3.7.5.2 Direct Expansion

Direct Expansion Cooling Efficiency (COP)

Applicability

Packaged DX equipment

Definition

The cooling efficiency of a direct expansion (DX) cooling system at ARI rated conditions as a ratio of output over input in Btu/h per W, excluding fan energy. Fan energy shall be modeled separately according to Section 3.7.3 of this document.

For all unitary and applied equipment where the fan energy is part of the equipment efficiency rating, the equipment COP shall be calculated as follows:

For EER equipment ratings

(Equation 3.7.5.2-1)

$COP_{nfcooling} = 7.84\times 10^{-8} \cdot EER \cdot Q + 0.338 \cdot EER$

For SEER equipment ratings

(Equation 3.7.5.2-2)

$COP_{nfcooling} = -0.0076 \cdot SEER^{2} + 0.3796 \cdot SEER$

where

COPnfcooling	Packaged equipment cooling efficiency without the fan
Q	the AHRI-rated cooling capacity in Btu/h
EER, SEER	The energy efficiency ratio or seasonal energy efficiency ratio determined at AHRI test conditions, including the fan.

Units

Unitless ratio

Input Restrictions

As designed. Specify the COPnf as described above based on SEER for packaged equipment with net cooling capacity less than 65,000 Btu/h from manufacturer’s literature. For equipment with capacity above 65,000 Btu/h, use EER.

Baseline Rules

Calculate COP_nfcooling using the EER or SEER from the table below:

Equipment Capacity	Rated Efficiency	Test Procedure
<65,000 Btu/h	9.7 SEER	ARI 210/240
≥65,000 Btu/h and <135,000 Btu/h	9.9 EER	ARI 340/360
≥135,000 Btu/h and <240,000 Btu/h	9.1 EER
>240,000 Btu/h	8.8 EER

Direct Expansion Cooling Efficiency Adjustment Curve

Applicability

Packaged DX equipment

Definition

A curve that varies the cooling efficiency of a direct expansion (DX) coil as a function of evaporator conditions, condenser conditions, and for small packaged equipment, part-load ratio.

(Equation 3.7.5.2-3)

$PLR = \frac{Q_{operating}}{Q_{available(t_{wb},t_{odb/wt})}}$

(Equation 3.7.5.2-4)

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

For air-cooled DX systems

(Equation 3.7.5.2-5)

$EIR\_FT = a + b\cdot t_{wb}+c\cdot t_{wb}^{2}+d\cdot t_{odb}+e\cdot t_{odb}^{2}+f\cdot t_{wb}\cdot t_{odb}$

For water-cooled DX systems

(Equation 3.7.5.2-6)

$EIR\_FT = a + b\cdot t_{wb}+c\cdot t_{wb}^{2}+d\cdot t_{wt}+e\cdot t_{wt}^{2}+f\cdot t_{wb}\cdot t_{wt}$

(Equation 3.7.5.2-7)

$P_{operating}=P_{rated}\cdot EIR\_FPLR\cdot EIR\_FT\cdot CAP\_FT$

Where

PLR	Part load ratio based on available capacity (not rated capacity)
EIR-FPLR	A multiplier on the rated power draw (P_rated) to account for the part load ratio. See the equation above and choose coefficients from Appendix H based on the type of equipment.
EIR-FT	A multiplier on the rated power draw (P_rated) to account for the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature. See the equation above and choose coefficients from Appendix H.
Q_operating	Present load on air conditioner (Btu/h)
Q_available	Air conditioner available capacity at present evaporator and condenser conditions (in Btu/h).
t_wb	The entering coil wet-bulb temperature (°F)
t_wt	The water supply temperature (°F)
t_odb	The outside-air dry-bulb temperature (°F)
P_rated	Rated power draw at ARI conditions (kW)
P_operating	Power draw at specified operating conditions (kW)

Note: if an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms.

Baseline Rules

Use default curves.

Number of Cooling Stages
Applicability	DX systems with multiple stages
Definition	This applies to systems with multiple compressors or multiple discrete stages of cooling. This system is a packaged unit with multiple stages of cooling. Systems with unequally sized compressors may have additional cooling stages.
Units	None (Integer)
Input Restrictions	As Designed
Baseline Rules	All baseline DX system single stage.

Total Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	This provides the total cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage cooling capacity is 4 tons (48,000 Btu/h) and the total cooling capacity is 8 tons (96,000 Btu/h), the capacity is expressed as “0.50” for that stage.
Units	Array of fractions
Input Restrictions	As Designed
Baseline Rules	Not Applicable

Sensible Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	This provides the sensible cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated sensible cooling capacity for the unit. For example, if the stage sensible cooling capacity is 3.5 tons (42,000 Btu/h) and the total sensible cooling capacity is 7 tons (72,000 Btu/h), the capacity is expressed as "0.5" for that stage."
Units	Array of fractions
Input Restrictions	As designed
Baseline Rules	Not applicable

Supply Air Temperature Reset by Stage
Applicability	DX systems with multiple stages
Definition	This provides the cooling supply air temperature setpoint deviation from the cooling design supply air temperature, specified in the building descriptor Cooling Supply Air Temperature.
Units	The temperature reset is expressed as an array, with each entry a deviation from the design supply air temperature. For example, an entry of “5” for a stage would indicate a 5°F reset (for example, 60°F from 55°F).
Input Restrictions	As designed. Array of temperature differences, in degrees F
Baseline Rules	Not Applicable

Number of Heating Stages
Applicability	DX systems with multiple stages
Definition	The number of heating stages provided by the system. Multiple stages could be provided via a heat pump or via a multiple-stage gas furnace.
Units	Integer
Input Restrictions	As designed
Baseline Rules	Not Applicable

Heating Capacity by Stage
Applicability	DX systems with multiple stages
Definition	This provides the total heating capacity of each heating stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage heating capacity is 48,000 Btu/h and the heating capacity is 96,000 Btu/h, the capacity is expressed as “0.50” for that stage.
Units	Array of fractions
Input Restrictions	As Designed
Baseline Rules	Not Applicable.

Heating Supply Air Temperature by Stage
Applicability	DX systems with multiple stages
Definition	This provides the heating supply air temperature setpoint deviation from the design heating supply air temperature, specified in the building descriptor Cooling Heating Supply Air Temperature.
Units	The temperature reset is expressed as an array, with each entry a deviation from the design supply air temperature. For example, an entry of “-10” for a stage would indicate a 10°F reset (for example, 95°F from 105°F).
Input Restrictions	As Designed. Array of temperature differences, in degrees F
Baseline Rules	Not Applicable

Supply Fan Low Speed Ratio
Applicability	DX systems with multiple stages and two-speed fans
Definition	This specifies the low fan speed setting on a Single Zone VAV system or DX system with multiple cooling stages.
Units	None (fraction)
Input Restrictions	As Designed
Baseline Rules	Not Applicable.

Supply Fan Low Power Ratio
Applicability	DX systems with multiple stages and two-speed fans
Definition	This specifies the fraction of full load fan power corresponding to low fan speed operation on a Single Zone VAV system or DX system with multiple cooling stages.
Units	None (fraction)
Input Restrictions	As Designed
Baseline Rules	Not Applicable.

Piping Insulation
Applicability	All projects
Definition	Thermal insulation on piping systems for service hot water, steam piping, chilled water for cooling and hot water for space heating.
Units	List (see above)
Input Restrictions	Not modeled
Baseline Rules	Not modeled

Minimum Unloading Ratio
Applicability	Packaged systems which use hot-gas bypass during low load conditions
Definition	The upper end of the hot-gas bypass operating range. This is the percentage of peak cooling capacity below which hot-gas bypass will operate.
Units	Ratio
Input Restrictions	As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered. A maximum of 0.5 is allowed for units with a peak cooling capacity of 240 kBtu/h (20 tons) or less, and a maximum value of 0.25 is allowed for units with a peak cooling capacity greater than 240 kBtu/h.
Baseline Rules	No hot-gas bypass

Minimum HGB Ratio
Applicability	Packaged systems which use hot-gas bypass during low load conditions
Definition	The lower end of the hot-gas bypass operating range. The percentage of peak cooling capacity below which hot-gas bypass will no longer operate (i.e. the compressor will cycle).
Units	Fraction between 0 and 1
Input Restrictions	As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.
Baseline Rules	Not applicable

Condenser Type
Applicability	All direct expansion systems including heat pumps
Definition	The type of condenser for a direct expansion (DX) cooling system. The choices are: Air-cooled Water-cooled Air-cooled with evaporative pre-cooler
Units	List (see above)
Input Restrictions	As designed
Baseline Rules	Baseline HVAC systems 1 through 6 are all air cooled.

Condenser Flow Type
Applicability	All direct expansion systems including heat pumps
Definition	Describes water flow control for a water-cooled condenser. The choices are: Fixed Flow Two-position Variable Flow
Units	List (see above)
Input Restrictions	Default to fixed flow. If the variable-flow is selected, the software must indicate that supporting documentation is required on the output forms.
Baseline Rules	Not applicable. Baseline DX systems are always air cooled.

Direct Expansion Cooling Efficiency

Applicability

Packaged DX equipment

Definition

The cooling efficiency of a DX cooling system at ARI rated conditions as a dimensionless ratio of output over input, excluding fan energy. The abbreviation used for this full-load efficiency is COP_nf.cooling.

Fan energy shall be modeled separately according to Section 3.7.3 of this document.

Units

Unitless

Input Restrictions

As designed. Calculated as follows:

$COP_{nf,cooling}=\frac{Q_{t,gross,rated}}{(\textit{Total Input Power}[\text{W}]-W_{fan})\times 3.412[(\text{Btu/h})/\text{W}]}$

(21)

Where:

Q_{t.gross.rated} = The AHRI rated total cooling capacity of a packaged unit

Baseline Building

For Baseline Systems 1, 2, 3, 4, 5 and6:

Use the COP_nf.cooling from Table 64 (Standard 90.1-2019, Table G₃.5.4) packaged terminal air conditioners for System 1 or packaged terminal heat pumps for System 2.Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be taken from Standard 90.1-2019 Tables G₃.5.1, G₃.5.2, and G₃.5.4, and shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Table 64. Efficiency Requirements For Baseline Systems with PTAC and PTHPs (efficiency ratings excluding supply fan power)

Equipment Capacity	Rated Efficiency (EER cooling, COP heating)
PTAC All Capacities (cooling mode)	3.2 COP_nfcooling
PTHP All Capacities (cooling mode)	3.1 COP_nfcooling
PTHP All Capacities (heating mode)	3.1 COP_nfheating

For Baseline Systems 3, 4, 5, 6:

Equipment cooling efficiencies for DX coils shall be modeled in accordance to Table 65 and Table 66 (Standard 90.1-2019 Table G₃.5.2 for System 4 and Table G₃.5.1 for Systems 3, 5 and 6), which specify COP_nf.coolingfor packaged air conditioners. Baseline HVAC system types 5 or 6 efficiencies taken from Table 65 shall be based on the cooling equipment capacity of a single floor when grouping identical floors.

Table 65. Performance Rating Method Air Conditioners: System 3 (efficiency ratings excluding supply fan power)

Equipment Type	Size Category	Heating Section Type	Subcategory or Rating Condition	Minimum Efficiency
Air conditioners, air cooled	<65,000 Btu/h	All	Single Package	3.0 COP_nf.cooling
	≥65,000 Btu/h and <135,000 Btu/h		Split-system and single-package	3.5 COP_nf.cooling
	≥135,000 Btu/h and<240,000 Btu/h			3.4 COP_nf.cooling
	≥240,000 Btu/h and <760,000 Btu/h			3.5 COP_nf.cooling
	≥760,000 Btu/h			3.6 COP_nf.cooling

Table 66. Performance Rating Method Electrically Operated Unitary and Applied Heat Pumps: System 4

Equipment Type	Size Category	Heating Section Type	Subcategory or Rating Condition	Minimum Efficiency
Air-cooled, (cooling mode)	<65,000 Btu/h	All	Single-package	3.0 COP_nf.cooling
	≥65,000 Btu/h and <135,000 Btu/h		Split-system and single-package	3.4 COP_nf.cooling
	≥135,000 Btu/h and<240,000 Btu/h			3.2 COP_nf.cooling
	≥240,000 Btu/h			3.1 COP_nf.cooling
Air-Cooled (heating-mode)	<65,000 Btu/h (cooling capacity)	All	Single-package	3.4 COP_nfheating
	≥65,000 Btu/h and <135,000 Btu/h (cooling capacity)		47°F db/43°F wb outdoor air	3.4 COP_nfheating
	≥65,000 Btu/h and <135,000 Btu/h (cooling capacity)		17°F db/15°F wb outdoor air	2.3 COP_nfheating
	≥135,000 Btu/h (cooling capacity)		47°F db/43°F wb outdoor air	3.4 COP_nfheating
	≥135,000 Btu/h (cooling capacity)		17°F db/15°F wb outdoor air	2.1 COP_nfheating

Direct Expansion Cooling Efficiency Temperature Adjustment Curve

Applicability

Packaged DX equipment

Definition

A curve that varies the cooling efficiency of a DX coil as a function of evaporator conditions, condenser conditions. For air cooled DX systems:

$EIR\_FT = a + b\times t_{wb}+c\times t_{wb}^{2}+d\times t_{odb}+e\times t_{odb}^{2}+f\times t_{wb}\times t_{odb}$

(22)

For water cooled DX systems:

$EIR\_FT = a + b\times t_{wb}+c\times t_{wb}^{2}+d\times t_{wt}+e\times t_{wt}^{2}+f\times t_{wb}\times t_{wt}$

(23)

$P_{operating}=P_{rated}\times EIR\_FPLR\times EIR\_FT\times CAP\_FT$

(24)

Where:

PLR = Part load ratio based on available capacity (not rated capacity)

EIR-FT = A multiplier on the EIR to account for the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature

Q_operating = Present load on heat pump (Btu/h)

Q_available = Heat pump available capacity at present evaporator and condenser conditions (in Btu/h)

t_wb = The entering coil wet-bulb temperature (°F)

t_wt = The water supply temperature (°F)

t_odb = The outside-air dry-bulb temperature (°F)

P_rated = Rated power draw at ARI conditions (kW)

P_operating = Power draw at specified operating conditions (kW)

Note: If an air cooled unit employs an evaporative condenser, t_odb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

Table 67. Cooling System Coefficients for EIR-FT

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (Other)
a	2.0280385	-1.8394760	-0.6550461	-1.0639310
b	-0.0423091	0.0751363	0.0388910	0.0306584
c	0.0003054	-0.0005686	-0.0001925	-0.0001269
d	0.0149672	0.0047090	0.0013046	0.0154213
e	0.0000244	0.0000901	0.0001352	0.0000497
f	-0.0001640	-0.0001218	-0.0002247	-0.0002096
Rated _CWT	Max 85°F, Min 60°F	Max 85°F, Min 60°F	NA	NA
Rated _EWBT	Max 57°F, Min 77°F	Max 57°F, Min 77°F	Max 77°F, Min 57°F	Max 77°F, Min 57°F
Rated _OADBT	NA	NA	Max 115°F, Min 75°F	Max 115°F, Min 75°F
Source: (CEC 2013)

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms.

Baseline Building

Use default curves specified above, also documented in COMNET Appendix H (COMNET 2017)

Direct Expansion Part-Load Efficiency Adjustment Curve

Applicability

Packaged systems with DX cooling

Definition

A normalized performance adjustment curve to the rated efficiency (energy input ratio [EIR]) that describes how the efficiency varies at part-load conditions. At a value of 1 (full load), the normalized efficiency is 1.

The default curves are given as follows as adjustments to the EIR:

$PLR=\frac{Q_{operating}}{Q_{available}(t_{wb}, t_{odb/wt})}$

(25)

$EIR_{FPLR}=a+b\times PLR+c\times PLR^2 + d\times PLR^3$

$PLF_{FPLR}=a+b\times PLR+c\times PLR^2 + d\times PLR^3$

(26)

Where:

PLR = Part load ratio based on available capacity (not rated capacity)

EIR-FPLR = A multiplier on the EIR to account for the part load ratio

Q_operating = Present load on heat pump (Btu/h)

Q_available = Heat pump available capacity at present evaporator and condenser conditions (in Btu/h)

twb = The entering coil wet-bulb temperature (°F)

twt = The water supply temperature (°F)

todb = The outside-air dry-bulb temperature (°F)

This curve may take the form of a part-load factor (PLF) or EIR-FLPR, which is the fraction of time the unit must run to meet the part-load for that hour. For example, at 40% of full load, the equipment might need to run 50% of the hour (for cycling losses).

Note that for small packaged equipment with SEER ratings <65,000 Btu/h, the part-load efficiency curve is set to no degradation, since the part-load degradation is built-into the DX cooling efficiency temperature adjustment curve (Air Source, other)

Default curves are provided for the different major classes of equipment.

Units

Coefficients

Input Restrictions

The coefficients should sum to 1 (within a small tolerance). This corresponds to a curve output of 1 for an input of 1. User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms.

Baseline Building

The baseline part-load efficiency adjustment curves are shown in the tables below:

Table 68.Cooling System Coefficients for EIR-FPLR

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (PSZ with Cap<65,000 Btu/h)	Air-Source (Other)
a	0.1250000	0.2012301	0.1250000	0	0.2012301
b	0.8750000	-0.0312175	0.8750000	1	-0.0312175
c	0.0000000	1.9504979	0.0000000	0	1.9504979
d	0.0000000	-1.1205105	0.0000000	0	-1.1205105

Table 69. Cooling System Coefficients for Part-Load Factor (PLF) Correlation (EnergyPlus)

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (PSZ with Cap<65,000 Btu/h)	Air-Source (Other)
a	0.85	0	0.85	1	0
b	0.15	5.1091	0.15	0	5.1091
c	0	-8.5515	0	0	-8.5515
d	0	4.4744	0	0	4.4744
Source: (CEC 2013)

Direct Expansion Number of Cooling Stages
Applicability	DX systems with multiple stages
Definition	This applies to systems with multiple compressors or multiple discrete stages of cooling. This system is a packaged unit with multiple stages of cooling. Systems with unequally sized compressors may have additional cooling stages.
Units	None (integer)
Input Restrictions	As designed
Baseline Building	All baseline DX systems are single stage

Total Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	This provides the total cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage cooling capacity is 4 tons (48,000 Btu/h) and the total cooling capacity is 8 tons (96,000 Btu/h), the capacity is expressed as “0.5” for that stage.
Units	Array of fractions
Input Restrictions	As designed
Baseline Building	Not applicable for baseline systems

Sensible Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	Provides the sensible cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated sensible cooling capacity for the unit. For example, if the stage sensible cooling capacity is 3.5 tons (42,000 Btu/h) and the total sensible cooling capacity is 7 tons (72,000 Btu/h), the capacity is expressed as “0.5” for that stage.
Units	Array of fractions
Input Restrictions	As designed
Baseline Building	Applicable baseline systems 1 through 6. The sensible cooling capacity of the baseline building is oversized by 15%. Sizing calculations shall be based on 1% dry-bulb and 1% wet-bulb design conditions.

Supply Fan Low Speed Ratio
Applicability	Single zone DX systems with multiple stages and two-speed fans or VAV fans
Definition	Specifies the low fan speed setting on a single zone VAV system or DX system with multiple cooling stages
Units	None (fraction)
Input Restrictions	As designed
Baseline Building	Not applicable

Supply Fan Low Power Ratio
Applicability	Single zone DX systems with multiple stages and two-speed fans or VAV fans
Definition	Specifies the fraction of full load fan power corresponding to low fan speed operation on a single zone VAV system or DX system with multiple cooling stages
Units	None (fraction)
Input Restrictions	As designed
Baseline Building	Not applicable

Piping Insulation
Applicability	All projects
Definition	Thermal insulation on piping systems for service hot water, steam piping, chilled water for cooling, and hot water for space heating
Units	List (see above)
Input Restrictions	Not modeled
Baseline Building	Not modeled

Minimum Unloading Ratio
Applicability	Packaged systems that use hot-gas bypass during low load conditions
Definition	The minimum unloading ratio is where the equipment capacity can no longer be reduced by unloading and must be false loaded to meet smaller cooling loads. A typical false loading strategy is hot-gas bypass. The minimum unloading ratio is the upper end of the hot-gas bypass operating range. This is the percentage of peak cooling capacity below the range in which hot-gas bypass will operate. The actual unloading ratio shall be set to 50% of the user-entered minimum unloading ratio, with hot-gas-bypass operating below this level.
Units	Ratio
Input Restrictions	As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.
Baseline Building	Not applicable

Minimum HGB Ratio
Applicability	Packaged systems that use hot-gas bypass during low load conditions
Definition	The lower end of the hot-gas bypass operating range. The percentage of peak cooling capacity below which hot-gas bypass will no longer operate (i.e., the compressor will cycle).
Units	Fraction (between 0 and 1)
Input Restrictions	As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.
Baseline Building	Not applicable

Condenser Type

Applicability

All DX systems including heat pumps

Definition

The type of condenser for a DX cooling system. The choices are:

Air-cooled
Water-cooled
Air-cooled with evaporative pre-cooler

Units

List (see above)

Input Restrictions

As designed

Baseline Building

Based on prescribed system type

Table 70. Baseline Building Condenser Type

Baseline Building System	Condenser Type
1 – PTAC	Air cooled
2 – PTHP	Air cooled
3 – PSZ AC	Air cooled
4 – PSZ HP	Air cooled
5 – PVAV reheat	Air cooled
6 – Packaged VAV with PFP Boxes	Air cooled
7 – VAV with Reheat	N/A
8 – VAV with PFP Boxes	N/A
9 – Heating and Ventilation	N/A
10 – Heating and Ventilation	N/A
11 – Single Zone VAV	N/A
12- Single Zone CAV HW	N/A
13- Single Zone- CAV ER	N/A

Condenser Flow Type
Applicability	All DX systems including heat pumps
Definition	Describes water flow control for a water cooled condenser. The choices are: Fixed flow Two-position Variable flow
Units	List (see above)
Input Restrictions	As designed. For variable or staged capacity equipment, the minimum-unload ratio must be set properly for the simulation program. NOTE: If the variable-flow is selected, the software must indicate that supporting documentation is required on the output forms.
Baseline Building	Not applicable

Direct Expansion Cooling Efficiency

Applicability

Packaged DX equipment

Definition

The cooling efficiency of a DX cooling system at AHRI rated conditions as a dimensionless ratio of output over input, excluding fan energy. The abbreviation used for this full-load efficiency is COP_nf.cooling.

Fan energy shall be modeled separately according to Section 3.7.3 of this document.

Units

Unitless

Input Restrictions

As designed. Calculated as follows:

$COP_{nf,cooling}=\frac{Q_{t,gross,rated}}{(\textit{Total Input Power}[\text{W}]-W_{fan})\times 3.412[(\text{Btu/h})/\text{W}]}$

(22)

Where:

Q_{t.gross.rated} = The AHRI rated total cooling capacity of a packaged unit

Baseline Building

G3.2 New Construction/Major Alterations

For Baseline Systems 1, 2, 3, 4, 5 and 6:

Use the COP_nf.cooling from Table 68 (Standard 90.1-2022, Table G3.5.4) packaged terminal air conditioners for System 1 or packaged terminal heat pumps for System 2.Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be taken from Standard 90.1-2022 Tables G3.5.1, G3.5.2, and G3.5.4, and shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Table 68. Efficiency Requirements for Baseline Systems with PTAC and PTHPs (efficiency ratings excluding supply fan power)

Equipment Capacity	Rated Efficiency (EER cooling, COP heating)
PTAC All Capacities (cooling mode)	3.2 COP_nfcooling
PTHP All Capacities (cooling mode)	3.1 COP_nfcooling
PTHP All Capacities (heating mode)	3.1 COP_nfheating

For Baseline Systems 3, 4, 5, 6:

Equipment cooling efficiencies for DX coils shall be modeled in accordance to Table 69 and Table 70 (Standard 90.1-2022 Table G3.5.2 for System 4 and Table G3.5.1 for Systems 3, 5 and 6), which specify COP_nf.coolingfor packaged air conditioners. Baseline HVAC system types 5 or 6 efficiencies taken from Table 69 shall be based on the cooling equipment capacity of a single floor when grouping identical floors.

G3.3 Minor Alterations

All direct expansion HVAC equipment included in the scope of the retrofit shall be modeled at the minimum efficiency levels, both part load and full load, in accordance with Sections 6.4. Where the efficiency rating includes supply fan energy, calculate the minimum COPnfcooling using the equation from Standard 90.1-2022 Section 12.5.2(c). Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Applicable equations from Standard 90.1-2022 Section 12.5.2(c):

COPnfcooling = 7.84E-8 × EER × Q + 0.338 × EER

COPnfcooling = –0.0076 × SEER² + 0.3796 × SEER

COPnfcooling = 0.3322 × EER – 0.2145 (for use with packaged terminal air conditioning (PTAC) and heat pump (PTHP) units)

Where:

COPnfcooling = Packaged equipment DX cooling energy efficiency

Q = AHRI-rated cooling capacity in Btu/h.

EER and SEER shall be at AHRI test conditions.

Table 69. Performance Rating Method Air Conditioners: G3.2 System 3 (efficiency ratings excluding supply fan power)

Equipment Type	Size Category	Heating Section Type	Subcategory or Rating Condition	Minimum Efficiency
Air conditioners, air cooled	<65,000 Btu/h	All	Single Package	3.0 COP_nf.cooling
	≥65,000 Btu/h and <135,000 Btu/h		Split-system and single-package	3.5 COP_nf.cooling
	≥135,000 Btu/h and<240,000 Btu/h			3.4 COP_nf.cooling
	≥240,000 Btu/h and <760,000 Btu/h			3.5 COP_nf.cooling
	≥760,000 Btu/h			3.6 COP_nf.cooling

Table 70. Performance Rating Method Electrically Operated Unitary and Applied Heat Pumps: G3.2 System 4

Equipment Type	Size Category	Heating Section Type	Subcategory or Rating Condition	Minimum Efficiency
Air-cooled, (cooling mode)	<65,000 Btu/h	All	Single-package	3.0 COP_nf.cooling
	≥65,000 Btu/h and <135,000 Btu/h		Split-system and single-package	3.4 COP_nf.cooling
	≥135,000 Btu/h and<240,000 Btu/h			3.2 COP_nf.cooling
	≥240,000 Btu/h			3.1 COP_nf.cooling
Air-Cooled (heating-mode)	<65,000 Btu/h (cooling capacity)	All	Single-package	3.4 COP_nfheating
	≥65,000 Btu/h and <135,000 Btu/h (cooling capacity)		47°F db/43°F wb outdoor air	3.4 COP_nfheating
	≥65,000 Btu/h and <135,000 Btu/h (cooling capacity)		17°F db/15°F wb outdoor air	2.3 COP_nfheating
	≥135,000 Btu/h (cooling capacity)		47°F db/43°F wb outdoor air	3.4 COP_nfheating
	≥135,000 Btu/h (cooling capacity)		17°F db/15°F wb outdoor air	2.1 COP_nfheating

Direct Expansion Cooling Efficiency Temperature Adjustment Curve

Applicability

Packaged DX equipment

Definition

A curve that varies the cooling efficiency of a DX coil as a function of evaporator conditions, condenser conditions. For air cooled DX systems:

$EIR\_FT = a + b\times t_{wb}+c\times t_{wb}^{2}+d\times t_{odb}+e\times t_{odb}^{2}+f\times t_{wb}\times t_{odb}$

(23)

For liquid cooled DX systems:

$EIR\_FT = a + b\times t_{wb}+c\times t_{wb}^{2}+d\times t_{wt}+e\times t_{wt}^{2}+f\times t_{wb}\times t_{wt}$

(24)

$P_{operating}=P_{rated}\times EIR\_FPLR\times EIR\_FT\times CAP\_FT$

(25)

Where:

PLR = Part load ratio based on available capacity (not rated capacity)

EIR-FT = A multiplier on the EIR to account for the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature

Q_operating = Present load on heat pump (Btu/h)

Q_available = Heat pump available capacity at present evaporator and condenser conditions (in Btu/h)

t_wb = The entering coil wet-bulb temperature (°F)

t_wt = The water supply temperature (°F)

t_odb = The outside-air dry-bulb temperature (°F)

P_rated = Rated power draw at ARI conditions (kW)

P_operating = Power draw at specified operating conditions (kW)

Note: If an air cooled unit employs an evaporative condenser, t_odb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

Table 71. Cooling System Coefficients for EIR-FT

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (Other)
a	2.0280385	-1.8394760	-0.6550461	-1.0639310
b	-0.0423091	0.0751363	0.0388910	0.0306584
c	0.0003054	-0.0005686	-0.0001925	-0.0001269
d	0.0149672	0.0047090	0.0013046	0.0154213
e	0.0000244	0.0000901	0.0001352	0.0000497
f	-0.0001640	-0.0001218	-0.0002247	-0.0002096
Rated _CWT	Max 85°F, Min 60°F	Max 85°F, Min 60°F	NA	NA
Rated _EWBT	Max 57°F, Min 77°F	Max 57°F, Min 77°F	Max 77°F, Min 57°F	Max 77°F, Min 57°F
Rated _OADBT	NA	NA	Max 115°F, Min 75°F	Max 115°F, Min 75°F
Source: (CEC 2013)

Units

Data structure

Input Restrictions

Where publicly accessible performance curves are available for as-designed equipment they may be used for the proposed design otherwise the default equations and coefficients given above should be used. If default curves are not used, supporting documentation is required.

Baseline Building

Use default curves specified above, also documented in COMNET Appendix H (COMNET 2017)

Direct Expansion Part-Load Efficiency Adjustment Curve

Applicability

Packaged systems with DX cooling

Definition

A normalized performance adjustment curve to the rated efficiency (energy input ratio [EIR]) that describes how the efficiency varies at part-load conditions. At a value of 1 (full load), the normalized efficiency is 1.

The default curves are given as follows as adjustments to the EIR6F:

$PLR=\frac{Q_{operating}}{Q_{available}(t_{wb}, t_{odb/wt})}$

(26)

$EIR_{FPLR}=a+b\times PLR+c\times PLR^2 + d\times PLR^3$

$PLF_{FPLR}=a+b\times PLR+c\times PLR^2 + d\times PLR^3$

(27)

Where:

PLR = Part load ratio based on available capacity (not rated capacity)

EIR-FPLR = A multiplier on the EIR to account for the part load ratio

Q_operating = Present load on heat pump (Btu/h)

Q_available = Heat pump available capacity at present evaporator and condenser conditions (in Btu/h)

twb = The entering coil wet-bulb temperature (°F)

twt = The water supply temperature (°F)

todb = The outside-air dry-bulb temperature (°F)

This curve may take the form of a part-load factor (PLF) or EIR-FLPR, which is the fraction of time the unit must run to meet the part-load for that hour. For example, at 40% of full load, the equipment might need to run 50% of the hour (for cycling losses).

Note that for small packaged equipment with SEER ratings <65,000 Btu/h, the part-load efficiency curve is set to no degradation, since the part-load degradation is built-into the DX cooling efficiency temperature adjustment curve (Air Source, other)

Default curves are provided for the different major classes of equipment.

Units

Coefficients

Input Restrictions

The coefficients should sum to 1 (within a small tolerance). This corresponds to a curve output of 1 for an input of 1. Where publicly accessible performance curves are available for as-designed equipment they may be used for the proposed design otherwise the default equations and coefficients given above should be used. If default curves are not used, supporting documentation is required.

Baseline Building

The baseline part-load efficiency adjustment curves are shown in the tables below:

Table 72.Cooling System Coefficients for EIR-FPLR

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (PSZ with Cap<65,000 Btu/h)	Air-Source (Other)
a	0.1250000	0.2012301	0.1250000	0	0.2012301
b	0.8750000	-0.0312175	0.8750000	1	-0.0312175
c	0.0000000	1.9504979	0.0000000	0	1.9504979
d	0.0000000	-1.1205105	0.0000000	0	-1.1205105

Table 73. Cooling System Coefficients for Part-Load Factor (PLF) Correlation (EnergyPlus)

Coefficient	Water-Source (Heat Pump)	Water-Source (Other)	Air-Source (PTAC)	Air-Source (PSZ with Cap<65,000 Btu/h)	Air-Source (Other)
a	0.85	0	0.85	1	0
b	0.15	5.1091	0.15	0	5.1091
c	0	-8.5515	0	0	-8.5515
d	0	4.4744	0	0	4.4744
Source: (CEC 2013)

Direct Expansion Number of Cooling Stages

Applicability

DX systems with multiple stages

Definition

This applies to systems with multiple compressors or multiple discrete stages of cooling. This system is a packaged unit with multiple stages of cooling. Systems with unequally sized compressors may have additional cooling stages.

Units

None (integer)

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

All baseline DX systems are single stage

G3.3 Minor Alterations

The number of stages modeled in the baseline for systems in which an air-side economizer is modeled must minimally comply with the requirement of Standard 90.1-2022 Section 6.5.1.3c Table 6.5.1 in the baseline. Table 74 below includes these requirements.

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 6.5.1.3c requirements are inapplicable.

Table 74. DX Cooling Stage Requirements for Modulating Airflow Units

Rating Capacity, Btu/h	Minimum Number of Mechanical Cooling Stage	Minimum Compressor Displacement^a
≥65,000 and <240,000	3	≤35% of full load
≥240,000	4	≤25% of full load

a. For mechanical cooling stage control that does not use variable compressor displacement the percent displacement shall be equivalent to the mechanical cooling capacity reduction evaluated at the full load rating conditions for the compressor.

Total Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	This provides the total cooling capacity of each cooling stage, at AHRI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage cooling capacity is 4 tons (48,000 Btu/h) and the total cooling capacity is 8 tons (96,000 Btu/h), the capacity is expressed as “0.5” for that stage.
Units	Array of fractions
Input Restrictions	As designed
Baseline Building	G3.2 New Construction/Major Alterations Not applicable for baseline systems. G3.3 Minor Alterations The number of stages modeled in the baseline for systems in which an air-side economizer is modeled must minimally comply with the requirement of Standard 90.1-2022 Section 6.5.1.3c Table 6.5.1 in the baseline. Table 74 above includes these requirements. 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 6.5.1.3c requirements are inapplicable.

Sensible Cooling Capacity by Stage
Applicability	DX systems with multiple stages
Definition	Provides the sensible cooling capacity of each cooling stage, at AHRI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated sensible cooling capacity for the unit. For example, if the stage sensible cooling capacity is 3.5 tons (42,000 Btu/h) and the total sensible cooling capacity is 7 tons (72,000 Btu/h), the capacity is expressed as “0.5” for that stage.
Units	Array of fractions
Input Restrictions	As designed.
Baseline Building	G3.2 New Construction/Major Alterations Not applicable. G3.3 Minor Alterations The number of stages modeled in the baseline for systems in which an air-side economizer is modeled must minimally comply with the requirement of Standard 90.1-2022 Section 6.5.1.3c Table 6.5.1 in the baseline. Table 74 above includes these requirements. 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 6.5.1.3c requirements are inapplicable.

Supply Fan Low Speed Ratio

Applicability

Single zone DX systems with multiple stages and two-speed fans or VAV fans

Definition

Specifies the low fan speed setting on a single zone VAV system or DX system with multiple cooling stages

Units

None (fraction)

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

Not applicable

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 and 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:

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.
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.

Table 75. Fan Airflow Control Thresholds

Cooling System Type	Fan Motor Size, hp	Mechanical Cooling Capacity, Btu/h
DX cooling	Any	≥65,000
Chilled-water and evaporative cooling	≥1/4	Any

Supply Fan Low Power Ratio
Applicability	Single zone DX systems with multiple stages and two-speed fans or VAV fans
Definition	Specifies the fraction of full load fan power corresponding to low fan speed operation on a single zone VAV system or DX system with multiple cooling stages
Units	None (fraction)
Input Restrictions	As designed
Baseline Building	G3.2 New Construction/Major Alterations Not applicable G3.3 Minor Alterations See the Supply Fan Low Speed Ratio descriptor directly above for modeling requirements.

Piping Insulation
Applicability	All projects
Definition	Thermal insulation on piping systems for service hot water, steam piping, chilled water for cooling, and hot water for space heating
Units	List (see above)
Input Restrictions	Not modeled
Baseline Building	Not modeled

Minimum Unloading Ratio

Applicability

Packaged systems that use hot-gas bypass during low load conditions

Definition

The minimum unloading ratio is where the equipment capacity can no longer be reduced by unloading and must be false loaded to meet smaller cooling loads. A typical false loading strategy is hot-gas bypass.

The minimum unloading ratio is the upper end of the hot-gas bypass operating range. This is the percentage of peak cooling capacity below the range in which hot-gas bypass will operate.

The actual unloading ratio shall be set to 50% of the user-entered minimum unloading ratio, with hot-gas-bypass operating below this level.

Units

Ratio

Input Restrictions

As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.

Baseline Building

G3.2 New Construction/Major Alterations

Not applicable

G3.3 Minor Alterations

The baseline should be modeled as minimally compliant with Standard 90.1-2022 Section 6.5.9. 0 shall be used in the baseline if hot-gas bypass is not allowed according to Standard 90.1-2022 Section 6.5.9. According to Standard 90.1-2022 Section 6.5.9, cooling systems shall not be modeled with hot-gas bypass or other evaporator pressure control systems unless the system is designed with multiple steps of unloading or continuous capacity modulation. The capacity of the modeled hot-gas bypass shall be limited as indicated in Table 76 for VAV units. Hot-gas bypass shall not be modeled for constant-volume units.

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 6.5.9 requirements are inapplicable.

Table 76. Hot-Gas Bypass Limitation

Rated Capacity	Maximum Hot-Gas Bypass, % of Total Capacity
≤240,000 Btu/h	15%
>240,000 Btu/h	10%

Minimum HGB Ratio
Applicability	Packaged systems that use hot-gas bypass during low load conditions
Definition	The lower end of the hot-gas bypass operating range. The percentage of peak cooling capacity below which hot-gas bypass will no longer operate (i.e., the compressor will cycle).
Units	Fraction (between 0 and 1)
Input Restrictions	As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.
Baseline Building	G3.2 New Construction/Major Alterations Not applicable G3.3 Minor Alterations The baseline should be modeled as minimally compliant with Standard 90.1-2022 Section 6.5.9. 0 shall be used in the baseline if hot-gas bypass is not allowed according to Standard 90.1-2022 Section 6.5.9. According to Standard 90.1-2022 Section 6.5.9, cooling systems shall not be modeled with hot-gas bypass or other evaporator pressure control systems unless the system is designed with multiple steps of unloading or continuous capacity modulation. The capacity of the modeled hot-gas bypass shall be limited as indicated in Table 76 for VAV units. Hot-gas bypass shall not be modeled for constant-volume units. 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 6.5.9 requirements are inapplicable.

Condenser Type

Applicability

All DX systems including heat pumps

Definition

The type of condenser for a DX cooling system. The choices are:

Air-cooled
Water-cooled
Air-cooled with evaporative pre-cooler

Units

List (see above)

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

Based on prescribed system type

Table 77. Baseline Building Condenser Type

Baseline Building System	Condenser Type
1 – PTAC	Air cooled
2 – PTHP	Air cooled
3 – PSZ AC	Air cooled
4 – PSZ HP	Air cooled
5 – PVAV reheat	Air cooled
6 – Packaged VAV with PFP Boxes	Air cooled
7 – VAV with Reheat	N/A
8 – VAV with PFP Boxes	N/A
9 – Heating and Ventilation	N/A
10 – Heating and Ventilation	N/A
11 – Single Zone VAV	N/A
12- Single Zone CAV HW	N/A
13- Single Zone- CAV ER	N/A

G3.3 Minor Alterations

Same as proposed except if the proposed design includes variable refrigerant flow heat pumps or single-zone systems with electric resistance heat, in these cases air source heat pumps shall be modeled in the baseline design in which case the condenser type is air cooled.

Condenser Flow Type
Applicability	All DX systems including heat pumps
Definition	Describes water flow control for a liquid cooled condenser. The choices are: Fixed flow Two-position Variable flow
Units	List (see above)
Input Restrictions	As designed. For variable or staged capacity equipment, the minimum-unload ratio must be set properly for the simulation program. NOTE: If the variable-flow is selected, the software must indicate that supporting documentation is required on the output forms.
Baseline Building	G3.2 New Construction/Major Alterations Not applicable G3.3 Minor Alterations Same as the proposed design.