Where should a voltage measurement be taken when the heating thermostat is in the closed position?

Taking a voltage measurement while the heating thermostat is in the closed position is crucial for diagnosing electrical systems in heating systems. When the thermostat is closed, it allows current to flow through the system, activating components such as the contactor or relay (referred to as the FR contacts in the question).

Measuring voltage across the FR coil indicates if the coil is receiving the correct voltage to energize and close the contacts. At the same time, measuring from the load side of the normally open FR contacts verifies that the power is being successfully delivered to the load, such as a furnace or heating element.

By measuring both across the FR coil and from the load side of the FR contacts, one can confirm that the whole pathway is functioning as intended. A measurement across the FR coil should show voltage indicating the coil is energized, while the voltage measured from the load side of the contacts should confirm that the load is receiving power when the thermostat is closed. Together, these measurements ensure that the entire control circuit and the load are operational. Hence, the choice indicating to measure both is accurate in diagnosing the system's functionality.