All,

A colleague brought up an interesting point about filters and fan BHp.

Regarding 90.1-2007, Table 6.5.3.1.1A, Option 2, what is your
interpretation of "bhp = the maximum combined fan brake horsepower" for
an AHU? And how does 90.1-2007 verify this?

90.1-2007 defines "fan system bhp" and "fan system design conditions";
however there isn't any clear mention of how exactly fan BHp is defined
with respect to filters (100% loaded filter delta-p, 66% loaded filter
delta-p, 33% loaded filter delta-p, etc.). The fan system would still
meet capacity requirements, just at a reduced filter pressure drop as
scheduled.

For example, a designer could define (and schedule) the fan BHp to
operate at a max of 66% of allowable filter pressure drop for reduced
fan energy, yet schedule a fan motor to be able to meet the fully loaded
filter pressure drop.

The ramifications of scheduling a lower fan BHp against 90.1-2007 is the
proposed system will now be operating more "efficiently", compared to
the baseline, assuming the filters are truly replaced at 66% of
allowable filter pressure drop (which likely would never happen in the
field!).

Any thoughts on this? Anyone on the 90.1-2007 committee that could shed
some insight?

Thanks,

R. Ryan Hammond PE, HBDP, BEMP, LEED AP