Visualise the "stack effect" that arises because warm air is light and cold air is heavy. Adjust the chimney height, the indoor-outdoor temperature difference and the opening size to see the stack pressure that lifts the air, the induced velocity and the natural ventilation flow update in real time.
Parameters
Height h (from neutral plane)
m
Effective height up to the chimney top (opening)
Indoor temperature T_in
°C
Air temperature inside the room or flue
Outdoor temperature T_out
°C
Air temperature outside the building
Opening area A
m²
Effective area of the supply/exhaust opening
Discharge coefficient C_d
Coefficient for the contraction loss at the opening
Atmospheric pressure P_atm
kPa
Atmospheric pressure used to evaluate air density
Results
—
Stack pressure ΔP (Pa)
—
Induced velocity V (m/s)
—
Ventilation flow Q (m³/h)
—
Indoor air density ρ_in (kg/m³)
—
Outdoor air density ρ_out (kg/m³)
—
Draft direction
—
Chimney section — neutral plane and airflow
The dashed line is the neutral plane (zero pressure difference). Warm indoor air leaves at the top while cold outside air enters at the bottom. The triangle on the right is the pressure-difference profile, linear with height.
Air density ρ from the ideal-gas law (P: atmospheric pressure in Pa, R = 287 J/(kg·K), T: absolute temperature in K). Stack pressure ΔP is the product of the density difference, gravity g and the effective height h.
Induced velocity V (C_d: discharge coefficient, ρ_avg: mean of indoor and outdoor density) and ventilation flow Q (A: opening area). ΔP > 0 means the warm indoor air rises and pulls cold outside air in at the bottom.
What is the Stack Effect (Chimney Draft) Simulator?
🙋
A wood-stove chimney pulls smoke up just fine even though nobody is blowing any air into it. How does that work?
🎓
That is the "stack effect". Put simply, warm air is light and cold air is heavy — that density difference is the cause. Inside the chimney the combustion gas is hot, so it is light; the cold air outside is heavy. A pressure difference builds up between the top and bottom of the chimney, and the light air inside is pushed steadily upward. Instead of a fan, the "temperature difference" is doing the job of a pump.
🙋
I see! So the taller the chimney, the better it pulls?
🎓
Exactly. The pressure difference is ΔP = (ρ_out − ρ_in)·g·h, proportional to the height h. Move the "Height h" slider on the left and the "ΔP vs height" chart below rises in a straight line. That is why old fireplaces had tall chimneys, and why in tall buildings the atriums and shafts that run tens of metres act, unintentionally, as a giant "chimney". The temperature difference matters too, so it pulls better on a cold day.
🙋
There's a dashed line labelled "neutral plane" in the diagram. What is that?
🎓
The neutral plane is the height where the indoor-outdoor pressure difference is exactly zero. When the inside is warm, above the neutral plane the indoor pressure is higher and air flows out, while below it the indoor pressure is lower and outside air is drawn in. The pressure difference grows linearly with the distance from the neutral plane — that is the triangular profile on the right. So upper floors leak air out more easily and lower floors take in more draughts.
🙋
Wait, it can reverse in summer? When I raise the outdoor-temperature slider, ΔP goes negative.
🎓
Good catch. The stack effect runs on the "temperature difference", so when the outside becomes hotter than the room, the outside air is now the lighter one. ΔP changes sign and you get a "downward draft" — cold air slides down the chimney, or smoke flows back out of the fireplace. An air-conditioned building in summer is the same: the flow runs opposite to winter. At the instant the outdoor temperature equals room temperature, ΔP is exactly zero. On the "ΔP vs outdoor temperature" chart below you can see the line cross zero right at Tout = Tin.
🙋
I've heard the stack effect causes trouble in tall buildings. What kind of problems exactly?
🎓
A common field complaint is that in winter the ground-floor entrance door is heavy and hard to open. The lower floors are the side drawing air in, so the door gets sucked inward. There are more serious issues too: warm air leaks steadily up to the top floors through elevator shafts, driving up heating costs, and in a fire smoke can spread to the top floor all at once. So in design the basic move is to break up the "vertically connected chimney-like space" with airtight layers, fire compartments and vestibules to suppress the stack effect.
Frequently Asked Questions
The pressure difference from the stack effect is dP = (rho_out - rho_in)*g*h, where rho_in and rho_out are the indoor and outdoor air densities, g is gravitational acceleration and h is the chimney height measured from the neutral plane. Air density comes from the ideal-gas law rho = P/(R*T) using the absolute temperature T and atmospheric pressure P. When the inside is warmer than outside, rho_in < rho_out and dP is positive, giving an upward draft. This tool shows dP together with the induced velocity and ventilation flow that follow from it.
The neutral pressure plane is the height at which the indoor-outdoor pressure difference is zero. With the stack effect, when the inside is warm, above the neutral plane the indoor pressure is higher than outside (air is pushed out), and below it the indoor pressure is lower (outside air is drawn in). The pressure difference grows linearly with the distance from the neutral plane. In this tool, h represents the height from the neutral plane up to the top of the chimney (or the opening).
The stack effect is driven by the air-density difference between inside and outside, so the smaller the temperature difference, the weaker the draft. In winter the inside is warm and the outside cold, giving a strong upward draft; in summer, as the outdoor temperature approaches room temperature, dP falls close to zero and the chimney pulls poorly. If the outside becomes warmer than the room, dP changes sign and a downward (reverse) draft can push smoke back into the room. Raise the outdoor temperature in this tool and you will see dP cross zero exactly at Tout = Tin.
In tall buildings the effective height that acts as the chimney height h is very large, so in winter the stack pressure dP can exceed several tens to over 100 Pa. This makes ground-floor entrance doors hard to open, increases leakage to upper floors through elevator shafts and raises the heating load, and lets smoke spread rapidly to upper floors in a fire. The basic countermeasure is to break up the continuous chimney-like space with airtight layers, fire compartments and vestibules so the stack effect is suppressed.
Real-World Applications
Heating-appliance chimneys and flues: Wood stoves, boilers and gas water heaters exhaust their combustion gas outdoors using the stack-effect draft, with no fan. The height, cross-section and insulation of the chimney are designed so that the draft pressure reliably draws in the air needed for combustion. If the flue cools down, the internal temperature difference shrinks and the draft weakens, so insulating the flue with a double-wall chimney is directly tied to a stable draft and to preventing condensation and soot build-up.
Natural ventilation design of buildings: Gymnasiums, atriums, greenhouses and factories combine a low supply opening with a high exhaust opening so the stack effect ventilates the space without any power. As in this tool, you estimate the ventilation flow from the opening area and the temperature difference, then size the openings and the height difference to meet the required air-change rate. In "hybrid ventilation" combined with wind-driven ventilation, the stack effect provides the base ventilation when there is no wind.
HVAC and airtightness design of tall buildings: In high-rise buildings the winter stack pressure is large and causes hard-to-open entrance doors, elevator doors that fail to close and an increased heating load from excessive leakage on upper floors. Design controls the stack effect by airtightening the envelope, compartmentalising each floor, using double-door vestibules and pressurising elevator shafts. Using this tool to grasp the order of magnitude of ΔP from height and temperature difference is the first step in deciding whether such measures are needed.
Fire and smoke control, and CAE analysis: In a fire, the stack effect carries smoke rapidly up to higher floors through stairwells and shafts. In life-safety and smoke-control studies, pressurised smoke control and compartmentation suppress this chimney-like flow. Detailed work uses airflow-network analysis or CFD, but pinning down the order of ΔP with a hand calculation like this tool gives a sanity check on whether the analysis results lie in a reasonable range.
Common Misconceptions and Pitfalls
A common misconception is that the stack effect is set by chimney height alone. ΔP is indeed proportional to the height h, but the indoor-outdoor temperature difference matters just as much. ΔP contains the density difference (ρ_out − ρ_in), which is roughly proportional to the temperature difference. Even a tall chimney produces almost no draft if the flue has cooled until its internal temperature equals the outside air. Conversely, even a short chimney pulls hard if combustion keeps the flue hot. Think of "height" and "temperature difference" as multiplying together. Move height and temperature separately in this tool to see this combined relationship.
Next, assuming "large ΔP means proportionally large ventilation flow". The induced velocity is V = C_d·√(2ΔP/ρ_avg), proportional to the square root of ΔP. So quadrupling ΔP only doubles the velocity and flow. Moreover, in a real building the supply and exhaust openings are connected in series, so the flow is set by the effective C_d·A combining the resistance of both openings. If one opening is extremely small, the flow is choked there no matter how large ΔP is. This tool is an estimate for a single opening; balancing supply and exhaust is a separate design task.
Finally, the misconception that the stack effect is a winter phenomenon and is irrelevant in summer. The stack effect is driven by the indoor-outdoor temperature difference, and its sign is set by the direction of that difference. In an air-conditioned building, when the summer outside air is warmer than the room, ΔP reverses and a "reverse stack" flow runs opposite to winter. In chimneys this becomes a downward draft that can make the exhaust flow backwards. The position of the neutral plane also shifts with the flow direction. In design it is important to evaluate ΔP for both summer and winter conditions and check the direction and magnitude of airflow throughout the year.
How to Use
Set chimney height (hNum) between 1–20 m using the slider or numeric input
Enter indoor temperature (tinNum) in °C (typically 18–25°C for occupied spaces) and outdoor temperature (toutNum) in °C
Define effective draft area (aNum) in cm² (0.5–50 cm² for residential chimneys); larger areas reduce velocity but increase flow
Read Stack pressure ΔP in Pa, Induced velocity V in m/s, and Ventilation flow Q in m³/h; positive draft indicates upward air movement
Worked Example
A residential chimney with h=8 m, tin=22°C (ρ_in≈1.204 kg/m³), tout=−2°C (ρ_out≈1.293 kg/m³), and a=12 cm² produces ΔP≈10.7 Pa, V≈2.1 m/s, and Q≈91 m³/h. Winter conditions (larger Δt) generate stronger draft; a 10 m tall fireplace chimney in 15°C indoor/−5°C outdoor conditions achieves approximately 15 Pa driving pressure, adequate for natural ventilation in historic buildings.
Practical Notes
Stack effect reverses (negative ΔP) when indoor temperature falls below outdoor; common in summer for passive cooling design
Doubling chimney height increases draft pressure by ~1.4× due to logarithmic density variation; tall atria (12+ m) in commercial buildings exploit this for zero-energy ventilation
Effective area includes bends and restrictions; a 200 mm square duct with 90° bend reduces a by ~30% compared to straight geometry
Air density depends on moisture content; humid indoor air (40% RH at 22°C) is ~0.3% lighter, slightly enhancing draft in humid climates