USGBC is asking me for a manual calculation on fan power. Can you walk me through how you calculate the fan power for a VAV system for LEED?
It’s a pretty straightforward process. However, it does require some bouncing around in ASHRAE 90.1 and that can lead to confusion, so after this walk-through you might simply prefer to use our fan calculator. One of the most complicated parts of the calculation is determining the filter credit. But remember that you should only apply the fan credit if ALL the supply air is filtered. (The workaround is discussed in the next segment)
It's best to use an example: Let’s say you have a sound attenuation device on your full supply air of 10,000 cfm and you have a MERV 13 filter on your 2,500 cfm of ventilation air (and only on your ventilation air).
The fan credit (from 90.1 TABLE 126.96.36.199.1B) is .15” for the sound attenuation device and .9” for a MERV 13-15 filter.
From the footnotes of TABLE 188.8.131.52.1a, we see that the credit only applies to the cfm that the air goes through in the filter or device, so the equation for A (in the footnotes) is:
A = sum of (PD× CFMD/4131).
In this case that yields:
A = .9*2500/4131 + .15*10,000/4131 = .908 hp
Then, from Table G184.108.40.206 for variable air volume systems you use:
BHP = CFMs*0.0013 + A
Thus, you have
BHP = .0013*10,000+A = .0013*10,000+.908 = 13.908 bhp
You then have to move to the KW, which is calculated from G220.127.116.11. Since it is VAV, you use the equation for systems 3-8, which is
TOTAL system fan power in watts = bhp*746/efficiency
KW = bhp*746/1000/efficiency = bhp*.746/efficiency