3.7.3.2 Supply Fans

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.

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.

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

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).

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.

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%.

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 (%)

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

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

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.

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

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.

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.

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.

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 total static pressure and fan efficiency.

Units

List: Power-Per-Unit-Flow, Design Pressure Drop, or Brake Horsepower

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

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 57. All other baseline system shall use the brake horsepower method. If that method is not available, static pressure method should be used.

G3.3 Minor Alterations

Use one of the methods. The same method should be used in the baseline and proposed.

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

G3.2 New Construction/Major Alterations

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 G3.2.2.2, not the cooling or heating capacity of the system as determined using the sizing factors, also specified in G3.2.2.2. Using the system cooling and heating capacity will result in oversized baseline system airflows and energy cost because of the oversizing factors used in G3.2.2.2.

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.

G3.3 Minor Alterations

Use as designed sizing assumptions. If unknown, default to the assumptions described under G3.2 for this descriptor.

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

G3.2 New Construction/Major Alterations

Based on the baseline system type, summarized in Table 59

G3.3 Minor Alterations

The baseline should be modeled as minimally compliant with Standard 90.1-2022 Section 6.5.3.2.1. According to Standard 90.1-2022 Section 6.5.3.2.1 each baseline cooling system listed in Table 75 shall be modeled to vary the supply airflow as a function of load with the following requirements:

• DX and chilled-water cooling units that control the capacity of the mechanical cooling directly based on space temperature shall be modeled with a minimum of two stages of fan control. Low or minimum speed shall not exceed 66% of full speed. At low or minimum speed, the fan system shall be modeled such that the fan power is no more than 40% of the fan power at full fan speed. Low or minimum speed shall be used during periods of low cooling load and ventilation-only operation. 
• All other units, including DX cooling units and chilled-water units that control the space temperature by modulating the airflow to the space, shall be modeled with modulating fan control. Minimum speed shall not exceed 50% of full speed. At minimum speed, the fan power shall be modeled at 30% of the fan power at full fan speed. Low or minimum speed shall be modeled during periods of low cooling load and ventilation-only operation.
• Units that include an air economizer to meet the requirements of Section 6.5.1 shall be modeled with a minimum of two speeds of fan control during economizer operation.

Exceptions to 6.5.3.2.1:

1. Modulating fan control is not required to be modeled in the baseline for chilled-water and evaporative cooling units with <1 hp fan motors if the units are not used to provide ventilation air and if the indoor fan cycles with the load.
2. If the volume of outdoor air required to meet the ventilation requirements of Standard 62.1 at low speed exceeds the air that would be delivered at the speed defined above then the minimum speed shall be modeled to provide the required ventilation air.

Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.3.2.1 requirements are inapplicable.

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

G3.2 New Construction/Major Alterations

Fans for parallel fan-powered boxes are not included in fan power calculations.

Table 57 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).

G3.3 Minor Alterations

90.1 G3.3.2.8 (d) requires that for fan systems included in the scope of the retrofit using the BHP method of modeling proposed fan power, BHP should be the same as the proposed design up to the limit prescribed in Section 6.5.3.1, whichever is smaller.

Exception to 90.1 G3.3.2.8 (d): When a proposed design includes energy recovery, and all the following conditions are met,

1. Standard 90.1-2022 Section 6.5.6 requirements are applicable based on the scope of the alteration and the language in Standard 90.1-2022 Section 6.1.4, and
2. Exhaust air energy recovery is not required to be modeled in the baseline according to Standard 90.1-2022 Section 6.5.6

then the fan power of the baseline system shall be equal to either the proposed design system or the fan power limit in Section 6.5.3.1 calculated without fan power credit for energy recovery, whichever is less.

Fans not included in the scope of the alteration should be modeled the same in the baseline and proposed.

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

G3.2 New Construction/Major Alterations

The baseline building is defined by Table 57. 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.

G3.3 Minor Alterations

90.1 G3.3.2.8 (d) requires that for fan systems included in the scope of the retrofit using the using the pressure drop method of modeling proposed fan power, the pressure drop should be the same as the proposed design up to the limit that corresponds to fan power allowances prescribed in Section 6.5.3.1, whichever is smaller.

Exception to 90.1 G3.3.2.8 (d): When a proposed design includes energy recovery, and all the following conditions are met,

1. Standard 90.1-2022 Section 6.5.6 requirements are applicable based on the scope of the alteration and the language in Standard 90.1-2022 Section 6.1.4, and
2. Exhaust air energy recovery is not required to be modeled in the baseline according to Standard 90.1-2022 Section 6.5.6

then the fan power of the baseline system shall be equal to either the proposed design system or the fan power limit in Section 6.5.3.1 calculated without fan power credit for energy recovery, whichever is less.

Fans not included in the scope of the alteration should be modeled the same in the baseline and proposed.

Applicability

All fan systems, except those specified using the power-per-unit-flow and BHP methods

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

G3.2 New Construction/Major Alterations

The baseline supply fan efficiency shall be 65%.

G3.3 Minor Alterations

If using the pressure drop method of modeling proposed fan power, same as proposed. If unknown, a default of 65% can be used.

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. Motor efficiency is required to meet the mandatory efficiency requirements in Standard 90.1-2022 Section 10.4.1 as applicable.

Baseline Building

G3.2 New Construction/Major Alterations

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 90.1-2022 (Table 60 below).

Table 60. Minimum Nominal Efficiency for Electric Motors (%) per Table G3.9.1

G3.3 Minor Alterations

For fan systems included in the scope of the retrofit, motor efficiency should be modeled with the minimum efficiency requirement of Standard 90.1-2022 Section 10.4.1 as applicable. Fractional horsepower fan motors (motors 1/12 hp or greater and less than 1 hp) shall be modeled with a motor efficiency of 70%. If any of the exceptions apply, then it shall be modeled the same in the baseline and proposed.

Fans not included in the scope of the alteration should be modeled the same in the baseline and proposed.

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

G3.2 New Construction/Major Alterations

Draw through.

G3.3 Minor Alterations

Same as proposed design.

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

G3.2 New Construction/Major Alterations

In the air stream.

G3.3 Minor Alterations

Same as the proposed design.

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 61 for the type of fan specified in the proposed design.

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 61

Table 61. Fan Curve Default Values

Baseline Building

G3.2 New Construction/Major Alterations

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.

G3.3 Minor Alterations

The baseline shall be modeled using the “Multi zone VAV with static pressure reset(c)” curve.

Exception: the fan part load curve should be modeled the same in the baseline and proposed if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.3.2.3 requirements are inapplicable.

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

G3.2 New Construction/Major Alterations

Applicable when the baseline building uses the power-per-unit-flow method. Fan power is determined using Table 57 of this document. This power is then multiplied by the supply fan ratio.

G3.3 Minor Alterations

90.1 G3.3.2.8 (d) requires that for fan systems included in the scope of the retrofit using the power-per-unit-flow method of modeling proposed fan power, the kW/CFM should be the same as the proposed design up to the limit that corresponds to fan power allowances prescribed in Section 6.5.3.1, whichever is smaller.

Exception to 90.1 G3.3.2.8 (d): When a proposed design includes energy recovery, and all the following conditions are met,

1. Standard 90.1-2022 Section 6.5.6 requirements are applicable based on the scope of the alteration and the language in Standard 90.1-2022 Section 6.1.4, and
2. Exhaust air energy recovery is not required to be modeled in the baseline according to Standard 90.1-2022 Section 6.5.6

then the fan power of the baseline system shall be equal to either the proposed design system or the fan power limit in Section 6.5.3.1 calculated without fan power credit for energy recovery, whichever is less.

Fans not included in the scope of the alteration should be modeled the same in the baseline and proposed.

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

G3.2 New Construction/Major Alterations

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.

G3.3 Minor Alterations

The baseline shall be modeled using the “Multi zone VAV with static pressure reset(c)” curve.

Exception: the fan part load curve should be modeled the same in the baseline and proposed if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.3.2.3 requirements are inapplicable.