Stack effect drives air upward through a building because warm air is lighter than cool: the draught speed is v = sqrt(2 x g x H x dT / T) for a height H between inlet and outlet and a temperature difference dT, and flow is about 0.6 x area x v. A 3 m rise with a 5 K difference gives only 0.6 pascals of pressure, so a wind of 1 metre per second can overrule it.
Part of Passive Solar Building Design
How a temperature difference between indoor and outdoor air moves air up through low and high openings and tall shafts, and how to size a thermal chimney.
Hot Air Rising Creates a Small Pressure Difference
The carrier describes it: position inlets low and outlets high; hot air rises and exits through the high openings, pulling cooler air in through the low openings. A thermal chimney is a dark-colored, sun-exposed vertical shaft that heats the air in it; the hot air rises and exits at the top, pulling air through the building with no wind required.
The physics: the pressure difference between a column of warm air and a column of outdoor air of height H is dp = rho x g x H x dT / T, where rho is air density (about 1.2 kg/m3), g is 9.81 m/s2, dT is the difference in temperature (kelvin, the same as degrees Celsius for a difference) and T is the absolute temperature in kelvin (about 300 K for 27 degrees Celsius). Flow through an opening of area A is Q = Cd x A x sqrt(2 x dp / rho) = Cd x A x sqrt(2 x g x H x dT / T), with Cd about 0.6 for a sharp-edged opening (a standard orifice coefficient; see the natural-ventilation chapter of the ASHRAE Handbook, Fundamentals). With an inlet and an outlet in series use the effective area from Cross-Ventilation Design.
What You Need
- Height — the vertical distance between the low inlet and the high outlet.
- A temperature difference — indoors or in the shaft, warmer than outdoors.
- A dark shaft facing the sun — masonry or metal. The carrier says a south or west-facing column in the Northern Hemisphere; in the Southern Hemisphere the sun-facing side is north, and west still takes the afternoon sun.
- A closable cap or damper — see winter below.
Method: Size It, Place It
Step 1: Maximize height Flow rises with the square root of height: four times the height doubles the speed. Put the outlet above the roof ridge.
Step 2: Make the openings large The inlet and the shaft cross-section should each be at least as large as the area you computed.
Step 3: Take inlet air from a cool, shaded place Cool air from the shaded side of the house or from an earth tube (see Earth Tubes & Ground Coupling).
Step 4: Protect the outlet from wind Fit a cap or cowl so that wind passing the top produces suction, not pressure.
Worked Example
A room 5 m x 4 m x 2.6 m (52 m3). Low inlet and high outlet 3 m apart. Indoor 5 degrees warmer than outdoor, T = 300 K. Each opening 0.25 m2.
- Speed: v = sqrt(2 x 9.81 x 3 x 5 / 300) = sqrt(0.981) = 0.99 m/s.
- Pressure: dp = 1.2 x 9.81 x 3 x 5 / 300 = 0.59 Pa.
- Two equal openings in series: A_eff = 0.25 / sqrt(2) = 0.177 m2.
- Flow: Q = 0.6 x 0.177 x 0.99 = 0.105 m3/s = 378 m3/h = 7.3 air changes per hour.
A thermal chimney with the same 0.25 m2 openings, an assumed 10 K excess in the shaft and H = 4 m: v = sqrt(2 x 9.81 x 4 x 10 / 300) = 1.62 m/s, dp = 1.57 Pa, Q = 0.6 x 0.177 x 1.62 = 0.172 m3/s = 619 m3/h = 12 air changes per hour. (The 10 K is an assumption; the excess depends on shaft glazing, size and sun.)
Compare with wind: the dynamic pressure of wind is 0.5 x rho x V2, which is 0.6 Pa at 1 m/s and 2.4 Pa at 2 m/s. A breeze of 1 m/s equals the whole stack effect of the first example, and 2 m/s exceeds even the chimney’s 1.6 Pa. Stack effect is a weak, steady force: a gust on the wrong side reverses it. That is why the outlet needs shelter from the wind. See The Chimney Effect for the same limit in a stove chimney.
Common Mistakes
- Making the shaft short — flow depends on height.
- Small inlet — the shaft is starved of air.
- A thermal chimney open all winter — it drains warm air from the house. Fit a damper or cap and close it when heating.
- Outlet in the lee of a taller roof — wind eddies push air back down.
Safety
A chimney or exhaust can pull combustion gas into the house
Any shaft or fan that removes air from a house lowers its pressure. If a stove or fire is burning, it can reverse the flue and draw smoke and carbon monoxide into the room, which is invisible and odourless. Keep a clear air supply for every fire, never run a thermal chimney against a lit stove, and check that stove flues draw upward. Dark metal pipes in sun also burn skin and can ignite thatch and dry timber at the roof penetration.
What’s Next
- Learn next: Earth Tubes & Ground Coupling — the cool air source
- Combine with: Ventilation & Summer Cooling — the daily cycle