Net draft-

CO at detector-

Air ratio λ-

Gas spilling-

Room-

Room O₂-

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Stove, Chimney Draft and Carbon Monoxide Lab Blender 5.2

A cabin heated by a simple steel stove (burzhuyka, буржуйка) with the flue pipe (димохід) through the wall. Watch the chimney draft pull air through the fire, see what a closed damper, a blocked flue, a fan or a sealed room do to the burn, and how carbon monoxide (чадний газ, CO) builds up or is diluted. Drag to orbit, wheel or pinch to zoom.

Safety first - this is a physics model, not a safety tool.

Fit working carbon monoxide alarms (EN 50291 / UL 2034) in rooms with a fuel-burning appliance and near sleeping areas; test them.
Never use a charcoal grill or brazier, a camping stove, a generator or an unflued heater indoors. Never close the flue damper while embers are still glowing.
Keep a fresh-air supply, and have chimneys and appliances installed, inspected and cleaned by a qualified person. Follow local authority and civil-protection guidance.
If an alarm sounds or you feel headache, dizziness or nausea: go outside into fresh air at once, call 112 (in Ukraine also 101 rescue service, 103 ambulance, or your local emergency number) and do not go back in until you are told it is safe. Ventilating is not a repair.

Scenarios

Stove and fuel

Appliance Fuel

Stove door open (reloading)

Chimney

Insulated (double-wall) pipe outside X-ray flue

Net chimney draft against the height of the outside riser, for the current fire, damper and weather (steady state, wall temperature solved at every point). Dashed: the 12 Pa that stove makers usually ask for; dot: now.

Room and ventilation

Tightness

Live state

Draft available-

Room underpressure-

Flue flow-

Gas speed in pipe-

Fuel burnt-

Fire power (input)-

Heat into the room-

Stove surface-

Gas leaving stove / chimney-

CO in flue gas (dry)-

CO2 / O2 in flue (dry)-

CO made / spilled-

Carbon to CO / soot-

Air change (fresh air)-

CO: stove | room | bedroom-

Room O2 / CO2-

Carbon monoxide in the three zones (ppm). Dashed lines: WHO 15-min guideline 86 ppm, WHO 1-h guideline 30 ppm, WHO 8-h guideline 8.6 ppm. Dotted: a 70 ppm alarm threshold.

Net draft (Pa, left) and flue gas mass flow (g/s, right). A negative flow means the chimney is running backwards.

Temperatures: room air, stove surface and gas leaving the chimney.

Where the fuel energy goes: into the room (green), up the chimney (blue) and left unburnt as CO and soot (red).

CO detector and exposure

Detector position

Detector reads-

Alarm-

Bars show the average at the detector over each window as a share of the WHO guideline (35 mg/m3 for 1 h = 30 ppm, 100 mg/m3 for 15 min = 86 ppm, 10 mg/m3 for 8 h = 8.6 ppm, 7 mg/m3 for 24 h = 6 ppm). The alarm follows the quickest response a UL 2034 alarm may have (average of 400 ppm for 4 min, 150 ppm for 10 min or 70 ppm for 60 min); real alarms are allowed to take up to 15 min, 50 min and 240 min respectively, and EN 50291 sets different points, so a real unit can be slower. CO mixes with room air (its density is 97 % of air), so distance and doors matter more than height: a detector near the stove reacts first, one in another room last. Place alarms as the maker says, in the same room as each appliance and in the sleeping area; nothing replaces a servicing check.

Experiment: how much ventilation is enough?

Question: if a stove leaks some of its carbon monoxide into the room, how many air changes per hour (ACH) keep the room under a guideline? The test takes the CO the current fire makes, lets the chosen share leak into the room, integrates the room with RK4 for many ACH values and finds the smallest ACH that stays below the limit, then compares it with the closed form ACH = G / (V C_limit).

Share of the stove's CO that leaks in Limit

Press the button.

Steady CO in the room against ACH: closed form (line) and RK4 time stepping (dots), the limit (dashed) and the ACH needed (vertical).

View

Independent numerical checks

The science

Why a chimney draws

Hot flue gas is lighter than the cold air outside. The two columns of different density weigh differently and the difference pushes gas up the pipe: Δp = g H (ρ_out - ρ_flue) with the ideal-gas density ρ = p M / (R T). The model walks along the pipe in 25 cm steps, cools the gas with T(s) = T_amb + (T - T_amb) exp(-U π D Δs / (ṁ c_p)) and adds up the weight difference for every vertical piece (against room air inside, against outside air above the wall). A 4 kW fire and a 3.5 m insulated riser give about 13 Pa, the value stove makers ask for; a horizontal run gives no draft. The draft is used up by the flow resistance Δp = ṁ² Σ K / (2 ρ A²) of the air inlet, fuel bed, damper, elbows, wall friction, soot blockage and the exit. Closing the damper makes K = (1/(0.7 φ) - 1)² huge, so the flow falls.

Combustion and carbon monoxide

The fuel is described by its ultimate analysis (C, H, O, ash, moisture). Per kg it needs n_O2 = n_C + n_H/4 - n_O/2 moles of oxygen, 6.0 kg of air for dry wood and 17.2 kg for methane. The air ratio is λ = O₂ supplied / O₂ needed. With enough air all carbon becomes CO₂. When oxygen is short, carbon can only go to CO (needs half the oxygen) or stay as soot, tar and unburnt hydrocarbons; the model gives hydrogen the first oxygen, then splits carbon between CO₂ and CO, and a third of the starved carbon leaves as CO, capped at 30 % of the fuel carbon as measured for smouldering stoves. Even with excess air, poor mixing leaves a little CO that rises steeply below λ ≈ 1.3: ε(λ) = ε₀ (1 + 6 exp(-(λ - 1)/0.35)) with ε₀ = 1.5 % for a simple stove, 0.4 % for a modern one and 0.04 % for a gas burner. This is a heuristic fitted to the size of measured emissions, not a flame-kinetics code. Wet fuel burns cooler and dirtier, heating value LHV = LHV_dry(1 - w) - 2.44 w (18.4 MJ/kg dry wood, about 14 MJ/kg at 20 % moisture). Unburnt CO (283 kJ/mol) and soot (394 kJ/mol) are subtracted from the heat.

What the stove really does with the heat

The hot gas gives heat to the steel body by the effectiveness-NTU law (NTU = UA_g / (ṁ c_p)); the body loses it to the room by natural convection plus radiation, and its temperature follows C_s dT_w/dt = Q_gas - Q_loss. The bare pipe in the room is a radiator too. The room warms by C dT/dt = Q - (UA + ρ c_p Q_air)(T - T_out), so every cubic metre of ventilation air costs heat: this is the trade-off that makes people close vents.

Why a closed damper, a sealed room or a fan are dangerous

Fire in a stove is not switched off by closing the air: a hot bed of embers keeps releasing flammable gases whatever the air supply (the model keeps 25 to 30 % of the set rate for as long as the bed is hot). If the chimney cannot carry the gas away, the surplus leaves through the stove door and air inlet into the room. The same happens when the room is so tight that a stove (and a kitchen extractor fan, which loses flow as the underpressure rises) lowers the room pressure by as much as the draft: Δp_net = Δp_draft + p_room falls to zero and the chimney runs backwards. Even a weak positive draft is not safe: wind gusts of a few pascals push the flow backwards part of the time. The model takes that as a normal distribution with spread 0.3 + 0.25 Δp_wind and lets 60 % of the gas roll out for the share of time the net draft is negative. Room pressure comes from a mass balance of leakage in, leakage out, fan and stove, solved by bisection.

Carbon monoxide in the room

CO is colourless and odourless and has almost the density of air. In a well-mixed volume dC/dt = G/V - ACH·C, so it grows as C(t) = (G/(V ACH))(1 - e^{-ACH t}) toward C_ss = G/(V ACH); the ventilation needed to hold a limit is ACH = G/(V C_limit). The page uses three connected zones (near the stove, the rest of the room, the bedroom behind the door) with an exchange 0.5 C_d A √(g' H) through the door gap or opening. Guidelines: WHO indoor air quality guidelines 2010 (100 mg/m³ 15 min, 35 mg/m³ 1 h, 10 mg/m³ 8 h, 7 mg/m³ 24 h; at 20 °C 1 ppm = 1.165 mg/m³) and the NIOSH limits (35 ppm 8-h average, 200 ppm ceiling, 1200 ppm immediately dangerous). Chronic and acute effects depend on the person; the model does not predict health outcomes.

Limits of this model

One cabin, one steel stove and a round pipe, calibrated by physical reasoning rather than by measurements of a particular appliance. Heat transfer coefficients, the CO partition and the spill rules are simplified; real chimneys have unsteady puffs, thermal stratification, temperature inversions, wind pressure on the roof, leaks at joints and the human factor. Products of incomplete combustion other than CO (soot, tar, nitrogen oxides, fine particles) are not modelled. No appliance, rule or setting in this page is a safety recommendation: follow the maker's instructions, a qualified installer and local regulations. The cabin is an original model; names of appliance types are generic.