Example: a 50 bhp fan load at 1.3 in.w.c. static pressure is reduced to 1.0 in. w.c. by adjusting the control set point. New motor load =

50 bhp X (1'°/i.3)y1'5 = 33.7 bhp and savings would be 50-33.7 = 16.3 bhp

NOTE: For a variety of reasons, the entire theoretical savings of VFD applications do not always come to pass. To avoid savings expectations that aren't met, de-rating the theoretical savings is suggested. The author uses a conservative factor of 0.7 in such estimates.

Circulating fan and pump motor loads will change throughout the day and by season, so a load profile should be used to quantify savings. Make two identical load profiles in a spreadsheet, using the same volumetric flow rates and different static pressures (in. w.c) for the existing and proposed systems

'6356 *eff

Pump bhp =



Savings are from avoided ventilation due to reduced occupancy

Calculate the summer and winter ventilation cost per person, using the OA CFM per person and seasonal weather data profiles With an occupancy profile, compare costs for constant vs. variable ventilation.

Building exhaust requirements and pressurization requirements will limit how far down the ventilation rate can go, regardless of the number of people.

Not all local building departments allow this.

Savings are climate dependent. In high humidity areas, ventilation cost is very high. In moderate and dry climates, it is much lower and may only be a cost during winter.

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