Cooling Load - Lighting

The heat load from lighting in a building is found by summing up the number of lights of each type and wattage, then converting the watts to Btu/hr, multiplying this number by the usage factor and the special allowance factor, as shown in the below equation.

Cooling Load Lighting for the Mechanical PE Exam

The wattage of the light is based on the manufacturer reported value for the lamps in the lighting fixture, without taking into account the ballast. The lighting use factor is the ratio of the time the lights will be in use. This factor is typically 1.0 for most applications like offices, classrooms, stores, hospitals, etc. The usage factor may vary for a movie theater or inactive storage space. The special allowance factor takes into account the heat from ballasts. This factor is typically 1.2 for fluorescent lights and 1.0 for incandescent lights due to the lack of ballasts in incandescent lights.

Heat Transfer Cooling Load Lighting for the Mechanical PE Exam

Finally, the space fraction is the fraction of the total heat from the lights that is transmitted to the space. Lights located at the ceiling may have a percentage of its heat transmitted into the plenum and not into the space. This means that the air conditioning system, if the return is ducted, will not see the percentage of the heat that is transmitted to the plenum. If the plenum is used as a return, then the air conditioning will see the total heat from the lighting. For example, the space fraction for a hung fluorescent light (non-ceiling) will be 1.0, because the light is completely into the space. On the other hand a ceiling recessed light could have a space fraction of 0.5, meaning that 50% of its heat is transmitted to the plenum and the other 50% is transmitted to the space.