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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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