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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.
Across Canada, all plastic venting systems used for gas appliances are required to be certified to ULC S636, as specified in Clause 8.9.6 of the CSA B149.1 Natural Gas and Propane Installation Code. This national code is adopted by every province and territory, sometimes with local modifications; however, the ULC S636 certification requirement applies nationwide. Enforcement of this standard is carried out by provincial safety authorities such as TSSA (Ontario), Technical Safety BC, and the Régie du bâtiment du Québec.
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.
How Flue Gas Behaves: Temperature, Pressure, and Condensation
Temperature and Condensate Formation
As gas cools while moving through the system, condensate forms. This moisture is slightly acidic and can corrode metal pipes over time. Centrotherm’s InnoFlue® Polypropylene venting systems resist these effects, offering long-lasting durability and simplified installation.
Products like AcidiRID are specifically engineered to neutralize acidic condensate from high efficiency heating appliances, protecting both the building and the environment. Its compact design and easily replaceable media bag make maintenance quick and hassle-free.
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 U.S. and ULC S636 in Canada, which verify resistance to temperature, pressure, and corrosion.
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.
Centrotherm’s Approach to Flue Gas Venting
Centrotherm’s flue gas venting products combine innovation, compliance, and installer convenience.
Safety and Compliance
Standard InnoFlue® is listed to ULC S636 standards in Canada.
Installation Efficiency
Lightweight materials and gasketed joints reduce labor time and callbacks.
Sustainability and Durability
Centrotherm’s polypropylene construction resists corrosion and extends system life, ensuring long-term performance backed by a 10-year limited warranty.
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.