3.7.3.3 Return/Relief Fans

Baseline building systems 3 through 8 and 11 through 13, shall have a return/relief fan when the proposed design has a return/relief fan fan.

Plenum Zone
Applicability	Any system with return ducts or return air plenum
Definition	A reference to the thermal block that serves as return plenum or where the return ducts are located
Units	Text, unique
Input Restrictions	As designed
Baseline Rules	Applicable when the baseline building has a return fan. Same as the proposed design when the proposed design has a plenum, otherwise, the return air ducts are assumed to be located in the space.

Return Air Path
Applicability	Any system with return ducts or return air plenum
Definition	Describes the return path for air. This can be one of the following: ducted return; plenum return; or direct-to-unit.
Units	List (see above)
Input Restrictions	As designed
Baseline Rules	Applicable when the baseline building has a return fan. For baseline building systems 1 and 2, the return air path shall be direct-to-unit. For baseline building systems 3 through 8 and 11 through 13, when the proposed design is direct-to-unit, the baseline building shall be ducted return, otherwise the baseline building return air path shall be same as proposed design.

Return/Relief Air Rated Capacity
Applicability	All systems with a return or relief fan
Definition	The design air flow fan capacity of the return or relief fan(s). This sets the 100% fan flow point for the part-load curve (see below).
Units	cfm
Input Restrictions	As designed
Baseline Rules	Applicable when the baseline building has a return fan. The return fan air Rated Capacity shall be equal to 90% of the baseline building supply fan capacity or the baseline building supply fan capacity less the minimum outdoor air, whichever is larger. The two special cases apply. For systems serving laboratory spaces, airflow rate shall be based on a supply-air-to-room temperature setpoint difference of 17°F or the required ventilation air or makeup air, whichever is greater. If the proposed design HVAC system airflow rate based on latent loads is greater than the design airflow rate based on sensible loads, then the same supply-air-to-room-air humidity ratio difference (gr/lb) used to calculate the proposed design airflow shall be used to calculate design airflow rates for the baseline building design.

Return/Relief Fan Brake Horsepower
Applicability	Any system with return or relief fans that uses the brake horsepower method
Definition	The design shaft brake horsepower of the return/relief fan(s)
Units	Brake horsepower (bhp)
Input Restrictions	As designed
Baseline Rules	Applicable when the baseline building has a return fan. The bhp of the return fan shall be the fan system brake horsepower (see Table 3.7.3-1) times the return fan ratio. In other words, brake horsepower is allocated in proportion to the proposed design.

Return/Relief Design Static Pressure
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The design static pressure for return 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 return fan does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Rules	The baseline Return/Relief Design Static Pressure shall be equal to the return/relief fan brake horsepower divided by the constant 6,356 and an assumed fan efficiency of 65%.

Return/Relief Fan Efficiency
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The efficiency of the fan at design conditions
Units	Unitless
Input Restrictions	As designed. The return/relief fan efficiency does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Rules	65%

Return/Relief Motor Efficiency
Applicability	All return fans, except those specified using the power-per-unit-flow method
Definition	The full-load efficiency of the motor serving the supply fan
Units	Unitless
Input Restrictions	As designed. Not applicable when the power-per-unit-flow method is used.
Baseline Rules	For baseline systems 1, 2, 9 and 10, motor efficiency shall be 80%. For other systems, the minimum value from Table 3.7.3-3 shall be used.

Motor Position
Applicability	All return fans
Definition	The position of the supply fan motor relative to the cooling air stream. The choices are: in the air stream or out of the air stream.
Units	List (see above)
Input Restrictions	As designed.
Baseline Rules	In the air stream

Fan Part-Flow Power Curve
Applicability	All return fans for variable flow fan systems.
Definition	A part-load power curve which represents the percentage full-load power draw of the supply fan as a function of the percentage 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	Not applicable for baseline building systems 1-, 9, 10, 12 and 13. The default curve for VSD fans shall be used for baseline building systems 5-8 that have a return/relief fan. The default shall be taken from COMNET Appendix H.

Return/Relief Fan Power Index
Applicability	Any system with a return fan
Definition	The supply fan power per unit of flow
Units	kW/cfm
Input Restrictions	As specified in the manufacturers’ literature
Baseline Rules	Applicable when the baseline building uses the power-per-unit-flow method. Fan power is determined from Table 3.7.3-2. This power is then multiplied by the return fan ratio.

90.1-2019

System design supply airflow rates for the baseline building design shall be based on a supply-air-to room-air temperature difference of 20°F or the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is greater. For systems 3, 4, and 11 through 13, if return or relief fans are specified for systems serving the corresponding zones in the proposed design, the baseline building design shall also be modeled with fans serving the same functions. For systems 5 through 8, if return or relief fans are specified in the proposed building, then the baseline systems serving the corresponding zones will be modeled with a return/relief fan.

Plenum Zone
Applicability	Any system with return ducts or return air plenum
Definition	A reference to the thermal zone that serves as return plenum or where the return ducts are located
Units	Text, unique
Input Restrictions	As designed
Baseline Building	Applicable when the baseline building has a return fan. Same as the proposed design when the proposed design has a plenum; otherwise, the return air ducts are assumed to be located in the space.

Return Air Path
Applicability	Any system with return ducts or return air plenum
Definition	Describes the return path for air. This can be one of the following: ducted return; plenum return; or direct-to-unit.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	For baseline building systems 1 and 2, the return air path shall be direct-to-unit. For baseline building systems 3 through 8 and 11 through 13 and when the proposed design is direct-to-unit, the baseline building shall be ducted return, otherwise the baseline building return air path shall be the same as proposed design.

Return/Relief Air Rated Capacity
Applicability	All systems with a return or relief fan
Definition	The design airflow fan capacity of the return or relief fan(s). This sets the 100% fan flow point for the part-load curve (see below).
Units	cfm
Input Restrictions	As designed
Baseline Building	Applicable when the baseline building has a return fan. Return or relief fans shall be sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.

Return/Relief Fan Brake Horsepower
Applicability	Any system with return or relief fans that uses the brake horsepower method
Definition	The design shaft brake horsepower of the return/relief fan(s)
Units	Brake horsepower (bhp)
Input Restrictions	As designed
Baseline Building	Applicable when the baseline building has a return fan. The bhp of the return fan shall be the system fan brake horsepower multiplied by the return fan ratio. In other words, brake horsepower is allocated in proportion to the proposed design.

Return/Relief Design Static Pressure
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The design static pressure for return fan system. This is important for both fan electric energy usage and duct heat gain calculations.
Units	Inches of water column (in. H₂O gauge)
Input Restrictions	As designed. The design static pressure for the return fan does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Building	Return fan static pressure = (Return fan BHP x 6356 x 0.65)/cfm return

Return/Relief Fan Efficiency
Applicability	Any system with return or relief fans that uses the static pressure method
Definition	The efficiency of the fan at design conditions; this is the static efficiency and does not include the efficiency loss of the motor
Units	Unitless
Input Restrictions	As designed. The return/relief fan efficiency does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Building	65%

Return/Relief Motor Efficiency
Applicability	All return fans, except those specified using the power-per-unit-flow method
Definition	The full-load efficiency of the motor serving the supply fan
Units	Unitless
Input Restrictions	As designed. Not applicable when the power-per-unit-flow method is used.
Baseline Building	For baseline systems 1, 2, 9, 10, 12, and 13 is 80%. For baseline systems 3 through 8 and 11, fan motor efficiency is determined 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.

Motor Position
Applicability	All return fans
Definition	The position of the supply fan motor relative to the cooling air stream. The choices are in the air stream or out of the air stream.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	In the air stream

*Fan Part-Flow Power Curve*
Applicability	All return fans for variable flow fan systems
Definition	A part-load power curve that represents the percentage full-load power draw of the return fan as a function of the percentage full-load 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	Not applicable for baseline building systems 1 through 4. The curve for “Multi zone VAV with static pressure reset curve” shall be used for baseline building systems 5 through 8 and system 11.

Return/Relief Fan Power Index
Applicability	Any system with a return fan
Definition	The return fan power per unit of flow
Units	kW/cfm
Input Restrictions	As specified in the manufacturers’ literature
Baseline Building	Applicable when the baseline building uses the power-per-unit-flow method. Fan power is determined using Table 53. This power is then multiplied by the return fan ratio.