Buoyancy-related pressures, varying with height in accordance air density, are calculated by MacroFlo on the basis of a uniform air density in each room (consistent with the stirred tank assumption).
The air pressure in a room is thus a linear function of height:
(5)
where
is the pressure (Pa) in room n at height h above ground level
is height above ground level (m)
is the air density in room n
= 9.81 m/s 2 is the acceleration due to gravity
For the outside air mass, both wind and buoyancy-induced pressure must be included:
(6)
where
is the external pressure (Pa) experienced by opening i
is the wind pressure (Pa) experienced by opening i
is the outside air density
Air densities are calculated as a function of temperature and humidity (see APsim Methods, Properties of Moist Air).
This variation of pressure with height causes height-dependent pressure imbalances between air masses at different temperatures, a phenomenon known as the stack effect.
At the start of a flow calculation the wind pressures are known but the buoyancy component of pressure in each room is only determined up to an additive constant. This constant, p n (0) , is established from the opening flow characteristics and the requirement for flow balancing in each room.
(5) 
is the pressure (Pa) in room n at height h above ground level 
is height above ground level (m) 
is the air density in room n 
= 9.81 m/s 2 is the acceleration due to gravity 
(6) 
is the external pressure (Pa) experienced by opening i 
is the wind pressure (Pa) experienced by opening i 
is the outside air density