Imagine heat as a red balloon filled with helium. When you walk through a house, you would
see them collect on the ceiling, peaks of vaulted ceilings, and upper corners of the room
if there is no air turbulence or drafts. When given the chance, they will drift up open
stairways, for they seek the highest perch.
Heat is a balloon without the plastic wrapper, and where it collects it exerts a pressure
on the surface of the ceiling. The amount of pressure depends on the difference in temperature
between the inside and outside of the space. The pressure is also greater than the 68°
used for the heat loss calculation. When the outdoor temp is 5°, the difference between
inside and outside can reach 100°. Just ask any painter who's had to work on an eight-foot
ladder or scaffold. This resulting difference is almost 50% greater than that calculated,
so the ceiling is losing one and a half times as much heat as calculated.
In multi-level structures, the flow of heat up stairways and through the first floor ceilings
reduces the requirements of the second floor while increasing the demand on the first floor
for heat. On any level, the collecting points for these "red balloons" become
good locations for return air intakes. Returning the heat increases the efficiency of the
system and reduces the loss to the outside. During the cooling season, the high returns
relieve the stratification that can develop as cool air tries to settle on the floor. As
all supplies and returns create drafts, high returns will have the least impact on occupant
comfort and increase system efficiency.