What substances should never mix in a confined space when installing a furnace?

In the context of furnace installation, mixing combustion air with supply and return air in a confined space can lead to dangerous situations. Combustion air is crucial for the efficient and safe operation of gas- or oil-fueled equipment as it is responsible for facilitating the combustion process. When this air mixes with supply and return air, it can compromise the furnace's ability to operate correctly, as it might receive insufficient air for combustion or create backdrafts that can pull harmful exhaust gases back into the living space. This can cause carbon monoxide buildup, creating serious health hazards for occupants.

Additionally, proper separation of combustion air from supply and return air helps ensure optimal combustion efficiency, which is essential for both performance and fuel economy. Thus, maintaining distinct air sources prevents inefficiencies and safety issues that might arise from improper mixing.

Understanding this principle underscores the importance of adhering to installation guidelines that prioritize safety and efficiency in heating systems, particularly in confined spaces.