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6.7.3 Fan Systems

Baseline Building Fan System Summary

The baseline building fan system is summarized in this section. See Figure 6.1.2-1 for the HVAC baseline building system mapping.

Total baseline building fan system power for the baseline building fan systems is given in Table 6.7.3-1 for ASHRAE Standard 90.1-2007 and Table 6.7.3-1 for ASHRAE Standard 90.1-2001. In these tables, "cfms" is the supply fan air flow at peak design conditions. This is calculated for the baseline building with the sizing procedure described in Figure 2.4-1 and Figure 2.5.2-1. This brake horsepower includes the supply fan, the return fan, and exhaust fans. Exhaust fans include kitchen hoods, toilets, fume hoods, and other miscellaneous fans that operate at design conditions.

Table 6.7.3-1: "Baseline Building Fan System – ASHRAE Standard 90.1-2007"

  System Types 1-2 System Types 3-4 System Types 5-8
Brake Horsepower (bhp) Not applicable 0.00094 x cfms + A 0.0013 x cfms + A
Fan Motor Efficiency (hm) Not applicable ASHRAE Standard 90.1-2007, Table 10.8 ASHRAE Standard 90.1-2007, Table 10.8
Fan Power (W) 0.3 x cfms  (bhp x 746)/$ \eta $m (bhp x 746)/$ \eta $m
The term “A” for system types 3-8, is calculated based on equipment in the proposed design using the procedure in Table 6.5.3.1.1B of ASHRAE Standard 90.1-2007. This accounts for various additional fan pressure drops associated with special conditions.

Table 6.7.3-2: "Baseline Building Fan System – ASHRAE Standard 90.1-2001"

  Fan Size System Types 1-4 System Types 5-8
Nameplate Horsepower (nhp) < 20,000 cfm 0.0012 x cfms + Pfilter + Pprocess + Prelief 0.0017 x cfms + Pfilter + Pprocess + Prelief
  => 20,000 cfm  0.0011 x cfms + Pfilter + Pprocess + Prelief 0.0015 x cfms + Pfilter + Pprocess + Prelief
Fan Motor Efficiency (hm)   ASHRAE Standard 90.1-2001, Table 10.2 ASHRAE Standard 90.1-2001, Table 10.2
Brake Horsepower (bhp)   nhp x $ \eta $m nhp x $ \eta $m
Fan Power (W)   (bhp x 746)/ $ \eta $m (bhp x 746)/ $ \eta $m
The terms Pfilter, Pprocess, and Prelief account for additional pressure drop. These are based on features in the proposed design such as special filtration, process fans, and relief or return fans. See Section 6.3.3.1 of ASHRAE Standard 90.1-2001 and the associated User's Manual for details on how to calculate these adders.

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 6.7.3-1 and Table 6.7.3-1 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

Supply Fans

Fan System Modeling Method
Applicability All fan systems
Definition

Software commonly models fans in three ways.

  • Power-per-unit-flow. The simple method is for the user to enter the electric power per unit of flow (W/cfm). This method is commonly used for unitary equipment and other small fan systems.
  • Static pressure. A more detailed method is to model the fan as a system whereby the static pressure, fan efficiency, part-load curve, and motor efficiency are specified at design conditions.
  • Break horsepower. A third method is to specify brake horsepower at design conditions instead of fan efficiency and static pressure. This is a variation of the second method whereby brake horsepower is specified in lieu of static pressure and fan efficiency. The latter two methods are commonly used for VAV and other larger fan systems.
Units List: power-per-unit-flow, static pressure or brake horsepower
Input Restrictions As designed. The power-per-unit-flow method shall be used when no fan performance data is available for the proposed design cooling system, e.g. only EER or SEER are available.
Baseline Rules 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 brake horsepower method.
Supply Fan Design Air Rated Capacity
Applicability All fan systems
Definition The design air flow rate of the supply fan(s) at design conditions. This building descriptor sets the 100% point for the fan part-load curve.
Units cfm
Input Restrictions As designed. This input should be at least as great as the sum of the design air flow specified for each of the thermal blocks that are served by the fan system. For multiple deck systems, a separate entry should be made for each deck.
Baseline Rules The program shall automatically size the air flow at each thermal block to meet the loads. The design air flow rate calculation shall be based on a 20 degree temperature differential between supply air and the room air. The supply fan design air flow rate shall be the sum of the calculated design air flow for the thermal blocks served by the fan system.
Fan Control Method
Applicability All fan systems
Definition A description of how the supply (and return/relief) fan(s) are controlled. The options include:
  • Constant volume
  • Variable-flow, inlet or discharge dampers
  • Variable-flow, inlet guide vanes
  • Variable-flow, variable speed drive (VSD)
  • Variable-flow, variable pitch blades
  • Variable-flow, other
  • Two-speed
  • Constant volume, cycling (fan cycles with heating and cooling)
Units List (see above)
Input Restrictions As designed
Baseline Rules Applicable to variable air volume systems
Based on the prescribed system type. Refer to the HVAC System Map in Figure 6.1.2-1.

Table 6.7.3-3: "Baseline Building Fan Control Method"

Baseline building System Fan Control Method
System 1 – PTAC Constant volume
System 2 – PTHP Constant volume
System 3 – PSZ-AC Constant volume
System 4 – PSZ-HP Constant volume
System 5 – Packaged VAV with Reheat Variable-flow, variable speed drive (VSD)
System 6 – Packaged VAV with PFP boxes Variable-flow, variable speed drive (VSD)
System 7 – VAV with Reheat Variable-flow, variable speed drive (VSD)
System 8 – VAV with PFP boxes Variable-flow, variable speed drive (VSD)
Supply Fan Brake Horsepower
Applicability All fan systems, except those specified using the power-per-unit-flow method
Definition The design shaft brake horsepower of the supply fan(s). This input does not need to be supplied if the Supply Fan kW is supplied.
Units Horsepower (hp)
Input Restrictions As designed. If this building descriptor is specified for the proposed design, then the Static Pressure and Fan Efficiency are not.
Baseline Rules See Table 6.7.3-1 for ASHRAE Standard 90.1-2007 and Table 6.7.3-1 for ASHRAE Standard 90.1-2001. These tables give the baseline building fan system brake horsepower. The brake horsepower for the supply fan is this value times the Supply Fan Ratio (see above).
Supply Fan Static Pressure
Applicability All fan systems, except those specified using the power-per-unit-flow method
Definition The design static pressure for the supply fan. 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 supply fan does not need to be specified if the supply fan brake horsepower (bhp) is specified.
Baseline Rules Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use Brake Horsepower.
Supply Fan Efficiency
Applicability All fan systems, except those specified using the power-per-unit-flow method
Definition The efficiency of the fan at design conditions
Units Unitless
Input Restrictions As designed. The supply fan efficiency does not need to be specified if the supply fan brake horsepower (bhp) is specified.
Baseline Rules Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use Brake Horsepower.
Supply Motor Efficiency
Applicability All supply 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 From Table 10.8 of ASHRAE Standard 90.1-2007 or Table 10.2 of ASHRAE Standard 90.1-2001
Fan Position
Applicability All supply fans
Definition The position of the supply fan relative to the cooling coil. The configuration is either draw through (fan is downstream of the coil) or blow through (fan is upstream of the coil).
Units List (see above)
Input Restrictions As designed.
Baseline Rules Draw through
Motor Position
Applicability All supply 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 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. The curve is typically represented as a quadratic equation with an absolute minimum power draw specified.
Units Unitless ratio
Input Restrictions

As designed. The default fan curve shall be selected from Equation (6.7.3-1) and Table 6.7.3-4 for the type of fan specified in the proposed design.

(6.7.3-1)

$ Greater \;of $

$ PLR = a + b \cdot FanRatio + c \cdot FanRatio^2 + d \cdot FanRatio^3 $

$ PLR = PowerMin $

where

PLR Ratio of fan power at part load conditions to full load fan power
PowerMin Minimum fan power
FanRatio Ratio of cfm at part-load to full-load cfm
a, b, c and d Constants from Table 6.7.3-4 below

Table 6.7.3-4: "Fan Curve Default Values"

 

Fan Type - Control Type a b c d %PowerMin
AF or BI riding the curvea 0.1631 1.5901 -0.8817 0.1281 70%
AF or BI with inlet vanesa 0.9977 -0.659 0.9547 -0.2936 50%
FC riding the curvea 0.1224 0.612 0.5983 -0.3334 30%
FC with inlet vanesa 0.3038 -0.7608 2.2729 -0.8169 30%
Vane-axial with variable pitch bladesa 0.1639 -0.4016 1.9909 -0.7541 20%
Any fan with VSD (use for baseline building)b 0.0013 0.1470 0.9506 -0.0998 20%
VSD with static pressure resetc -0.0031 0.0991 1.0268 -0.1128 20%
Data Sources: 
a.     ECB Compliance Supplement, public review draft, Version 1.2, March 1996, but adjusted to be relatively consistent with the curve specified in the PRM.   
b.     The fan curve for VSD is specified in Table G3.1.3.15
c.     Advanced VAV System Design Guide, California Energy Commission, CEC Publication 500,-03-082 A-11, April 2005, but adjusted to be relatively consistent with the curve specified in the PRM..
Baseline Rules Not applicable for baseline building systems 1-4. The curve for VSD fans shall be used for baseline building systems 5-8 with no adjustment for static pressure setpoint reset.
Supply Fan kW
Applicability Fan systems that use the power-per-unit-flow method
Definition The supply fan power per unit of flow.
Units kW/cfm
Input Restrictions

As designed or specified in the manufacturers’ literature. For units with rated total cooling capacities less than 120,000 Btu/h, the user may default to a value calculated as follows:

(6.7.3-2)

$$ Fan_{kw} = 0.365 \times \frac{Q_{rated}}{30,000} $$

where

Fankw The supply fan power (kW)
Qrated The rated total cooling capacity (Btu/h)
Baseline Rules Applicable when the baseline building uses the power-per-unit-flow method. Fan power is determined using Table 6.7.3-1 for ASHRAE Standard 90.1-2007 and Table 6.7.3-1 for ASHRAE Standard 90.1-2001. This power is then multiplied by the supply fan ratio.

Relief Fans

The baseline building has a return fan when the baseline building system is type 3 through 8 and the proposed design has a return 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 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 the baseline building supply fan capacity less exhaust air flow.
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 6.7.3-1 and Table 6.7.3-1) 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 Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use Brake Horsepower.
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 Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use Brake Horsepower.
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 From Table 10.8 of ASHRAE Standard 90.1-2007 or Table 10.2 of ASHRAE Standard 90.1-2001
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 Equation (6.7.3-1) and Table 6.7.3-4 for the type of fan specified in the proposed design.
Baseline Rules Not applicable for baseline building systems 1-4. The curve for VSD fans shall be used for baseline building systems 5-8 that have a return/relief fan.
Return/Relief Fan kW
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 using Table 6.7.3-1 for ASHRAE Standard 90.1-2007 and Table 6.7.3-1 for ASHRAE Standard 90.1-2001. This power is then multiplied by the return fan ratio.

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, otherwise the baseline building shall use the brake horsepower 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 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. 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.
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 6.7.3-1 or Table 6.7.3-1 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 Not applicable. When static pressure and fan efficiency are entered for the proposed design, the baseline building shall use brake horsepower.
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 As designed. The exhaust fan efficiency does not need to be specified if the return fan brake horsepower (bhp) is specified.
Baseline Rules Not applicable. When static pressure and fan efficiency are entered for the proposed design, the baseline building shall use brake horsepower.
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 10.8 of ASHRAE Standard 90.1-2007 or Table 10.2 of ASHRAE Standard 90.1-2001
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 Equation (6.7.3-1) and Table 6.7.3-4 for the type of fan specified in the proposed design.
Baseline Rules The baseline building fan curve shall be selected from Equation (6.7.3-1) and Table 6.7.3-4 for the type of fan specified in the proposed design.
Exhaust Fan KW
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 fan system power from Table 6.7.3-1 or Table 6.7.3-1