Effect of air infiltration on furnace performance Considering real-world leakage around doors, joints, and flues, what is the typical impact of cold air infiltration on furnace operation and efficiency?

Introduction / Context:
(Note: “Air filtration” in many texts is a typographical variant for “air infiltration.”) Infiltration is the unwanted ingress of ambient air into the furnace. It alters combustion conditions and increases stack losses, undermining fuel efficiency.

Given Data / Assumptions:

• Cold outside air leaks into a hot furnace chamber or flue.
• We assess its effect on thermal efficiency and indicative symptoms.

Concept / Approach:
Infiltration introduces extra nitrogen and oxygen not accounted for in combustion control. This cools the combustion products and increases total flue-gas mass, thereby raising sensible heat carried to the stack per unit of useful heat delivered to the load. Net result: lower thermal efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Energy balances show higher flue-gas flow and lower average gas temperature in the furnace region; overall efficiency drops even if stack temperature may or may not increase depending on control settings.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing controlled excess air with uncontrolled infiltration; the former can be optimized, the latter should be minimized via sealing and pressure balance.

Final Answer:
reduces its thermal efficiency.