What should be checked first if a furnace does not turn on?

When troubleshooting a furnace that does not turn on, the first step should be to check the power source and thermostat settings. This is critical because if the furnace is not receiving power, it simply cannot operate, regardless of the condition of other components. Ensuring the thermostat is set correctly, and that it's functioning properly is equally important, as a misconfigured thermostat can prevent the furnace from receiving the signal to turn on.

Verifying the power supply includes checking circuit breakers or fuses to make sure that power is flowing to the unit, while confirming that the thermostat is set to a temperature that would prompt the furnace to ignite. If either of these parameters is incorrect, the furnace will remain off despite the overall system being in good condition.

Other issues, such as leaks in the expansion tank or the condition of the furnace filter, are important but would usually be assessed after confirming that the basic operational aspects of the furnace are functional. The age of the furnace provides insight into its reliability and potential need for replacement or major repairs, but it does not impact its capacity to turn on immediately. Therefore, focusing initially on the power source and thermostat settings is the most logical and effective approach.