How many heating systems should a single flue typically be used for?

A single flue is designed to safely vent the exhaust gases from one heating system at a time. Each heating system produces combustion gases that require proper venting to prevent backdraft or accumulation of harmful gases inside the living space. Using a single flue for more than one heating system can lead to the potential for dangerous conditions, such as the improper venting of gases due to the combined outputs of multiple systems, which may not be properly balanced or compatible. This emphasizes the importance of appropriate flue sizing and configuration to ensure the safe and effective operation of heating appliances. Therefore, the correct answer reflects that a single flue should typically be used for one heating system only.