A fired furnace or boiler has to move a large volume of air in and an even larger volume of hot flue gas out, and it does that with fans. A forced draft fan sits on the air side and blows fresh combustion air into the burners, while an induced draft fan sits on the gas side and draws the products of combustion up the stack. The two do opposite jobs and put the firebox under opposite pressures, and understanding that difference explains why some furnaces run pressurized, some run under suction, and larger units run with both fans working together. This page contrasts the FD fan and the ID fan, how they are coordinated, and how their run and airflow signals feed the burner management system.
Forced vs induced draft fans in one line: A forced draft (FD) fan pushes combustion air into the windbox and burners, putting the firebox under positive pressure, while an induced draft (ID) fan pulls flue gas out through the stack, putting the firebox under negative pressure or suction. A furnace may use only an FD fan (pressurized), only natural or induced draft (suction), or both fans coordinated in a balanced-draft arrangement. Both fans provide run status and airflow proofs that the burner management system requires before it will permit a purge and light-off.
The forced draft fan is the supply fan. It draws ambient air, raises its pressure, and delivers it into the windbox, the plenum that feeds each burner through registers and dampers. Because the FD fan is pushing air into a more or less closed box, it tends to pressurize the firebox above atmospheric, and a furnace that relies on the FD fan alone is called a pressurized or positive-pressure furnace. The advantage is a simpler, cheaper arrangement with only one fan handling clean, cool air, which is easier on the fan and its bearings than handling hot, dirty gas.
The induced draft fan is the exhaust fan. It sits downstream of the furnace, often after the convection section and any air heater or emissions equipment, and it pulls the flue gas through the whole path and up the stack. Because the ID fan is drawing gas out of the firebox, it holds the firebox below atmospheric pressure, so a furnace that relies on the ID fan alone runs under suction and is called a negative-pressure furnace. The ID fan handles hot, particle-laden gas, so it is a heavier, more expensive machine and is more exposed to erosion and fouling than the FD fan.
The pressure sign matters for leakage. In a pressurized furnace, any gap in the casing leaks hot flue gas and unburned fuel outward toward operators, which is a safety and housekeeping concern, so pressurized designs demand tight, well-maintained casings. In a suction furnace, the same gap leaks ambient air inward, which is safer for people but dilutes the flue gas, cools the process, wastes ID fan power on that infiltrated air, and can upset combustion analysis at the outlet. Neither leakage direction is free, and the choice of draft type is partly a choice about which leakage problem a plant would rather live with.
Large boilers and furnaces frequently run both fans together in what is called a balanced-draft arrangement. The forced draft fan supplies combustion air on the inlet and the induced draft fan removes flue gas on the outlet, and the two are coordinated so the firebox sits at a slightly negative pressure, typically just below atmospheric. Holding a small suction keeps the safety benefit of inward leakage, so hot gas does not blow out at casing seams, while the FD fan still provides the deliberate, measured airflow needed for clean combustion.
Coordinating the pair is a control problem because the two fans interact. The FD fan is usually driven by the demand for combustion air, which follows firing rate and the desired air-fuel ratio, so as firing increases the FD fan opens up to admit more air. The ID fan is then driven by furnace pressure, adjusting its own damper or speed to pull out exactly as much gas as the FD fan is pushing in, which keeps the firebox at its slight negative setpoint. In effect the FD fan sets the amount of air and the ID fan trims the pressure, and the two loops must be tuned so a change in one does not send the furnace pressure swinging.
That interaction is also why a balanced-draft unit needs protective logic beyond simple loops. If firing suddenly stops but the ID fan keeps pulling hard, the firebox pressure can collapse far more negative than intended and threaten the structure, so these units carry high-speed furnace-pressure protection that runs the ID fan back or trips it when pressure excursions get out of hand. The coordination is therefore not just about efficiency and clean burning, but about keeping the firebox pressure inside the mechanical limits the furnace was built to withstand.
Before any fuel reaches a burner, the burner management system has to prove that air is actually moving through the furnace, and the draft fans are central to that proof. The BMS checks that the required fans are running, usually through motor run status or a proven-running contact, and that they are producing real airflow rather than spinning with a closed damper or a slipped coupling. Airflow itself is proven with dedicated measurements such as a windbox-to-furnace differential, a duct flow element, or an air pressure switch, so that a running fan motor alone is never taken as proof that air is present.
Those proofs gate the purge. The furnace must be swept with a defined number of air changes before light-off to clear any accumulated fuel, and the BMS will only start and complete that purge while the fans are proven running and airflow is at or above the required purge rate. Losing a fan or losing airflow during the purge resets it, because a purge that was not actually moving air did not clear anything. This is a hard interlock precisely because the whole point of the purge is to remove an explosive accumulation before an ignition source appears.
The same signals become continuous firing permissives once the burners light. If a required draft fan trips or airflow falls below the safe minimum during normal operation, the BMS treats it as a loss of combustion air path and drives a master fuel trip, cutting fuel because fuel without adequate air is the recipe for both incomplete combustion and a later explosion. When a furnace is monitored remotely, a cloud SCADA platform such as Merobix carries fan run status, airflow, and furnace draft alongside the fuel and flame signals, so an operator away from the site can see whether an FD or ID fan trip was the first event in a shutdown and confirm that the air path was proven before the burners ever fired.
A forced draft fan pushes fresh combustion air into the windbox and burners on the inlet side, tending to pressurize the firebox, while an induced draft fan pulls hot flue gas out through the stack on the outlet side, tending to put the firebox under suction. The FD fan handles clean cool air and is lighter, and the ID fan handles hot dirty gas and is heavier. A furnace may use one or both, and when both run together they are coordinated to hold the firebox slightly negative.
A furnace driven by a forced draft fan alone runs positive because the fan pushes air in faster than gas leaves, so any casing gap leaks hot gas outward. A furnace driven by an induced or natural draft runs negative because gas is pulled out, so any gap leaks ambient air inward. Positive pressure gives a simpler single-fan design but demands tight casings for safety, while negative pressure is safer for people but dilutes and cools the flue gas through infiltration.
The burner management system will not permit a pre-ignition purge unless the required draft fans are proven running and airflow is at or above the purge rate, because the purge exists to sweep accumulated fuel out of the furnace and a purge without real airflow clears nothing. The same fan run and airflow signals become continuous firing permissives, so losing a fan or losing airflow during operation drives a master fuel trip.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.