How can a technician check the function of a thermostat?

A technician can check the function of a thermostat by testing the resistance between the W (heat) and C (common) terminals. This method is effective because it allows the technician to verify that the thermostat is correctly sending a signal to turn the heating system on. When the thermostat is set to heat and is functioning properly, there should be a measurable resistance indicating that the circuit is closed, allowing current to flow and activate the system. If there is no resistance or an open circuit, it suggests that the thermostat may be faulty or that there is an issue in its wiring.

Other methods like measuring the voltage across T and R can provide information, but they don't specifically confirm the thermostat's ability to send a signal when calling for heat as effectively as checking the resistance between W and C. Observing the system's heating cycles and checking the thermostat setting can indicate whether the system is operating correctly, but they do not directly test the internal operation and signaling function of the thermostat itself.