What type of current does flame rectification change from alternating to direct?

Flame rectification refers to the process that occurs in a burner management system, where the presence of a flame is detected through the electrical properties of the flame itself. When a flame is present, it allows a small amount of current to flow through, resulting in a change from alternating current (AC) to direct current (DC). This change occurs because the flame acts like a rectifier, enabling current to pass in one direction only, thus indicating that combustion is occurring as intended.

The other options, such as gas flow, thermal energy, and mechanical energy, do not pertain to this phenomenon. Gas flow refers to the movement of gas, thermal energy relates to heat transfer, and mechanical energy involves physical movement or work done, none of which involve the rectification process. Therefore, the change from alternating to direct current is specifically linked to the electrical current associated with flame detection in HVAC systems.