QIYUE FAN Selection Knowledge
Forced Draft Fan vs Induced Draft Fan: The Simple Difference
Direct answer: A forced draft fan supplies combustion air before the furnace. An induced draft fan pulls flue gas through the boiler and downstream exhaust system.
In a balanced-draft boiler, both fans may operate together, but they are not interchangeable. Their system position, handled medium, pressure-loss sources, temperature and material requirements are different.
Forced Draft · FD
Pushes Combustion Air In
Usually located before the furnace or burner. The FD fan handles ambient or preheated combustion air and overcomes air-side resistance.
Induced Draft · ID
Pulls Flue Gas Out
Usually located after the boiler or gas-cleaning equipment. The ID fan handles flue gas and downstream exhaust resistance.
FD Fan vs ID Fan Pressure Logic
Positive-Pressure Air Supply vs Negative-Pressure Flue-Gas Draft
In practical boiler terminology, the FD side is generally the positive-pressure combustion-air side, while the ID side is the negative-pressure flue-gas side. The final fan still has to be selected from the actual operating point and complete system resistance.
FD Fan Function
Combustion-Air Supply
Pushes air through the inlet system, damper, air preheater, burner or register and toward the combustion zone.
ID Fan Function
Flue-Gas Exhaust
Pulls gas through the boiler, dust collector, scrubber, ductwork, damper and chimney-side resistance.
Selection Principle
Pressure Is Not Enough
Airflow, pressure basis, temperature, density, dust, corrosion, material, drive and motor data must be reviewed together.
Forced Draft Fan vs Induced Draft Fan
What Is the Difference Between FD and ID Fans?
The main difference is the fan's position in the process air path. That position determines what the fan handles and which pressure losses and working conditions must be included in selection.
Forced Draft Fan · FD
Clean-Air / Preheated-Air Side
Usually installed before the furnace or boiler. It must overcome the resistance created by filters, dampers, air-preheaters, ducts, burners and combustion-air registers.
Induced Draft Fan · ID
Hot Flue-Gas / Exhaust Side
Usually installed after the boiler or treatment equipment. Temperature, dust, ash, moisture, abrasion, corrosion and downstream system resistance require greater attention.
FD / ID Comparison
Forced Draft and Induced Draft Fan Selection Comparison
Do not select a boiler fan from the words FD, ID or from motor power alone. Compare the complete operating condition before confirming the fan model, impeller, material, drive and motor.
| Selection Point |
Forced Draft Fan |
Induced Draft Fan |
| System Position |
Before furnace or combustion zone |
After boiler, collector or scrubber depending on process route |
| Handled Medium |
Clean or preheated combustion air |
Hot flue gas, dust, ash, moisture or corrosive gas |
| Pressure-Loss Sources |
Filter, damper, air preheater, supply duct and burner |
Boiler, collector, scrubber, duct, damper and chimney |
| Main Risks |
Insufficient combustion air or unstable burner operation |
Temperature, abrasion, corrosion or insufficient draft |
| Configuration Focus |
Airflow, pressure, preheat temperature, speed and control |
Gas volume, temperature, dust, material, wear and maintenance |
| Typical Duty |
Boiler forced draft, furnace air supply and burner support |
Boiler induced draft, flue-gas exhaust and dust-removal systems |
Balanced Draft Boiler Route
How FD and ID Fans Work Together
In a balanced-draft boiler, the FD fan supplies combustion air while the ID fan removes flue gas and helps maintain the required furnace draft. The two fans work in the same process but perform different duties.
04
Collector
Dust control
Important: Whether a boiler uses FD only, ID only or balanced draft, fan selection must be based on the real airflow, pressure basis and complete system resistance.
Selection Notes
What to Confirm Before Selecting an FD or ID Fan
The process position determines whether the duty is forced draft or induced draft. The final fan model is then determined by the operating point and working condition.
For FD Fans
Combustion-Air Side
Confirm combustion-air volume, burner resistance, duct resistance, air-preheater condition, air temperature, fan speed, motor data and control method.
For ID Fans
Flue-Gas Side
Confirm gas volume, continuous and peak temperature, dust or ash loading, moisture, corrosion, collector resistance and chimney loss.
For Both Types
Installation & Electrical
Confirm static or total pressure basis, gas density, rotation, outlet angle, drive arrangement, voltage, frequency, quantity and maintenance space.
QIYUE FAN Product Family
Choose the Product Family After Confirming the Duty
FD and ID describe operating duties. They do not replace full fan model selection. Confirm the process route and operating point first, then review the suitable product family.
Combustion Air
Boiler Forced Draft Fans
For clean-air or preheated-air supply on the forced-draft side.
View Boiler FD Fans →
Flue-Gas Exhaust
Boiler Induced Draft Fans
For hot flue-gas extraction and negative-pressure draft duties.
View Boiler ID Fans →
Boiler Fan Series
G/Y4-73 Boiler Fan
G4-73 and Y4-73 configurations serve different clean-air and flue-gas duties.
View G/Y4-73 →
Selection Data
What Information Should Be Sent for FD or ID Fan Selection?
For an accurate quotation, send the actual duty and installation information. For replacement projects, also send the existing fan drawing, nameplate and duct layout if available.
01
Duty
Airflow and static or total pressure.
02
Medium
Clean air, preheated air or flue gas.
03
Condition
Temperature, density, dust, ash, moisture and corrosion.
04
Installation
Rotation, outlet angle, drive, motor data and quantity.
Do not select by motor power first. Motor power is checked after airflow, pressure, speed, efficiency, gas density and the actual operating point are confirmed.
Continue by System Position
Review the Complete FD or ID System
Flue-Gas Side
Boiler Induced Draft System
Review boiler resistance, dust collector, scrubber, duct, damper, chimney and flue-gas condition on the ID side.
Frequently Asked Questions
FAQ About Forced Draft and Induced Draft Fans
Is a forced draft fan the same as an induced draft fan?
No. An FD fan supplies combustion air before the furnace, while an ID fan extracts flue gas after combustion. Their positions, media and pressure-loss conditions are different.
What is the function of a forced draft fan?
It supplies combustion air required by the burner or furnace and overcomes resistance in the upstream air-supply route.
What is the function of an induced draft fan?
It pulls flue gas through the boiler and downstream equipment, supports the required negative draft and moves gas toward the discharge route.
Which fan is associated with positive and negative pressure?
The FD side is generally the positive-pressure air-supply side and the ID side the negative-pressure flue-gas side. Exact pressure references depend on the system design.
Which fan usually handles more dust and temperature?
The induced draft fan usually requires more attention to temperature, ash, moisture, abrasion and corrosion because it operates downstream of combustion.
Can the same centrifugal fan series be used for FD and ID?
Some series can be configured for different duties, but the actual configuration must match airflow, pressure, temperature, medium, material, speed, drive and installation direction.
What pressure losses should be included for an ID fan?
Include the boiler or furnace, dust collector, scrubber, ducts, elbows, dampers, chimney and required discharge condition.
Is induced draft vs forced draft the same comparison?
Yes. The wording is reversed, but the comparison is still between downstream flue-gas exhaust duty and upstream combustion-air supply duty.
Boiler Fan Selection Support
Need Help Selecting a Forced Draft or Induced Draft Fan?
Send the application, airflow, static or total pressure, temperature, medium condition, gas density, dust load, fan position, motor data, quantity and existing installation information.
