What kind of contact do most thermostats contain to control the heating system in a conditioned space?

Most thermostats contain thermally actuated contacts to control the heating system in a conditioned space. These types of contacts utilize the expansion or contraction of materials in response to temperature changes. When the temperature in the space reaches a predetermined set point, the thermal element within the thermostat moves, opening or closing the circuit that signals the heating system to turn on or off.

This method of control is effective because it allows for automatic and precise adjustments to the heating system based on the ambient temperature. Thermally actuated thermostats can use various mechanisms, such as bimetallic strips or gas-filled sensors, to respond to temperature changes smoothly and accurately maintain the desired indoor climate.

The other options represent different methods of actuation not typically used in standard thermostats. Electrically actuated devices rely on an external power source, which is less common in basic thermostats. Pneumatically actuated thermostats use compressed air systems, often seen in larger commercial heating setups rather than typical residential environments. Manually operated thermostats require user intervention to adjust settings, reducing the automation and efficiency of temperature control compared to their thermally actuated counterparts.