How furnace pressure (draft) is normally controlled In fuel-fired furnaces, the internal pressure/draft is most commonly adjusted by which element in the flue/stack system?

Introduction / Context:
Maintaining a slight negative pressure (draft) inside furnaces prevents hot gases from escaping through doors and joints and ensures stable combustion air flow. Operators usually control this draft with simple, robust devices in the flue path.

Given Data / Assumptions:

• Conventional furnace with stack or duct to an I.D. fan or natural draft chimney.
• Objective: maintain target draft (e.g., a few mm water column negative) at the furnace.

Concept / Approach:
Draft is a function of total system resistance and the suction provided by the stack or I.D. fan. The most straightforward way to set furnace pressure is by modulating the outlet flow resistance using a damper located in the flue/stack. While variable-speed I.D. fans can also adjust draft, the standard and most common control element remains the damper.

Step-by-Step Solution:

Verification / Alternative check:
Operating procedures typically specify setting the stack damper to maintain draft readings on the furnace pressure gauge.

Why Other Options Are Wrong:

• Air or fuel pressure regulators: Primarily control combustion stoichiometry, not furnace draft directly.
• I.D. fan speed: Can control draft where VFDs are installed, but dampers are the conventional, more common method.
• Secondary air swirl vanes: Affect mixing, not overall furnace static pressure.

Common Pitfalls:
Over-adjusting burner air/fuel instead of the flue damper, leading to unstable flame and poor efficiency.

Final Answer:
Damper position in the flue/stack