A furnace has been operating well during mild winters but fails to provide continuous heat during very cold days. What is the MOST CORRECT diagnosis?

When a furnace operates well during mild winters but struggles to deliver consistent heat during extremely cold conditions, the most likely diagnosis is that the furnace was undersized in heating capacity.

Heating systems are designed to maintain comfortable temperatures within a home based on specific climatic conditions, including the coldest temperatures typically experienced in that area. If the furnace cannot keep up with the demands of heat during prolonged cold spells, it may indicate that the unit does not have sufficient capacity to handle the higher thermal load required in those conditions. This undersizing could result from inaccurate calculations during the initial installation or a system that is no longer adequate due to changes in the living space, such as increased insulation or additional windows.

On the other hand, while issues like a faulty thermostat or inadequate gas supply can certainly affect performance, they would likely result in erratic or inconsistent operation at all temperatures rather than a specific failure during cold weather. Therefore, deciding that the furnace was undersized aligns directly with the symptoms of insufficient heating during extreme cold.