What is the ideal exhaust temperature for an atmospheric furnace? Key Insights for Efficiency

The temperature of furnace exhaust, also known as flue gas temperature, is influenced by several factors, including the type of furnace, its design, and operating conditions. For an atmospheric furnace equipped with a drafthood and properly set blower speed, the exhaust temperature typically ranges between 415°F and 525°F. This range corresponds to a temperature difference (ΔT) of about 75 to 85°F between the exhaust and the surrounding air. Achieving this range ensures efficient combustion and proper venting of exhaust gases.

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Key Points Explained:

1. Furnace Type and Design:

   • The reference specifically mentions an atmospheric furnace, which is a common type of residential heating system. These furnaces rely on natural draft to expel exhaust gases, and their design includes a drafthood to manage airflow and prevent backdrafting.
   • The presence of a blower ensures proper air circulation and combustion efficiency, which directly impacts the exhaust temperature.

2. Delta T (ΔT) and Its Importance:

   • Delta T (ΔT) refers to the temperature difference between the flue gas and the ambient air. For an atmospheric furnace, the ideal ΔT is 75 to 85°F.
   • Maintaining this ΔT ensures that the furnace operates efficiently, minimizing heat loss and preventing issues such as condensation in the flue.

3. Flue Gas Temperature Range:

   • The flue gas temperature for an atmospheric furnace typically falls within the range of 415°F to 525°F.
   • This range is consistent with proper combustion and venting conditions. If the temperature is too low, it may indicate incomplete combustion or excessive heat loss. If it is too high, it could suggest overfiring or insufficient airflow.

4. Factors Affecting Exhaust Temperature:

   • Blower Speed: Proper blower speed settings are critical for maintaining the correct ΔT and flue gas temperature. Incorrect settings can lead to inefficient combustion and improper venting.
   • Drafthood Design: The drafthood ensures that the furnace operates under the correct draft conditions, which helps maintain the desired exhaust temperature.
   • Combustion Efficiency: Efficient combustion produces exhaust gases within the specified temperature range. Poor combustion can result in lower or higher temperatures.

5. Implications for Equipment and Consumable Purchasers:

   • Energy Efficiency: Purchasers should ensure that the furnace is designed to maintain the correct ΔT and flue gas temperature, as this directly impacts energy efficiency and operating costs.
   • Safety and Compliance: Proper exhaust temperatures are essential for safe operation and compliance with building codes and standards.
   • Maintenance Considerations: Regular maintenance, including checking blower speed and drafthood operation, is necessary to sustain optimal exhaust temperatures and overall furnace performance.

By understanding these key points, equipment and consumable purchasers can make informed decisions about furnace selection, operation, and maintenance to ensure efficient and safe performance.

Summary Table:

Key Aspect	Details
Furnace Type	Atmospheric furnace with drafthood and blower for natural draft venting.
Flue Gas Temperature Range	415°F to 525°F
Delta T (ΔT)	75 to 85°F (difference between flue gas and ambient air).
Critical Factors	Blower speed, drafthood design, and combustion efficiency.
Importance	Ensures energy efficiency, safety, compliance, and optimal performance.

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