Gas Fitter Practice Test

When the high limit terminals read 110V during a no heat call, which component can be inferred as open?

1. Gas valve coil

2. Thermostat

3. High limit

4. Motor

The correct answer is: High limit

When the high limit terminals read 110V during a no heat call, this indicates that power is present at these terminals, suggesting that the high limit switch is not closing as it should. The high limit switch is designed to break the circuit when the temperature exceeds a specified limit, preventing overheating. If it is open and 110V is being detected, it suggests that the system is preventing further operation as a safety measure. In this scenario, the thermostat is typically responsible for signaling a call for heat; if it were open or malfunctioning, you would not expect the high limit terminals to receive power. The gas valve coil cannot be inferred to be open based solely on the high limit reading as it is dependent on multiple factors. The motor is also not directly indicated by the 110V reading at the high limit terminals since the motor's operation is generally independent of the state of the high limit switch in terms of voltage readings. Thus, the presence of 110V at the high limit terminals during a no heat situation leads to the conclusion that the high limit switch must be open as it is functioning as a protective mechanism to shut down the system when it detects conditions that could lead to unsafe operation.