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Definition
When calculating the effect an electrical load has on the HVAC systems, it is the demand and not the connected load, which is converted to heat energy. The difference between these two figures is often very large. The connected load includes the wattage for all lighting and equipment loads for each receptacle. The difference between the demand and connected load is often referred to as diversity. Diversity is taken since not all outlets will be used simultaneously.

In addition, each piece of equipment connected may also use considerably less energy than its rating (i.e. a laser printer does not usually print continuously).

Effect of Electrical Loads on HVAC Systems
When considering the effect of watts per square foot in regard to the HVAC systems, it is important to understand that both heating and cooling are affected. Cooling is obvious; the heating effect may not be, e.g.: in an exterior space any reduction in the electrical load could require that heat be added during the colder months to offset this reduction.

The quantity of air (CFM) and supply air temperature required are also affected by changes in watts per square foot. This is particularly true for interior spaces where the majority of the load is electrical in the form of lights and equipment.

Present Trends
The present trend in electrical load for lighting is decreasing while floor power is increasing. This is the result of more efficient lighting sources offset by the increased use of PC's and printers. The once central computer room is now being spread throughout the office.

As a guide for general office space, lighting demands are usually about two watts per square foot and floor power is from 4 to 6 watts per square foot depending on occupancy.

Systems Effect
All projects should be checked for any additional heating needed although most projects only require supplemental cooling. Supplemental cooling can be achieved by adding either small air handling units (AHU) above the ceiling or larger units located in closets throughout the space.

The additional cooling required to serve these AHU's can be generated in several ways. The most common of these are described below:

Chilled Water - If a chilled water source is available, this is the simplest way to provide supplemental cooling. It also provides the greatest flexibility for control but does require that chilled water be available throughout the year.

An additional advantage is that no refrigerant gases are needed at the unit. This is becoming increasingly important since most refrigerants are considered environmentally unacceptable.

As an alternative to central chilled water there are many types of small air-cooled and water cooled chillers available to the designer.

Condenser Water - As with chilled water the condensing source has to be available throughout the year. A disadvantage is that the refrigeration equipment is local and greater maintenance is needed.

Air Cooled - These are available as integral units or split systems. They require an outside air source for the condenser. Refrigerant gases are a potential problem. New codes being considered may also require that the refrigerant piping be regularly inspected for leaks.