Stack effect is the ventilation in buildings and chimneys that results from thermal differences between indoor and outside temperature. The greater the thermal difference and the height of the structure, the greater the stack effect. The stack effect is also referred to as the "chimney effect" or as "natural ventilation". Good ventilation (right) and bad ventilation (left). ...

In a completely sealed enclosure, thermal differences between the inside and outside will result in a pressure difference, because warm air is less dense than cold air. If the enclosure is not sealed, the less dense warm air will rise because it is being displaced by the denser cold air (= thermal buoyancy). Therefore, stack effect is a form of convection or natural ventilation.

Convection is the transfer of heat by currents within a fluid. ...

Stack effect in buildings

Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed.

In a modern high-rise building with a sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it. Especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke. High-rise is a 1975 novel by J. G. Ballard. ... Building envelope refers to the exterior surface that encloses the interior space of a building. ...

Stack effect in chimneys

The stack effect in chimneys is similar to that in buildings, except it involves hot flue gases having large temperature differences with the ambient and outside air. Furthermore, chimneys typically provide little obstruction for the flue gas along its length. These facts can create a strong stack effect in chimneys, so much so, that in old buildings using a fireplace for heating it draws in more cold outside air than can be heated by the fireplace, resulting in a net heat loss. A chimney is a system for venting hot gases and smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. ... A flue is a pipe or channel for conveying exhaust gases from a fireplace, furnace, boiler, or generator. ... A fireplace with a burning fire. ...

See: Chimney#Chimney draught or draft

A chimney is a system for venting hot gases and smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. ...

The driving force for the stack effect

There is a pressure difference between the outside air and the air inside the building caused by the difference in temperature between the outside air and the inside air. That pressure difference ( ΔP ) is the driving force for the stack effect and it can be calculated with the equation presented below. ^{[1] [2]} The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building. For multi-floor, high-rise buldings, h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference^[1] explains how the NPL affects the stack effect in high-rise buildings.

For chimneys where air is on the outside and combustion flue gases are on the inside, the equation will only provide an approximation and h is the height of the chimney.

SI units:

where:
ΔP	= available pressure difference, in Pa
a	= atmospheric pressure, in Pa
h	= height or distance, in m
To	= absolute outside temperature, in K
Ti	= absolute inside temperature, in K

U.S. customary units:

where:
ΔP	= available pressure difference, in psi
a	= atmospheric pressure, in psia
h	= height or distance, in ft
To	= absolute outside temperature, in °R
Ti	= absolute inside temperature, in °R

Cover of brochure The International System of Units. ... The pascal (symbol: Pa) is the SI unit of pressure. ... metre or meter, see meter (disambiguation) The metre is the basic unit of length in the International System of Units. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... U.S. customary units, commonly known in the United States as English units or standard units, are units of measurement that are currently used in the U.S., in some cases alongside units from SI (the International System of Units—the modern metric system). ... Pounds-force per square inch (lbf/in²) is a non-SI unit of pressure. ... A human foot - Enlarge to view legend For other uses, see Foot (disambiguation). ... Rankine is a now rarely used temperature scale named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859. ...

The flow induced by the stack effect

The draught or draft flow rate induced by the stack effect can be calculated with the equation presented below. ^{[1] [2]} The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building and A is the flow area of the openings. For multi-floor, high-rise buildings, A is the flow area of the openings and h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference^[1] explains how the NPL affects the stack effect in high-rise buildings.

For chimneys where air is on the outside and combustion flue gases are on the inside, the equation will only provide an approximation. Also, A is the cross-sectional flow area and h is the height of the chimney.

SI units:

where:
Q	= stack effect draught/draft flow rate, m³/s
A	= flow area, m²
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)
g	= gravitational acceleration, 9.807 m/s²
h	= height or distance, m
Ti	= average inside temperature, K
Te	= outside air temperature, K

U.S. customary units:

where:
Q	= stack effect draught/draft flow rate, ft³/s
A	= area, ft²
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)
g	= gravitational acceleration, 32.17 ft/s²
h	= height or distance, ft
Ti	= average inside temperature, °R
Te	= outside air temperature, °R

Cover of brochure The International System of Units. ... The cubic meter (symbol m³) is the SI derived unit of volume. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The acceleration due to gravity denoted g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... The metre, or meter, is a measure of length. ... U.S. customary units, commonly known in the United States as English units or standard units, are units of measurement that are currently used in the U.S., in some cases alongside units from SI (the International System of Units—the modern metric system). ... The cubic foot (symbols ft³, cu. ... A square foot is by definition the area enclosed by a square with sides each 1 foot long. ... A human foot - Enlarge to view legend For other uses, see Foot (disambiguation). ... Rankine is a now rarely used temperature scale named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859. ...

See also

• HVAC
• Chimneys

HVAC (pronounced either H-V-A-C or, occasionally, H-VAK) is an initialism/acronym that stands for heating, ventilation and air-conditioning. This is sometimes referred to as climate control. ... Chimney stacks on a Newcastle upon Tyne building A chimney is a system for venting hot gases and smoke from a stove, furnace or fireplace to the outside atmosphere. ...

References

1. ^a b c d Natural Ventilation Lecture 2
2. ^a b Natural Ventilation Lecture 3
3. ASHRAE Fundamentals Handbook