functionality allows the thermostat to regulate the temperature by alternately activating either the heating or cooling system, which is crucial for maintaining comfort in a dwelling.

In contrast, other options like SPST, DPST, and DPDT do not suit the application described because they either handle fewer functions (like SPST) or involve a more complex setup with multiple poles and throws that are unnecessary for controlling a single heating and cooling stage.