What should be the next step if the thermostat on a mid-efficiency gas furnace calls for heat, the igniter glows but burners do not light?

When a thermostat signals that heat is needed and the igniter is glowing but the burners do not ignite, the most logical next step is to test the gas valve and manifold pressure. This action is critical because if the gas valve is malfunctioning or if there is insufficient gas pressure reaching the burners, they won't be able to ignite, even if the igniter is functioning correctly.

Testing the gas valve involves ensuring that gas is reaching the burners properly and is regulated at the correct pressure for combustion. If no gas is flowing or if there’s an issue with the pressure, the burners will not ignite. Resolving gas supply issues is essential for the furnace to function properly and safely.

While replacing or cleaning the igniter or checking connections could be valid steps in other scenarios, in this case, they would not address the primary concern of whether gas is reaching the burners. Prioritizing the verification of gas supply helps to efficiently troubleshoot and resolve the heating issue.