What Is Flue Gas? A Practical Guide for HVAC Professionals

Flue gas is the mixture of gases produced when fuel is burned in appliances such as boilers, furnaces, or tankless water heaters. In heating appliances, these gases must be carefully vented through an exhaust or flue system to prevent buildup of carbon monoxide and other combustion by-products inside a building. 

Proper venting is essential not only for safety but also for the longevity and efficiency of heating appliances.

What Is Flue Gas? The Definition You Need

Flue gas is primarily composed of nitrogen, carbon dioxide, water vapor, and small amounts of carbon monoxide and nitrogen oxides. The flue or boiler exhaust pipe is the conduit that channels these gases safely outdoors. 

The term flue is often used interchangeably with vent, but technically a flue refers to the inner passage carrying exhaust, while the vent system includes all connected parts such as elbows, appliance adapters, and termination fittings.

Why Flue Gas Matters to Contractors and Manufacturers

When flue gases are not properly vented, they can cause: 

• Reduced appliance efficiency.
• Corrosion inside vent materials.
• Dangerous indoor air contamination.

In municipalities where relevant mechanical standards are enforced, manufacturers and installers must ensure systems meet UL 1738 listing requirements. In non-code markets, systems should be designed, installed, and maintained to ensure safe operation and venting performance in accordance with the appliance manual. 

Flue, Vent, and Exhaust - What’s the Difference?

While they’re related, these components serve distinct purposes: 

• Flue: The pipe or duct that carries combustion gases from the appliance.
• Exhaust vent: The complete system designed to transport and discharge flue gas outdoors.
• Boiler exhaust pipe: The system that carries combustion gases safely from the boiler outside.

Understanding these distinctions helps manufacturers design safe and efficient venting systems. 

Pressure Categories and Vent Performance

Different appliances operate under positive or negative pressure. The flue vent system must handle the corresponding stress and maintain an airtight seal to prevent gas leakage. 

Designing a Safe Appliance Exhaust Vent

Material and Listing Requirements

Materials used in heating appliance venting systems must meet applicable safety standards. In municipalities where relevant mechanical standards are enforced, this includes UL 1738 in the US and ULC S636 in Canada, which verify resistance to temperature, pressure, and corrosion. In non-code markets, materials should still be selected and installed to ensure safe, reliable operation in accordance with the appliance. 

Centrotherm’s InnoFlue® Lite is a lightweight, CE EN 14471–certified polypropylene vent system engineered for safe and reliable use with Category II and IV gas-fired appliances (U.S.) and Class II C appliances (Canada). While it does not include the flame retardants required for UL 1738 certification, InnoFlue® Lite is specifically designed to withstand flue gas exhaust temperatures up to 230°F (110°C) without risk of degradation, ensuring long-lasting performance and durability.

System Layout and Condensate Management

Exhaust runs should minimize bends and include proper condensate drains to protect the appliance. 

Sealing and Joint Integrity

Centrotherm’s InnoFlue® uses gasketed connections that provide a gas-tight seal, simplifying installation while reducing field errors.

Common Venting for Multiple Appliances

In multi-unit buildings, shared vent systems must be engineered for balanced pressure and certified for combined exhaust loads. 

Key Takeaways for Heating Appliance Professionals including HVAC specialists

Flue gas management is central to heating appliance safety and efficiency. By understanding how combustion gases behave and ensuring your exhaust systems comply with required codes and safety requirements, you can protect end users and enhance appliance performance.

Need help selecting the right flue gas system for your next project? Contact our team below. We also offer training for your team! 

Get in touch today.