General. Refer to par. 18.104.22.168 for the selection of
outdoor and indoor design conditions. Manual procedures provided
below for determining heating and cooling loads are generally
only applicable to small systems (e.g., heating systems less than
200,000 Btu per hour and cooling systems less than 10 tons).
Computer programs are available that will provide more precise
load determinations and the time of day with the highest cooling
load. The highest heating load is assumed to occur just before
dawn; therefore, this should be considered in the design heating
Q = U * A * (Ti - To)
Q = Btu/hr heat loss by transmission,
U = heat transfer coefficient (look this up in a
handbook for your particular wall, floor, roof, etc.),
A = area of the surface (wall, window, roof, etc.),
Ti = inside design temperature, and
To = outside design temperature.
Use this formula to compute heat transmission losses
from each element of the building skin (e.g., walls, windows,
roof, etc.). Note that attic and crawl space and ground
temperature are different from outdoor temperatures.
Infiltration and Ventilation. To determine the heating
load use the larger of the infiltration and ventilation loads.
Outdoor air provided for ventilation should exceed the air
exhausted by 10 to 15 percent to minimize infiltration. The
designer must use judgment on the amount of excess supply air to
include based on number and type of windows and doors.
Q = 1.10 * CFM * (Ti - To)
CFM = cubic feet per minute of outdoor air, and
Q = the sensible heat loss, Btu/hr.