3.7.3.2 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, 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 are available for the proposed design cooling system, e.g. only EER or SEER are available.
Baseline Rules	The baseline building fan system for Systems 1, 2, 9, and 10 is specified using the power-per-unit-flow method. For systems 3-8 and 11-13, the break horsepower method shall be used for the baseline building.

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 return air for zones. The design supply air flow rate is the larger of the flow rate required to meet space conditioning requirements and the required ventilation flow rate. (The ventilation requirements may dictate supply air flow rate requirements for laboratory spaces and other spaces with special process requirements.) 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. For laboratory spaces, the design air flow rate calculation shall be based on a 17 degree temperature differential rather than 20. For baseline systems 9 and 10, the design air flow rate calculation shall be based on the temperature difference between a supply air temperature setpoint of 105 °F and the design space heating temperature setpoint, the minimum outdoor air flow rate or the airflow rate required to comply with applicable codes, whichever is greater. If the proposed design includes a fan(s) to provide non-mechanical cooling (ventilation only), the baseline building design shall include a separate fan to provide non-mechanical cooling, sized and controlled the same as the proposed building design.

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

Prescribed based on the baseline building HVAC system type. See the table below

Baseline Building System	Fan Control Method
1 PTAC	Constant volume
2 PTHP
3 PSZ-AC
4 PSZ-HP
5 PVAV Reheat	Variable-flow, variable speed drive (VSD)
6 PVAV with PFP boxes
7 VAV Reheat
8 VAV with PFP boxes
9 HV Furnace	Constant Volume
10 HV Electric	Constant Volume
11 SZ-VAV	Variable-flow, variable speed drive (VSD)
12 CZ-CV-HW	Constant Volume
13 CZ-CV-ER	Constant Volume

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). Fans for parallel fan-powered boxes are not included in fan power calculations.
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 required.
Baseline Rules	See Table 3.7.3-2

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 the brake horsepower modeling method.

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 the brake horsepower modeling method.

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	Table 3.7.3-4.

Table 3.7.3-4 – Nominal Efficiency for Electric Motors (%)

Horsepower	Efficiency	Horsepower	Efficiency	Horsepower	Efficiency
1	85.5	15	92.4	75	95.4
1.5	86.5	20	93.0	100	95.4
2	86.5	25	93.6	125	95.4
3	89.5	30	93.6	150	95.8
5	89.5	40	94.1	>= 200	95.8
7.5	91.7	50	94.5
10	91.7	60	95.0

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. If data is unavailable, a default fan curve shall be selected from COMNET Appendix H for the fan type used in the proposed design.

(Equation 3.7.3-1)

$$Greater\ of$$

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

$$PLR=PowerMin$$

where

PLR	Ratio of fan power at part load conditions to full load fan power
PowerMin	Minimum fan power ratio
FanRatio	Ratio of cfm at part-load to full-load cfm
a, b, c, d	Coefficients from Appendix H

Baseline Rules

Only applicable baseline building systems 5-8 and 11. The part load for the baseline building fan shall be determined from either Table 3.7.3-6 for Equation 3.7.3-1 with a = .0013, b = 0.1470, c = 0.9506 and d = 0.0998.

Table 3.7.3-5 Baseline Building VSD Performance

Fan Part-Load Ratio	Fraction of Full-Load Power
0.00	0.00
0.10	0.03
0.20	0.07
0.30	0.13
0.40	0.21
0.50	0.30
0.60	0.41
0.70	0.54
0.80	0.68
0.90	0.83
1.00	1.00

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:

(Equation 3.7.3-2)

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

where

Fan_kw	The supply fan power (kW)
Q_rated	The rated total cooling capacity (Btu/h)

Baseline Rules

Applicable to baseline systems 1, 2, 9, and 10. See Table 3.7.3.2

90.1-2019

Fan System Modeling Method
Applicability	All fan systems
Definition	Software commonly models fans in three ways. 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. A more detailed method is to model the fan as a system whereby the static pressure, fan efficiency, and motor efficiency are specified at design conditions. 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.
Units	List: Power-Per-Unit-Flow, Design Pressure Drop, or Brake Horsepower
Input Restrictions	As designed
Baseline Building	Systems 1, 2, 9, and 10 should use power per unit flow if allowed by the software. If not allowed used one of the other methods as described in Table 54. All other baseline system shall use the brake horsepower method. If that method is not available, static pressure method should be used.

Supply Fan Design Air Rated Capacity
Applicability	All fan systems
Definition	The design airflow 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
Baseline Building	The software shall automatically size the system airflow to meet the baseline building loads 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 the applicable codes or accreditation standards, whichever is greater. The baseline system airflow is determined by the load to be met by the airflow and the 20°F (11°C) temperature difference. The loads to be used would be the design load as determined by the sizing runs specified in Section G₃.1.2.2, not the cooling or heating capacity of the system as determined using the sizing factors, also specified in G₃.1.2.2. Using the system cooling and heating capacity will result in oversized baseline system airflows and energy cost because of the oversizing factors used in G₃.1.2.2. See additional discussion in Section 3.6.5.1 for VAV systems. spaces with special process requirements.) The supply fan design airflow rate shall be the sum of the calculated design airflow for the thermal zones served by the fan system. For laboratory spaces, the design airflow rate calculation shall be based on a 17°F temperature differential rather than 20°F. For baseline systems 9 and 10, the design supply airflow rates shall be based on the temperature difference between a SAT setpoint of 105°F and the design space heating temperature setpoint, the minimum outdoor airflow rate or the airflow rate required to comply with applicable codes, whichever is greater. If the proposed design HVAC system airflow rate based on latent loads greater than the same based on sensible loads, then the same supply-air-to-room humidity ratio difference (gr/lb) used to calculate the proposed design airflow should be used to calculate the design airflow rates for the baseline building.

Fan Control Method
Applicability	All fan systems
Definition	A description of how the supply (and return/relief) fan(s) is 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 Building	Based on the baseline system type, summarized in Table 55

Table 55. Baseline Building Fan Control Method

System No.	System Type	Fan Control
System 1	Package terminal air conditioner	Constant volume
System 2	Packaged terminal heat pump	Constant volume
System 3	Packaged roof top air conditioner	Constant volume
System 4	Packaged roof top heat pump	Constant volume
System 5	Packaged rooftop VAV with reheat	Variable volume, variable speed drive (VSD)
System 6	Packaged rooftop VAV with PFP boxes and reheat	Variable volume, VSD
System 7	Packaged rooftop VAV with reheat	Variable volume, VSD
System 8	VAV with parallel fan-powered boxes and reheat	Variable volume, VSD
System 9	Warm air furnace, gas fired	Constant volume
System 10	Warm air furnace, electric	Constant volume
System 11	Single zone VAV	Variable volume, VSD
System 12	Single zone system (CHW and HW boiler)	Constant volume
System 13	Single zone system (CHW and electric resistance)	Constant volume

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 required.
Baseline Building	Fans for parallel fan-powered boxes are not included in fan power calculations. Table 53 of this document gives 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. H₂O)
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 Building	The baseline building is defined by Table 53. This approach only works if the system has only a supply fan. If return/exhaust fans are also present in the system, then the supply fan bhp needs to be calculated and divided amongst the supply, return, and exhaust fans.

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; this is the static efficiency and does not include motor losses
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 Building	The baseline supply fan efficiency shall be 65%.

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 Building

For systems 1, 2, 9, and 10, motor efficiency is assumed to be 80%. The motor efficiency for systems 3 through 8 is determined from Table G₃.9.1 of ASHRAE Standard 90.1-2019 (Table 56 below).

Table 56. Minimum Nominal Efficiency for Electric Motors (%)

Shaft Input Power	Full-Load Motor Efficiency, %
1	82.5
1.5	84
2	84
3	87.5
5	87.5
7.5	89.5
10	89.5
15	91
20	91
25	92.4
30	92.4
40	93
50	93
60	93.6
75	94.1
100	94.5
125	94.5
150	95
200	95

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 Building	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 Building	In the air stream

Fan Part-Flow Power Curve

Applicability

All variable flow fan systems

Definition

A part-load power curve that represents the percentage full-load power draw of the supply fan as a function of the percentage full-load airflow. The curve is typically represented as a cubic equation with an absolute minimum power draw specified.

Units

Unitless ratio

Input Restrictions

The fan curve shall be selected from Equation (10) and Table 57 for the type of fan specified in the proposed design.

$$\text{Greater of}$$

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

$$PLR=PowerMin$$

(10)

Where:

PLR = Ratio of fan power at part load conditions to full load fan power

PowerMin = Minimum fan power ratio

FanRatio = Ratio of cfm at part-load to full-load cfm

a, b, c and d = Constants from Table 57

Table 57. Fan Curve Default Values

Fan Type - Control Type	A	B	c	d	%Power_Min
Multi Zone VAV with Airfoil (AF) or Backward Incline (BI) riding the curve^(a)	0.1631	1.5901	-0.8817	0.1281	70%
Multi Zone VAV with AF or BI with inlet vanes^(a)	0.9977	-0.659	0.9547	-0.2936	50%
Multi Zone VAV with Forward Curved (FC) fans riding the curve^(a)	0.1224	0.612	0.5983	-0.3334	30%
Multi Zone VAV with FC with inlet vanes^(a)	0.3038	-0.7608	2.2729	-0.8169	30%
Multi Zone VAV with vane-axial with variable pitch blades^(a)	0.1639	-0.4016	1.9909	-0.7541	20%
Multi Zone VAV with VSD and fixed SP setpoint^(b)	0.0013	0.1470	0.9506	-0.0998	20%
Multi zone VAV with static pressure reset^(c)	0.04076	0.0881	-0.0729	0.9437	10%
Single zone VAV fan^(d)	0.027828	0.026583	-0.087069	1.030920	10%
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) This is the good SP reset VSD fan curve from the advanced VAV design guide used for MZVAV systems. (d) This is the perfect SP reset VSD fan curve from the advanced VAV design guide used for SZVAV systems. http://www.energy.ca.gov/2003publications/CEC-500-2003-082/CEC-500-2003-082-A-11.PDF

Baseline Building

Not applicable for baseline building systems 1 through 4. Baseline systems 5 through 8 will use the curve for “Multi zone VAV with fixed static pressure setpoint” curve. System 11 shall use the “Single zone VAV fan” curve. Constant volume fans are used for systems 9, 10, 12, and 13 and hence the descriptor is not applicable.

Supply Fan Power Index (kW/cfm)
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
Baseline Building	Applicable when the baseline building uses the power-per-unit-flow method. Fan power is determined using Table 53 of this document. This power is then multiplied by the supply fan ratio.

Static Pressure Reset Controls
Applicability	All VAV fan systems. Baseline systems 5 through 8.
Definition	Static pressure reset controls, reset the fan static pressure for VAV systems-based zone damper position. For systems with DDC of individual zone boxes reporting to the central control panel, static pressure setpoint shall be reset based on the zone requiring the most pressure.
Units	Unitless
Input Restrictions	As designed. If static pressure reset is implemented in the proposed system, the curve for “Multi zone VAV with static pressure reset curve” shall be used.
Baseline Building	Not applicable for baseline building systems 1 through 4 or 9, 10, 11, 12, and 13. The curve for “Multi Zone VAV with VSD and fixed SP setpoint”’ shall be used for baseline building systems 5 through 8.