Stack Effect Pressure and the Neutral Pressure Plane

The driving force is nothing but the density difference between a warm column of air inside and a cold column outside, integrated over height -- so a tall building in a cold climate develops a large pressure with no wind and no fans involved at all. The relation is LINEAR in height and roughly linear in the temperature difference, which is why the same weather that is a curiosity in a two-storey house is a design problem in a tower: ten times the height is ten times the pressure. The neutral plane is the useful half of the answer. Below it the building is negative to outside and air comes in; above it the building is positive and air goes out, carrying whatever moisture the indoor air holds into the assemblies at the top of the building. That is why the top floors get the condensation problems and the bottom floors get the cold draughts, and why sealing the top of a building changes the pressure everywhere below it. The plane sits at mid-height only when the leakage is symmetric, and it moves toward whichever opening is larger -- a building with a leaky lobby and a tight roof has its neutral plane low, and most of the building is then positive. That fraction is an input here rather than an assumption, because it is the part that a real building rarely satisfies. The consequences are practical rather than theoretical: elevator and stairwell doors that will not close against the pressure, shafts that behave as chimneys, smoke that moves the wrong way in a fire, and combustion appliances low in the building fighting a draft they were not designed for. This is a steady-state calculation on ENTERED temperatures at a single condition: it does not model wind, which routinely exceeds stack effect and can reverse it locally, mechanical pressurization, the leakage distribution that actually sets the neutral plane, or the interaction between floors that compartmentation changes entirely. ASHRAE Fundamentals, the smoke control design where one exists, and the mechanical engineer of record govern.

total stack pressure difference = 0.0188 x 1898.3-equivalent constant form: dP (Pa) = 3460 x height (m) x (1/T_outdoor - 1/T_indoor) in kelvin, which in US units is 1898.3 x height (ft) x (1/T_outdoor - 1/T_indoor) in degrees Rankine; the neutral plane splits that total between the bottom and the top in proportion to its position.

The ASHRAE Fundamentals stack-effect relation by name, in its US-unit form. The constant 1898.3 Pa per foot per reciprocal Rankine is the SI 3460 converted at 0.3048 m per foot and 1.8 Rankine per kelvin, both exact.

One published relation and two exact conversions.

Estimate. AHJ and licensed professional govern.

Field names used by the API: height_ft, indoor_temp_f, outdoor_temp_f, neutral_plane_fraction, tall_building_height_ft, total_pressure_pa, total_pressure_inwc, neutral_plane_ft, bottom_pressure_pa, top_pressure_pa, tall_pressure_pa

• No wind, no mechanical systems real buildings combine all threeASHRAE Fundamentals
• Neutral plane position is entered it depends on where the leakage sits, which this does not modela zonal diagnostic
• Air density from temperature only humidity and altitude are not carriedthe full relation