Ventilation-controlled fire

A ventilation-controlled fire is a fire in an enclosed space whose rate of burning is limited by the supply of air rather than by the amount of fuel that is burning. In such a fire the combustion process is governed by the available oxygen, or by the lack of it. It is the counterpart of a fuel-controlled fire, in which there is enough oxygen and the fire size depends on the rate at which the fuel releases flammable gases. A ventilation-controlled fire develops when the fire is large and the room has only limited openings for the inflow of air and the outflow of smoke.

In a ventilation-controlled fire, more fuel is pyrolysed than can be burned with the oxygen available in the room. The hot fire gases then contain almost no oxygen. The surplus fuel leaves the room with the gas flow and may burn outside, which produces the flames seen coming out of windows and doors. A fully developed fire after flashover is usually ventilation-controlled. Its heat release rate is then set mainly by the supply of fresh air through openings such as windows and doors. The book Introduction to Fire Safety Engineering notes that most compartment fires are ventilation-controlled.

Fuel control and ventilation control

Every fire in a room starts as a fuel-controlled fire. In the early stage the fire is small compared with the room, and there is plenty of oxygen. A fire in the open air is always fuel-controlled, because it always has enough oxygen. In the growth stage the fire is described by its own rate of energy release, with little influence from the compartment.

As the fire grows, the openings of the room begin to restrict the airflow. The fire then moves from overventilated to underventilated, or fuel-rich, burning. In a post-flashover fire, the rate of air flow into the room and therefore the peak rate of burning become limited. The limit is set by the size, shape and position of the openings, or by the rate of mechanical ventilation. Flashover usually brings the fire to its fully developed state, in which all fuel in the room is involved. Not all the fuel gases can then burn inside the room, because the air supply is limited.

The type of control can change during a fire. Immediately after flashover, small items and easily pyrolysed materials burn very quickly, which gives a strongly ventilation-controlled regime with long flames from the openings. Later the pyrolysis rate falls until conditions in the room are close to stoichiometric, and the flames from the openings become much shorter. At some stage the fire becomes fuel-controlled again. In the decay stage, the fire may also change from ventilation control to fuel control as the fuel is consumed.

Openings themselves change as well. Doors and windows may be opened, and a closed window may fail partly or completely during the growth phase. In the World Trade Center towers in 2001, the fires were generally ventilation limited: they burned and spread only as fast as windows broke. Experiments for the same investigation showed the effect of ventilation control directly. When three office workstations burned together in a compartment, the total mass loss was three times that of the earlier tests with a single workstation. The peak heat release rate, however, was only about 50 per cent higher, which reflected the ventilation-limited nature of the tests.

How the openings limit the fire

Once a hot layer fills most of a room, the inflow of air through a window or door depends mainly on the size of the opening. Above a gas temperature of about 150 °C, the flow hardly depends on temperature. It is then proportional to the area of the opening multiplied by the square root of its height. This product is known as the ventilation factor. Its first use to analyse post-flashover test data is attributed to Kawagoe.

The commonly used constant gives an air inflow of 0.5 kg/s for each unit of ventilation factor, while calculated values range from 0.40 to 0.61 depending on the discharge coefficient. For most fuels, burning with 1 kg of air releases about 3000 kJ of heat. The heat release rate that the inflowing air can support with complete combustion is called the stoichiometric heat release rate:

$${\dot{Q}}_{\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}}=3000\cdot 0.5A_0\sqrt{H_0}=1500A_0\sqrt{H_0}$$

Symbol	Description	Unit
${\dot{Q}}_{\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}}$	maximum heat release rate inside the room that the inflowing air can support	kW
$A_0$	area of the opening	m²
$H_0$	height of the opening	m
$A_0\sqrt{H_0}$	ventilation factor	m5/2

The relation applies to fires close to or after flashover in rooms with a door or window, when the smoke layer reaches down to near the floor. The International Fire Engineering Guidelines (2005) apply such relations with the total area of the vents and their effective height. In mechanically ventilated rooms, the air supply is set by the ventilation rate of the system instead.

As a worked example, the SFPE Handbook considers a room 7 m wide, 4 m deep and 2.43 m high with one open door of 0.76 m by 2.05 m and no other ventilation. The door area is 1.56 m², and the peak heat release rate of the fire is estimated at 4.5 MW. The task is to decide whether the fire is ventilation-controlled and how much carbon monoxide can reach other rooms.

$${\dot{Q}}_{\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}}=1500\times 1.56\times \sqrt{2.05}=1500\times 2.23=3350\mathrm{k}\mathrm{W}$$

The air inflow is 0.5 times 2.23, or 1.12 kg/s. The door can therefore support a fire of only about 3.3 MW inside the room. The estimated fire of 4.5 MW is larger than this, so the fire is ventilation-controlled. The handbook expresses the same result as an equivalence ratio of 1.3, meaning that 1.3 times as much fuel is burning as the incoming air can consume. The difference of about 1.2 MW represents fuel gases that cannot burn inside the room and leave it unburnt.

The same ventilation factor also sets a lower bound for flashover. A method by Babrauskas gives a minimum heat release rate for flashover of 600 times the ventilation factor. For the door in the example this is about 1.3 MW, well below the stoichiometric 3.3 MW. Early experiments with wood fires in rooms with small windows found that the fires burned at roughly stoichiometric rates. Empirical observations on such wood fires gave a burning rate of 0.09 kg/s per unit of ventilation factor.

Effect on the products of combustion

The degree of ventilation control is expressed by the equivalence ratio. It compares the fuel burning rate with the air supply, relative to the ratio needed for complete combustion. A fire is overventilated when the ratio is below 1 and underventilated when it is above 1.

Ventilation control changes the chemistry of the fire. Well-ventilated fires are generally of little concern for toxic gases, whereas underventilated burning produces high levels of products of incomplete combustion, in particular carbon monoxide. In hood experiments, carbon monoxide production was minimal below an equivalence ratio of 0.6. It increased above this value and levelled out above about 1.2 for most fuels. Hydrogen and unburned hydrocarbons also appeared as the ratio approached 1. In the worked example above, the carbon monoxide yield inside the room is 0.14 kg per kilogram of fuel burned.

Once the oxygen in the room is used up, incomplete combustion becomes more likely, and the yields of carbon monoxide and soot of hydrocarbon fuels rise strongly. Material data measured in well-ventilated conditions then no longer apply. Soot yields in common tables are pre-flashover values, measured with an excess of oxygen. In New Zealand, the Verification Method C/VM2 (second edition, 2025) therefore prescribes higher yields once a design fire becomes ventilation limited or reaches flashover. From that moment the soot yield is 0.14 kg/kg and the carbon monoxide yield 0.40 kg/kg.

The unburnt gases also create other hazards. Mixed with oxygen, they form a combustible mixture that needs only ignition energy to burn. If fresh air suddenly enters a closed room in which such gases have built up, for example when a door is opened, the gases can burn explosively in a backdraft.

Burning outside the room

In a ventilation-controlled fire, the excess fuel does not disappear. When the pyrolysis rate exceeds what the air supply can burn, unburnt volatiles are ejected from the room, and in some cases the inward airflow is reduced. A significant part of the heat is then released outside the room, and the heat release rate inside it falls. The maximum heat release rate inside the room occurs when the pyrolysis rate and the air supply are close to stoichiometric.

Flames outside the openings are a characteristic feature of a developed fire, because part of the pyrolysis gases burns outside the room. These hot, fuel-rich gases can also flow into adjacent rooms, mix with air and form a separate burning zone, known as external burning. Burning outside the room generally reduces the carbon monoxide and smoke carried to other parts of a building. In the worked example, the equivalence ratio of 1.3 is below the value of 1.6 at which external burning was first observed in tests. The carbon monoxide formed in the room is therefore assumed to flow on to other rooms.

Use in design and regulation

Fire engineering guidelines commonly calculate both limits and take the lower one. The International Fire Engineering Guidelines (2005) state that the heat release rate of a fully developed fire is the lesser of the value for ventilation control and the value for fuel control. For cellulosic fuels, they estimate the air inflow as 0.52 kg/s per unit of ventilation factor. The stoichiometric fuel-to-air ratio of about 5.7 is increased by a factor of about 1.3 under ventilation-limited conditions. With an effective heat of combustion of 18 MJ/kg, this gives a heat release rate of 2.16 MW per unit of ventilation factor. In Australia, the Fire Safety Verification Method data sheets (2020) use the same principle of the lesser of the two.

In New Zealand, C/VM2 (second edition, 2025) lets a design fire grow until flashover, until its peak heat release rate, until sprinkler activation or until it becomes ventilation limited. The ventilation limit is the heat release rate at which the predicted energy release first falls below the design fire because of a lack of oxygen. It is determined with a fire model. For ventilation-limited cases, the input is then ramped to 1.5 times this limit over 15 seconds. Growing the fire further to its tabulated peak would generate an excessive amount of unburned fuel in the model and could cause numerical problems.

Ventilation control also governs how long a fire lasts. The duration of a fully developed fire can be estimated as the fire load divided by the burning rate. A smaller ventilation factor gives a lower burning rate and a longer fire. By comparison, domestic furniture burning freely has a fire duration of about 20 minutes. Burning rate data published for single items reflect fuel-controlled burning. They may not apply where ventilation control limits the burning or radiation from the room increases it.

Modelling and limitations

Fire models handle ventilation control in simplified ways. In the zone model CFAST, the pyrolysis rate is assumed to stay the same when oxygen limits the fire, but only part of the fuel burns. The heat release rate is the lesser of the value set by the fuel supply and the value supported by the oxygen entrained into the fire. By default, burning stops when the oxygen mass fraction falls to 0.15.

The book Introduction to Fire Safety Engineering notes that the course of a ventilation-controlled fire is so complex that it can hardly be predicted in detail. Most models deal with it poorly. The user must therefore state what the model does when oxygen runs short. The options are to let all gases burn inside the room, to assume flames out of the openings so that the excess burns outside, or to stop combustion. Many zone models, especially those for large spaces, ignore a fire that later becomes ventilation-controlled, and this should be stated as a limitation. In computational fluid dynamics, modelling of ventilation-controlled fires is limited and model-dependent, and it should be reported as an uncertainty.

The simple relations have limits of their own. The Swedish method for compartment temperatures assumes that the energy release is below stoichiometric and ignores excess fuel burning outside. Most of its validation tests used wood cribs, which burn more slowly and produce less excess fuel than furnishings. The test series used to validate fire models for US nuclear plants covered equivalence ratios of only 0 to 0.6, all in the overventilated range. Underventilated fires therefore fall outside the range covered by that validation.

See also

• Compartment fire
• Burning rate
• Smoke explosion
• Oxygen depletion
• Stoichiometry
• External flaming

References

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