Tech A says, "To ensure that a heating thermostat is in the closed position, a voltage check at 'R' and 'W' should read 24 volts." Tech B says, "To ensure that a heating thermostat is in the closed position, a voltage check at 'C' and 'W' should read 24 volts." Which is correct?

To verify the correct functioning of a heating thermostat in a typical low-voltage heating system, it is important to understand how voltage is measured across the terminals. Tech A's statement about checking the voltage between 'R' (the power terminal) and 'W' (the heating terminal) to find 24 volts reflects a common method to ensure that the thermostat is calling for heating. However, this measurement does not confirm that the thermostat is in a closed position; it merely indicates that the heating command is being sent.

On the other hand, Tech B's assertion to check for 24 volts between 'C' (the common terminal) and 'W' is the correct approach to ensure that the thermostat is indeed closed. When the thermostat closes, it completes the circuit, allowing voltage to be present between these two terminals. This confirms that the heating system is being activated and that the thermostat is functioning properly.

Thus, Tech B's explanation provides an accurate method for determining the operational status of a heating thermostat, which is why this statement is correct.