Which condition is most likely problematic if both gas furnaces demonstrate decreased burner flame size when operating together?

When both gas furnaces exhibit decreased burner flame size while operating simultaneously, the most likely issue is an inadequate gas supply pressure. This condition indicates that the gas supply cannot meet the combined demand of both furnaces, leading to a reduction in flame size as the furnaces attempt to operate under insufficient fuel pressure.

In typical systems, each appliance relies on a consistent and adequate gas pressure to maintain its designed operational efficiency. If the gas pressure is lower than required, it can result in reduced burner performance, which manifests as a smaller flame size. This situation may occur due to several reasons, such as undersized gas lines, blockages, or issues with the gas meter.

Normal operating behavior would not result in decreased flame sizes in gas-burning appliances; they are designed to function properly under optimal conditions. A malfunctioning thermostat typically controls the heating cycle rather than influencing gas pressure, while faulty ignition systems would result in difficulties igniting burners rather than a decrease in flame size once lit. Thus, identifying inadequate gas supply pressure is crucial for resolving the issue at hand.