Which type of liquid or gas must not be used as a heat transfer fluid?

The choice of flammable materials as heat transfer fluids is critical from a safety and code compliance perspective. Using flammable liquids or gases poses significant risks of fire and explosion, especially in systems where high temperatures are present. The North Carolina Mechanical Code emphasizes safety and encourages the use of fluids that minimize hazards.

Non-flammable, neutral, and stable fluids are more desirable because they reduce the risk of combustion and help ensure safe operation of heating systems. Non-flammable fluids, like certain types of water or synthetic oils, do not ignite easily, which is essential for preventing accidents. Neutral fluids are chemically stable and do not react with other substances, which is important for maintaining the integrity of the system. Stable fluids are those that resist changes in pressure and temperature, thereby ensuring consistent performance and minimizing the risk of leaks or breakdowns.

In conclusion, the use of flammable materials as heat transfer fluids is prohibited due to inherent safety risks, making the choice of a non-flammable or stable fluid crucial for system safety and compliance with mechanical codes.