Direct Expansion Cooling Efficiency (COP) |
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Applicability |
Packaged DX equipment |
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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
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Units |
Unitless ratio |
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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. |
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Baseline Rules |
Calculate COPnfcooling using the EER or SEER from the table below:
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Direct Expansion Cooling Efficiency Adjustment Curve |
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Applicability |
Packaged DX equipment |
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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
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. |
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Units |
Data structure |
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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. |
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Baseline Rules |
Use default curves. |
Number of Cooling Stages |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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Applicability |
All direct expansion systems including heat pumps |
Definition |
The type of condenser for a direct expansion (DX) cooling system. The choices are:
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Units |
List (see above) |
Input Restrictions |
As designed |
Baseline Rules |
Baseline HVAC systems 1 through 6 are all air cooled. |
Condenser Flow Type |
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Applicability |
All direct expansion systems including heat pumps |
Definition |
Describes water flow control for a water-cooled condenser. The choices are:
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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 |
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Applicability |
Packaged DX equipment |
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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 COPnf.cooling. Fan energy shall be modeled separately according to Section 3.7.3 of this document. |
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Units |
Unitless |
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Input Restrictions |
As designed. Calculated as follows:
Where: Qt.gross.rated = The AHRI rated total cooling capacity of a packaged unit |
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Baseline Building |
For Baseline Systems 1, 2, 3, 4, 5 and6: Use the COPnf.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)
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 COPnf.cooling for 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 COPnf.cooling |
≥65,000 Btu/h and <135,000 Btu/h |
Split-system and single-package |
3.5 COPnf.cooling |
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≥135,000 Btu/h and<240,000 Btu/h |
3.4 COPnf.cooling |
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≥240,000 Btu/h and <760,000 Btu/h |
3.5 COPnf.cooling |
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≥760,000 Btu/h |
3.6 COPnf.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 COPnf.cooling |
≥65,000 Btu/h and <135,000 Btu/h |
Split-system and single-package |
3.4 COPnf.cooling |
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≥135,000 Btu/h and<240,000 Btu/h |
3.2 COPnf.cooling |
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≥240,000 Btu/h |
3.1 COPnf.cooling |
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Air-Cooled (heating-mode) |
<65,000 Btu/h (cooling capacity) |
All |
Single-package |
3.4 COPnfheating |
≥65,000 Btu/h and <135,000 Btu/h (cooling capacity) |
47°F db/43°F wb outdoor air |
3.4 COPnfheating |
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17°F db/15°F wb outdoor air |
2.3 COPnfheating |
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≥135,000 Btu/h (cooling capacity) |
47°F db/43°F wb outdoor air |
3.4 COPnfheating |
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17°F db/15°F wb outdoor air |
2.1 COPnfheating |
Direct Expansion Cooling Efficiency Temperature Adjustment Curve |
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Applicability |
Packaged DX equipment |
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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:
For water cooled DX systems:
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 Qoperating = Present load on heat pump (Btu/h) Qavailable = 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) Prated = Rated power draw at ARI conditions (kW) Poperating = 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. Table 67. Cooling System Coefficients for EIR-FT
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Units |
Data structure |
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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. |
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Baseline Building |
Use default curves specified above, also documented in COMNET Appendix H (COMNET 2017) |
Direct Expansion Part-Load Efficiency Adjustment Curve |
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Applicability |
Packaged systems with DX cooling |
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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[1]:
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 Qoperating = Present load on heat pump (Btu/h) Qavailable = 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. |
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Units |
Coefficients |
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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. |
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Baseline Building |
The baseline part-load efficiency adjustment curves are shown in the tables below: Table 68.Cooling System Coefficients for EIR-FPLR
Table 69. Cooling System Coefficients for Part-Load Factor (PLF) Correlation (EnergyPlus)
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[1] The EIR is the ratio of energy used by the system to cooling capacity in the same units. It is the reciprocal of the coefficient of performance (COP). EnergyPlus uses a part-load factor correlation for PLF as a function of PLR. The EnergyPlus PLF is related to the DOE-2 EIR(PLR) by the following: EIR-FPLR = PLR / PLF.
Direct Expansion Number of Cooling Stages |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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Applicability |
All DX systems including heat pumps |
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Definition |
The type of condenser for a DX cooling system. The choices are:
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Units |
List (see above) |
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Input Restrictions |
As designed |
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Baseline Building |
Based on prescribed system type Table 70. Baseline Building Condenser Type
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Condenser Flow Type |
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Applicability |
All DX systems including heat pumps |
Definition |
Describes water flow control for a water cooled condenser. The choices are:
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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 |