3.7.3.4 Exhaust Fan Systems

Exhaust fans include toilet exhaust, kitchen exhaust, as well as fume hoods in laboratories and other spaces. Some systems typically operate at constant flow, while flow varies for other systems depending on, for instance, the position of the sash for fume hoods. Exhaust fan flow is specified and scheduled for each thermal block. An exhaust fan system may serve multiple thermal blocks. The baseline building has exhaust fans when the proposed design has exhaust fans. The exhaust air flow is the same for the baseline building and the proposed design.

Exhaust Fan Name
Applicability	All exhaust systems serving multiple thermal blocks
Definition	A unique descriptor for each exhaust fan. This should be keyed to the construction documents, if possible, to facilitate plan checking. Exhaust rates and schedules at the thermal block level refer to this name.
Units	Text, unique
Input Restrictions	Where applicable, this should match the tags that are used on the plans.
Baseline Rules	The baseline building will have an exhaust system that corresponds to the proposed design. The name can be identical to that used for the proposed design or some other appropriate name may be used.

Exhaust Fan System Modeling Method
Applicability	All exhaust fan systems
Definition	Software commonly models fans in three ways. See definition for supply system modeling method.
Units	List: power-per-unit-flow, static pressure or brake horsepower
Input Restrictions	As designed
Baseline Rules	If the proposed design uses the power-per-unit-flow method, the baseline building shall also use this method.

Exhaust Fan Rated Capacity
Applicability	All exhaust systems
Definition	The rated design air flow rate of the exhaust fan system. This building descriptor defines the 100% flow case for the part-flow curve. Actual air flow is the sum of the flow specified for each thermal block, as modified by the schedule for each thermal block.
Units	cfm
Input Restrictions	As designed
Baseline Rules	Same as the proposed design.

Fan Control Method
Applicability	All exhaust fan systems
Definition	A description of how the exhaust fan(s) are controlled. The options include: Constant volume Two-speed Variable-flow, inlet or discharge dampers Variable-flow, inlet guide vanes Variable-flow, variable speed drive (VSD) Variable-flow, variable pitch blades
Units	List (see above)
Input Restrictions	As designed, however, when exhaust fan flow at the thermal block level is varied through a schedule, one of the variable-flow options shall be specified.
Baseline Rules	The baseline building exhaust fan control shall generally be the same as the proposed design, with the following exceptions. For laboratories that have exhaust flow of 5,000 cfm or more, the baseline building exhaust flow shall vary in response to scheduled fume hood exhaust and lab zone airflow schedules. Fume hood exhaust flow control shall be the same as design, and general exhaust (relief) shall be VSD. If a kitchen zone design exhaust rate is greater than 5,000 cfm, the baseline building system serving the kitchen shall be system type 5 or 7 with a demand ventilation system on 75% of the exhaust air. The system shall reduce exhaust and replacement air system airflow rates by 50% for one half of the kitchen occupied hours.

Exhaust Fan Schedule
Applicability	All exhaust fan systems
Definition	A schedule that indicates when the exhaust fan system is available for operation. Exhaust fan flow is specified at the thermal block level.
Units	Data structure: schedule, on/off
Input Restrictions	The exhaust fan system shall be available during all periods when one or more thermal blocks served by the system are scheduling exhaust.
Baseline Rules	Same as the proposed design

Exhaust Fan Brake Horsepower
Applicability	All exhaust fan systems
Definition	The design shaft brake horsepower of the exhaust fan(s).
Units	Brake horsepower (bhp)
Input Restrictions	As designed
Baseline Rules	The bhp for the baseline building is the total system fan horsepower from Table 3.7.3-2 times the exhaust fan ratio.

Exhaust Fan Design Static Pressure
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The design static pressure for exhaust fan system. This is important for both fan electric energy usage and duct heat gain calculations.
Units	Inches of water column (in. H₂0)
Input Restrictions	As designed. The design static pressure for the exhaust fan does not need to be specified if the exhaust fan brake horsepower (bhp) is specified.
Baseline Rules	The exhaust fan static pressure for the baseline building would be equal to the total fan system static pressure (specified in section Supply Fan Static Pressure) multiplied by the exhaust fan ratio. Exhaust Fan Static Pressure = (Exhaust Fan BHP x 6356 x 0.65)/cfm exhaust

Exhaust Fan Efficiency
Applicability	Any exhaust fan system that uses the static pressure method
Definition	The efficiency of the exhaust fan at rated capacity
Units	Unitless
Input Restrictions	For kitchen exhaust fans, the fan efficiency is prescribed at 50%. As designed for other exhaust fan systems. The exhaust fan efficiency does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Rules	The baseline exhaust fan efficiency is 65%.

Exhaust Fan Motor Efficiency
Applicability	All exhaust fan systems
Definition	The full-load efficiency of the motor serving the exhaust fan
Units	Unitless
Input Restrictions	As designed
Baseline Rules	From Table 3.7.3-4.

Fan Part-Flow Power Curve
Applicability	All variable flow exhaust fan systems
Definition	A part-load power curve which represents the ratio full-load power draw of the exhaust fan as a function of the ratio full-load air flow.
Units	Unitless ratio
Input Restrictions	As designed. The default fan curve shall be selected from COMNET Appendix H for the type of fan specified in the proposed design.
Baseline Rules	The baseline building fan curve shall be selected from COMNET Appendix H for the type of fan specified in the proposed design.

Exhaust Fan Power Index
Applicability	All exhaust systems
Definition	The fan power of the exhaust fan per unit of flow. This building descriptor is applicable only with the power-per-unit-flow method.
Units	W/cfm
Input Restrictions	As designed.
Baseline Rules	The baseline building exhaust fan power index shall be the fan system power from Table 3.7.3-2 multiplied times the exhaust fan ratio.

90.1-2019

Exhaust fans include toilet, kitchen and laboratory exhaust. Some systems typically operate at constant flow, while flow varies for other systems depending on, for instance, the position of the sash for fume hoods. Exhaust fan flow is specified and scheduled for each thermal zone. An exhaust fan system may serve multiple thermal zones. The baseline building has exhaust fans when the proposed design has exhaust fans. The design exhaust airflow is the same for the baseline building and the proposed building.

Exhaust Fan Name
Applicability	All exhaust systems serving multiple thermal zones
Definition	A unique descriptor for each exhaust fan. This should be keyed to the construction documents, if possible, to facilitate plan checking. Exhaust rates and schedules at the thermal zone level refer to this name.
Units	Text, unique
Input Restrictions	Where applicable, this should match the tags that are used on the plans
Baseline Building	The baseline building will have an exhaust system that corresponds to the proposed design. The name can be identical to that used for the proposed design or some other appropriate name may be used.

Exhaust Fan System Modeling Method
Applicability	All exhaust fan systems
Definition	Software commonly models fans in three ways. See definition for supply system modeling method.
Units	List: Power-Per-Unit-Flow, Static Pressure, or Brake Horsepower
Input Restrictions	As designed
Baseline Building	If the proposed design uses the power-per-unit-flow method, the baseline building shall also use this method, otherwise the baseline building shall use the static pressure method

*Exhaust Fan Rated Capacity*
Applicability	All exhaust systems
Definition	The rated design airflow rate of the exhaust fan system. This building descriptor defines the 100% flow case for the part-flow curve. Actual airflow is the sum of the flow specified for each thermal zone, as modified by the schedule for each thermal zone.
Units	cfm
Input Restrictions	As designed
Baseline Building	Same as the proposed design except that baseline building kitchen exhaust rate may not exceed the maximum levels defined by Section 3.6.5.3 of this document

*Fan Control Method*
Applicability	All exhaust fan systems
Definition	A description of how the exhaust fan(s) are controlled. The options include: Constant volume Two-speed Variable-flow, inlet or discharge dampers Variable-flow, inlet guide vanes Variable-flow, VSD Variable-flow, variable pitch blades
Units	List (see above)
Input Restrictions	As designed, however, when exhaust fan flow at the thermal zone level is varied through a schedule, one of the variable-flow options shall be specified
Baseline Building	The baseline building exhaust fan control shall generally be the same as the proposed design. Exceptions: Exhaust fans s serving laboratory spaces shall reduce the exhaust air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards as described in Section 3.6.5.3 under descriptor Laboratory: Exhaust Minimum Airflow Rate.

*Exhaust Fan Schedule*
Applicability	All exhaust fan systems
Definition	A schedule that indicates when the exhaust fan system is available for operation. Exhaust fan flow is specified at the thermal zone level.
Units	Data structure: schedule, on/off
Input Restrictions	As designed
Baseline Building	Same as the proposed design for exhaust fans not serving kitchen or laboratory spaces For kitchen and laboratory zone exhaust, refer to Section 3.6.5.3 of this document

*Exhaust Fan Brake Horsepower*
Applicability	All exhaust fan systems
Definition	The design shaft brake horsepower of the exhaust fan(s)
Units	Brake horsepower (bhp)
Input Restrictions	As designed
Baseline Building	The bhp for the baseline building is the total system fan horsepower from Table 53 of this document multiplied by the exhaust fan ratio

*Exhaust Fan Design Static Pressure*
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The design static pressure for exhaust fan system. This is important for both fan electric energy usage and duct heat gain calculations.
Units	Inches of water column (in. H₂O)
Input Restrictions	As designed for exhaust fans not serving kitchens. The design static pressure for the exhaust fan does not need to be specified if the exhaust fan bhp is specified.
Baseline Building	The exhaust fan static pressure for the baseline building would be equal to the total fan system static pressure (specified in section Supply Fan Static Pressure) multiplied by the exhaust fan ratio Exhaust Fan Static Pressure = (Exhaust Fan BHP x 6356 x 0.65)/cfm exhaust

*Exhaust Fan Efficiency*
Applicability	Any exhaust fan system that uses the static pressure method
Definition	The efficiency of the exhaust fan at rated capacity; this is the static efficiency and does not include losses through the motor
Units	Unitless
Input Restrictions	As designed. The exhaust fan efficiency does not need to be specified if the return fan bhp is specified.
Baseline Building	For baseline system, fan efficiency is 65%

Exhaust Fan Motor Efficiency
Applicability	All exhaust fan systems
Definition	The full-load efficiency of the motor serving the exhaust fan
Units	Unitless
Input Restrictions	As designed
Baseline Building	From Table G₃.9.1 of Standard 90.1-2019 for the next motor size greater than the bhp calculated, through the process described above, using a totally enclosed fan motor at 1800 rpm

*Fan Part-Flow Power Curve*
Applicability	All variable flow exhaust fan systems
Definition	A part-load power curve that represents the power draw of the exhaust fan as a function of the airflow
Units	Unitless ratio
Input Restrictions	As designed. The default fan curve shall be selected from Equation (10) and Table 57 for the type of fan specified in the proposed design.
Baseline Building	Same as proposed

*Exhaust Fan Power Index*
Applicability	All exhaust systems
Definition	The fan power of the exhaust fan per unit of flow. This building descriptor is applicable only with the power-per-unit-flow method.
Units	kW/cfm
Input Restrictions	As designed
Baseline Building	The fan system power from Table 53 multiplied by the exhaust fan ratio