What should be measured at the furnace to check if the thermostat is functioning?

Measuring the resistance between the W (heat) and C (common) terminals at the furnace is a reliable way to check if the thermostat is functioning correctly. The W terminal connects to the heating control circuit, and when the thermostat calls for heat, it closes the circuit and creates a path for current to flow to the furnace. Testing the resistance between these terminals helps to confirm whether the thermostat is correctly signaling the furnace to turn on.

A low resistance reading indicates that the thermostat is successfully completing the circuit. If there is an open circuit or very high resistance, it suggests that the thermostat may be faulty and not allowing the furnace to receive the command to heat. This direct measurement at the furnace provides clear feedback on the operational status of the thermostat, making it an essential step in troubleshooting heating issues.

In contrast, measuring voltage at the compressor or the current draw of the fan motor pertains more to the cooling function of the HVAC system and does not directly reflect the operation of the thermostat in the heating cycle. Similarly, determining the temperature difference across the coils typically relates to performance assessment rather than the functional status of the thermostat itself. Thus, the approach of testing resistance between the W and C terminals is the most relevant method for diagnosing the thermostat's function regarding heating