Fan Ratios | COMNET

When the proposed design has exhaust fans (toilets or kitchens), return fans, or fume hood exhaust systems, the baseline building has the same systems. The brake horsepower determined from Table 3.7.3-1 is allocated to these baseline building fan systems proportionally to the allocation in the proposed design. The allocation of brake horsepower to the supply fan, the return and any exhaust or fume hood fans is based on the ratios described below.

Supply Fan Ratio
Applicability	Systems that serve thermal blocks that have exhaust, fume hoods, kitchen exhaust or return fans
Definition	The ratio of supply fan brake horsepower in the proposed design to total fan system brake horsepower for the proposed design at design conditions
Units	Unitless ratio
Input Restrictions	Derived from other building descriptors
Baseline Rules	Same as proposed design

Return Fan Ratio
Applicability	Systems that serve thermal blocks that have exhaust, fume hoods, kitchen exhaust or return fans
Definition	The ratio of return fan brake horsepower in the proposed design to total fan system brake horsepower for the proposed design at design conditions
Units	Unitless ratio
Input Restrictions	Derived from other building descriptors
Baseline Rules	Same as proposed design

Exhaust Fan Ratio
Applicability	Systems that serve thermal blocks that have exhaust, fume hoods, kitchen exhaust or return fans
Definition	The ratio of exhaust fan brake horsepower in the proposed design to total fan system brake hp for the proposed design at design conditions. Exhaust fans include toilet exhaust, kitchen hoods and other miscellaneous exhaust. Fume hood exhaust is treated separately.
Units	Unitless ratio
Input Restrictions	Derived from other building descriptors. In the event that a common exhaust system serves thermal blocks that are served by different HVAC systems, the brake horsepower shall be divided in proportion to design cfm.
Baseline Rules	Same as proposed design