Fig. 3 Ð Estimating Block Load
Fig. 4 Ð Floor Plan of Typical Zones
for Single-Story Office Building
LOAD CONSIDERATIONS
Lighting Ð Even though lighting loads (Watts/sq ft) are con- siderably lower in today's buildings, the lighting is by far the largest load component.
It is necessary, therefore, to pay close attention to getting an accurate estimate of the lighting requirements.
In estimating the lighting load, special consideration should be given to evaluating storage effect and the performance of return air ceiling plenums. Both of these items reduce the peak room load from lights and delay the time at which the stored heat becomes a load on the central equipment.
Oversizing Ð Oversizing of variable volume systems re- sults in unused equipment capacity and worse performance at part load, not in increased system air¯ow. The actual sys- tem operation will re¯ect the actual system load, not the de- sign load. If conservative data, safety factors, or provision for future loads are included in the design estimate, the ac- tual system air¯ow will not be increased. The equipment will be capable of handling an increased load should it ever ex- ist, but will automatically throttle back to handle only the actual load at that moment.
It is recommended that safety factors not be included in load calculations; they are not included in the following method.
Air Motion, Ventilation and Odor Dilution Ð Air motion, ventilation, and odor dilution deserve special attention in the design of a VAV system. The designer must visualize the correct system operating condition in order to evaluate the adequacy of these items at either full or
Room air motion is determined by the supply air quantity and the diffuser induction ratio. The minimum room air ve- locity is higher if building humidity and temperature are higher. The design cfm at peak cooling load in any zone should be not less than the minimum shown below:
DESIRED | DESIGN CFM |
ROOM TEMPERATURE | AT PEAK COOLING |
(F) | (cfm/sq ft) |
78 | 0.7 |
75 | 0.4 |
These minimums are based on using the Carrier Modu- line diffuser, which has very high performance; competitive diffusers require a higher cfm/sq ft.
The outside air cfm requirement at maximum design con- ditions may be determined by local building code. If the out- side air cfm to the central air handler is adequate to maintain a low overall building odor level, the odor level in a par- ticular space will depend upon the odors generated locally in that space and the supply air¯ow to that space. A space with high odor generation (a conference room with much smok- ing) should be provided with a separate exhaust system to increase the air ¯ow through the space for odor dilution. The only way to increase the VAV air¯ow to that space would be to add reheat to increase the room sensible heat, which is unacceptable from an energy conservation standpoint.
The following odor dilution cfm (either VAV supply or supplemental exhaust cfm) is usually adequate:
Private or General Office Ð 0.25 cfm/sq ft Major Conference Room Ð 1.0 cfm/sq ft
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