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Heating, Cooling Loads and Energy Use
The design of any commercial building HVAC system requires a
licensed professional engineer and must be done according to all
other aspects of the building as a system.
Evaluating commercial building loads is complex and usually time
consuming. A number of software programs are available to help
designers proceed with this evaluation. However, preliminary design
for simple buildings can still be evaluated using hand calculations
or rudimentary spreadsheet programs
Heating and Cooling Load Calculation
The first step in the sizing process usually involves calculating
each zone’s peak heating and cooling load as well as the
whole-building peak loads. The following factors typically need
to be considered when performing these calculationsSolar gains
through windows: Standard double-glazed windows can let up to
75 percent of this energy penetrate the building, where it becomes
a cooling load. Additional window treatments such as tinted and
reflective glazing, shading and draperies can further reduce solar
gains.
Internal gains from occupants (including latent heat for cooling
purposes): Each adult will typically generate about 75 W of sensible
energy and 55 W of latent energy.
Internal gains from lighting and equipment: Lighting power is
often about 20 W/m² in office buildings but can be as high
as 40 to 50 W/m².
The equipment load (also called plug-load) is often in the 2-
to 5-W/m² range but can be as high as 15 to 20 W/m².
Outside air loads (sensible and latent) from ventilation and infiltration:
All buildings should meet at least the minimum outside air requirements
imposed in their local jurisdiction. The amount of outside air
is often taken from ASHRAE Standard 62. A typical value for outside
airflow rate is 15 L/s/occupant.
Heat gains or losses through windows, walls, floors and roofs:
These gains are mostly important for heating load calculation
but may still have some impact for the cooling load, especially
the windows, and heat gain. The amount of heat transfer through
these components can be estimated using the following formula:
Heat gain/loss = Area X (surface temperature outside – surface
temperature inside)/RSI value.
Using the very simplified values and formulas presented here
can help in getting a rough estimate of a zone heating and cooling
load. Some other important points to know about load calculations
are as follows:
The heating load calculation must be done without credit for
occupants and internal gains, since this load usually occurs at
night.
Zone loads are calculated with consideration only to the zone’s
peak gains (i.e., solar) or losses (for heating).
Each zone’s peak loads may occur at different moments. However,
for hand calculations, cooling loads are usually calculated during
the hottest day of the summer for three different times for each
zone. The greatest of the calculated loads are selected as the
zone peak loads.
Heating loads need only to be evaluated at the heating design
temperature, since no credit for solar gain or internal gain is
considered. However, since some areas in the core of a building
may require cooling at all times, these zones may need to consider
internal gain even under winter design conditions.
Whole-building loads are calculated considering all zones’
loads. The whole-building peak loads may not occur at the same
moment as that of any of its zones. Precise determination of the
time of occurrence of the whole-building load requires either
extensive hand calculations or, more realistically, an hourly
computer simulation. Approximate cooling block load can be estimated
using the greatest of the sum of all zone loads for the three
time periods previously considered.
Design temperatures must be obtained from a reliable source, such
as ASHRAE Handbook Fundamentals, 2001.
Typical values for building heating load range from 20 to 120
W/m². Cooling loads generally vary from 50 W/m² for
buildings in cool climates with little internal gains to 200 W/m²
or more for commercial buildings in hot climates with high internal
gains.
For a thorough calculation of the zones and whole-building loads,
one of the following three methods presented in ASHRAE should
be employed:
Transfer Function Method (TFM): This is the most complex of the
methods proposed by ASHRAE and requires the use of a computer
program or advanced spreadsheet.
Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF):
This method is derived from the TFM method and uses tabulated
data to simplify the calculation process. The method can be fairly
easily transferred into simple spreadsheet programs but has some
limitations due to the use of tabulated data.
Total Equivalent Temperature Differential/Time-Averaging (TETD/TA):
This was the preferred method for hand or simple spreadsheet calculation
before the introduction of the CLTD/CLF method.
Cooling Loads (Heat gain Loads)
Heat gain from occupants in the building
Heat gain from office equipment
Heat gain from restaurant equipment
Heat gain from electric motors
Cooling Load Check Figures
Cooling Heating Load Calculation
Software
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