Baseline System Configuration (PRM System Selection Dialog) 90.1 2007
System Selection
90.1-2007 Section G3.1.1
90.1-2010 Section G3.1.1
Residential building types include dormitory, hotel, motel, and multifamily. Residential space types include guest rooms, living quarters, private living space, and sleeping quarters. Other building and space types are considered non-residential (Table G3.1.1A)
Where no heating system is to be provided or no heating energy source is specified, use the “Electric and Other” heating source classification (Table G3.1.1A)
Where attributes make a building eligible for more than one baseline system type, use the predominant condition to determine the system type for the entire building (Table G3.1.1A)
For laboratory spaces with a minimum of 5,000 cfm of exhaust, use system type 5 or 7 and reduce the exhaust and makeup air volume to 50% of design values during unoccupied periods (Table G3.1.1A)
For all-electric buildings, the heating shall be electric resistance (Table G3.1.1A)
If a building has both residential and non-residential with their total conditioned floor area both greater than 20,000 ft2, then the HVAC system type is determined separately for the residential and non-residential portions. (G3.1.1 Exception a)
Purchased Heat
90.1-2007 Section G3.1.1.1
90.1-2010 Section G3.1.1.1
When the proposed building systems use purchased hot water or steam, the baseline building shall use the same hot water or steam sources and be modelled without on-site boilers, electric heat, or furnaces. Costs shall be based on actual utility rates for the purchased steam or hot water.
In 90.1-2010, an exception to G3.1.3.5 states that pump power shall be modelled as 14 W/gpm for baseline systems using purchased heat. Users will need to adjust the Specific pump power in ApacheHVAC accordingly.
Preheat Coils
90.1-2007 Section G3.1.2.3
90.1-2010 Section G3.1.2.4
Preheat coils included in the proposed design need to be modelled as part of the baseline system. Prototype systems 7f and 8b include preheat coils as examples. This preheat coil and its controller can be copied into a baseline system in ApacheHVAC to meet this requirement of the performance rating method.
Figure 1: Preheat coil copied from Prototype System 7f into PRM Baseline System 5.
Demand Control Ventilation
90.1-2007 Section 6.4.3.9
90.1-2010 Section 6.4.3.9
Demand control ventilation (DCV) is required for all ventilation systems with design outdoor air capacities greater than 3,000 cfm serving areas larger than 500 ft2 and having an average design occupancy density exceeding 40 people per 1,000 ft2. This typically includes assembly spaces such as theaters, meeting rooms, ballrooms, etc. Systems with a design outdoor airflow less than 1200 cfm are excluded from this requirement.
In ApacheHVAC, PRM Baseline Systems 9, 10, and 11 include pre-defined options for DCV. Users can enable DCV on these systems by checking the “Demand-controlled ventilation using zone CO2 sensors” box on the Zone Ventilation & Exhaust tab of the System Parameters dialog.
Figure 2: Zone Ventilation & Exhaust tab of the System Parameters dialog with DCV option highlighted.
For PRM Baseline Systems in ApacheHVAC that do not include the option for DCV, modellers may use Prototype System 5b as an example for adding the damper and controllers to the baseline system.
Figure 3: Prototype System 5b with network and controllers relevant to DCV selected
As an alternative to DCV (defined in 90.1-2007 & 2010 Section 6.4.3.9 Exception a), systems may be provided with air-to-air heat recovery systems complying with 90.1-2007 & 2010 Section 6.5.6.1.
Economizers
90.1-2007 Section G3.1.2.6
90.1-2010 Section G3.1.2.7
Whether or not the baseline building HVAC system has an economizer depends on the type of system for the baseline building. HVAC systems 1, 2, 9, and 10 will never have an economizer; systems 3 through 8 will have an economizer if the building is not located in climate zone 1A, 1B, 2A, 3A or 4A. See exceptions for gas-phase air cleaning and supermarket open refrigerated casework systems.
When an economizer is required, it must have a high-limit shutoff switch (90.1-2007 G3.1.2.7 and 90.1-2010 G3.1.2.8) that senses dry-bulb temperature and shuts off economizer operation (reduces outdoor air to the minimum required for ventilation) when the outdoor temperature exceeds the values show in 90.1-2007 & 2010 Table G3.1.2.6B. The specific high-limit shutoff temperature depends on the climate zone.
In ApacheHVAC, PRM Baseline Systems 3 through 8 have an economizer with the dry-bulb temperature high-limit set to 70°F by default. The high-limit temperature can be changed (or the economizer can be disabled) by accessing the System Parameters tab of the System Parameters dialog.
Figure 4: System Parameters tab of the System Parameters dialog with economizer option highlighted
Dry-bulb high-limit economizers may increase energy use in dry or cold climates when spaces are humidified. This does not preclude modeling the baseline system with humidification and economizer operation.
Exhaust Air Energy Recovery
90.1-2007 Section G3.1.2.10
Exhaust air energy recovery is required for fan systems that are larger than 5,000 cfm and which also require a significant amount of outdoor air (70% or more of the supply air volume).
90.1-2010 Section G3.1.2.11
The exhaust air energy recovery requirement is defined relative to climate zone, supply air flow rate, and the percentage of outdoor airflow at full design airflow rate, per Table 6.5.6.1.
In both 90.1-2007 and 90.1-2010, the energy recovery system in the baseline building shall have a recovery effectiveness of at least 50%. The energy recovery system does not negate the requirement for an outdoor air economizer when economizer is required for the baseline building system. Furthermore, the baseline building system must permit air to bypass the energy recovery system during economizer operation. There are exceptions to the heat recovery requirements for specific climates, semi heated spaces with no air-conditioning, systems that exhaust toxic fumes or kitchen grease, systems serving laboratories with exhaust rates of 5,000 cfm or greater, and other conditions (see Exceptions to G3.1.2.10 in 90.1-2007, and Exceptions to 6.5.6.1 in 90.1-2010).
PRM Baseline systems 3 through 11 in ApacheHVAC include the option for airside energy recovery but it is disabled by default. To engage the pre-defined energy recovery, check the “Energy recovery preconditioning of outside air” box on the System Parameters tab of the System Parameters dialog.
Figure 5: System Parameters tab of the System Parameters dialog with energy recovery option highlighted
Equipment Efficiencies
90.1-2007 Section G3.1.2.1
90.1-2010 Section G3.1.2.1
The minimum efficiencies for HVAC equipment (6.4.1) must be used for the applicable equipment in the baseline building design. This includes any part load efficiencies if these are specified. Minimum HVAC equipment efficiencies are defined in sections 6.4.1.1, 6.4.1.2, 6.4.1.3, and 6.4.1.4 of 90.1-2007 and 90.1-2010.
The pre-defined DX Cooling types provided in ApacheHVAC are set up to meet 90.1 PRM requirements for Baseline systems. There are 11 pre-defined systems available:
5 for Packaged Single-Zone (PSZ) systems for different size ranges and associated COPs
3 for Packaged Terminal Air-Conditioning (PTAC) for different size ranges and associated COPs
3 for Packaged Terminal Heat Pumps (PTHP) for different size ranges and associate COPs (only 2 PTHP size ranges are provided for 90.1-2010)
The COP values in the pre-defined DX Cooling types match 90.1 requirements, as adjusted per California Title-24 ACM Manual methods to remove the supply fan power from the EER that was determined for a packaged unit at ARI conditions. The DX Cooling Type dialog provides the EER with supply air fan from 90.1 Chapter 6 tables only in the reference name of the DX Cooling Type. The actual number used in the input field for that dialog is the COP without the fan, i.e., after applying the Title-24 ACM method for removing the supply fan power.
Hot Water Supply Temperature Reset
90.1-2007 Section G3.1.3.4
90.1-2010 Section G3.1.3.4
For baseline building systems 1, 5, and 7, the hot water supply temperature shall be reset based on the outdoor dry-bulb temperature. When the outdoor temperature is 20°F and below, the supply temperature shall be a constant 180°F. When the outdoor temperature is 50°F and above, the supply temperature shall be 150°F. When the outdoor temperature is between 20°F and 50°F, the supply temperature shall be ramped in a proportional manner between 180°F and 150°F.
In ApacheHVAC, the PRM Baseline Systems 1, 5, and 7 include this hot water supply temperature reset by default.
Figure 6: Default hot water loop supply water temperature control settings for a PRM Baseline System 5
Hot Water Pumps
90.1-2007 Section G3.1.3.5
90.1-2010 Section G3.1.3.5
For systems 1, 5, and 7 (except for purchased hot water or steam), the baseline building design hot water pump power shall be 19 W/gpm. The hot water loop shall be modelled as primary-only with continuous variable flow. Systems serving 120,000 ft2 or more shall be modelled with variable speed drives, otherwise the systems shall be modelled as riding the pump curve.
Default hot water loops for PRM Baseline Systems in ApacheHVAC are primary-only variable flow, have a specific pump power of 19 W/gpm, and use a constant speed pump curve. When the area of the building served by the hot water loop exceeds 120,000 ft2, users will need to change the pump performance curve to variable speed using the pull-down menu in the Hot water loop dialog.
Figure 7: Default hot water loop pump configuration for a PRM Baseline System 5
In 90.1-2010, an exception to G3.1.3.5 states that pump power shall be 14 W/gpm for systems using purchased heat. Users will need to adjust the Specific pump power in ApacheHVAC accordingly.
Chiller Selection
90.1-2007 Section G3.1.3.7
90.1-2010 Section G3.1.3.7
For baseline system types 7 and 8, the number and type of electric water-cooled chillers to be modelled is determined as a function of building peak cooling load. When the building peak cooling load is less than or equal to 300 tons, a single screw chiller is required. When the building peak cooling load exceeds 300 tons but is not larger than 600 tons, two equally sized screw chillers are to be used.
For models where the baseline building peak cooling load exceeds 600 tons, a minimum of 2 equally sized centrifugal chillers must be used. No chiller, however, can be greater than 800 tons so the addition of more equally sized chillers may be required on large projects.
PRM Baseline Systems 7 and 8 in ApacheHVAC include a single water-cooled centrifugal chiller by default. For baseline buildings where the peak cooling load does not exceed 300 tons, no additional chillers are needed, but the chiller curve selection will need to be updated to a screw-type chiller.
If the peak cooling load in the baseline building exceeds 300 tons but is not larger than 600 tons, users can add a second chiller to the chilled water loop by using the Copy button on the Chiller set tab of the Chilled water loop dialog. Both chillers will need screw curve sets (not the default centrifugal curves) so it is recommended that this change be made prior to using the Copy button. Users should ensure that the chillers are equally sized and can do so quickly by setting the % CHWL capacity to 50% for each chiller.
Figure 8: Chiller set tab of the Chilled water loop dialog illustrating two equally sized chillers for a PRM Baseline System 7 serving a peak building cooling load of 500 tons
If the peak cooling load in the baseline building exceeds 600 tons, users can copy the default chiller until there are enough chillers on the chilled water loop to satisfy the building peak cooling load without any chiller exceeding 800 tons in size. Every loop must have at least 2 chillers and all chillers must be equally sized.
For example, a model where the baseline building peak cooling load is 700 tons requires 2 x 350 ton chillers. Where the baseline building peak cooling load is 3,000 tons, the model requires 4 x 750 ton chillers.
Figure 9: Chiller set tab of the Chilled water loop dialog illustrating four equally sized chillers for a PRM Baseline System 7 serving a peak building cooling load of 3,000 tons
It should be noted that the default baseline chiller curves come from the generic curve sets in DOE-2.2 and are based on entering condenser temperature (ECT). The manufacturer-specific curves available to users in the Performance Curve library are based on leaving condenser temperature (LCT). If LCT curves are used for proposed chillers, users may consider following the steps below to better isolate the results of actual differences in the baseline and proposed chiller plants (e.g., differences in chiller COP, design sizing, supply water temperature set points, etc.):
1. Define appropriate curve set for Proposed chiller & test via simulation to confirm that the chiller is modeling as expected.
2. Save a copy of that chiller with a new name for the Baseline chiller.
3. Revise the COP at the rated condition to match the ASHRAE 90.1 requirements for minimum chiller performance in Chapter 6.
4. Confirm that the IPLV is at least that required for 90.1. Data needed for IPLV calculations can be found by reading the values at the four required load ranges from the 2D plots in the ApacheHVAC Performance Curves Library interface. Derivation of IPLV for each chiller curve set will be included in a future release of the VE.
In 90.1-2010, an exception to G3.1.3.7 states that systems using purchased chilled water shall be modelled using different guidelines as defined in G3.1.1.3. If you have questions about modifying the PRM Baseline Systems in ApacheHVAC to meet these requirements, please contact support@iesve.com.
Chilled Water Supply Temperature Reset
90.1-2007 Section G3.1.3.9
90.1-2010 Section G3.1.3.9
For baseline building systems 7 and 8, the chilled water temperature shall be reset on an hourly basis, based on the outdoor air temperature. When the outdoor temperature is 80°F or greater, the supply temperature shall be a constant 44°F. When the outdoor temperature is 60°F or below, the supply temperature shall be 54°F. When the outdoor temperature is between 60°F and 80°F, the supply temperature shall ramp between 44°F and 54°F in a proportional manner. If there is dehumidification requirement of the cooling coils, the maximum reset of chilled water temperature should be calculated according to the dehumidification requirement and cooling coils performance.
In ApacheHVAC, the PRM Baseline Systems 7 and 8 include this chilled water supply temperature reset by default.
Figure 10: Default chilled water loop supply water temperature control settings for a PRM Baseline System 7
Chilled Water Pumps
90.1-2007 Section G3.1.3.10
90.1-2010 Section G3.1.3.10
For baseline building systems 7 and 8, pump power shall be 22 W/gpm. Chilled water pumps in systems with a cooling capacity of 300 tons or more shall be modelled as primary/secondary systems with variable speed drives on the secondary pumping loop. Chilled water pumps in systems with less than 300 tons cooling capacity shall be modelled as primary/secondary systems with the secondary pump riding the pump curve.
Default chilled water loops for PRM Baseline Systems in ApacheHVAC are primary/secondary configuration with constant flow on the primary loop. The primary loop has a specific pump power of 4.4 W/gpm with the pump riding the pump curve and the secondary loop has a specific pump power of 17.6 W/gpm with a variable speed curve. When the cooling capacity of the baseline building is less than 300 tons, users will need to change the pump performance curve on the secondary loop to constant speed using the pull-down menu in the Chilled water loop dialog.
In 90.1-2010, an exception to G3.1.3.10 states that pump power shall be 16 W/gpm for systems using purchased chilled water. Users will need to adjust the Specific pump power in ApacheHVAC accordingly.
Heat Rejection
90.1-2007 Section G3.1.3.11
90.1-2010 Section G3.1.3.11
For baseline building systems 7 and 8, the heat rejection device shall be an axial fan cooling tower with two-speed fans. Baseline cooling tower power should be calculated using Table 6.8.1G. In ApacheHVAC, PRM baseline systems 7 and 8 have a cooling tower with a two-speed fan by default. The fan power is calculated based on heat rejection load and the default fan electric input ratio. Users should edit the cooling tower fan power to meet the requirements of Table 6.8.1G.
Condenser water design supply temperature shall be 85°F or a 10°F approach to design wet-bulb temperature, whichever is lower. The design temperature rise shall be 10°F. The tower shall be controlled to maintain a 70°F leaving water temperature when weather permits, floating up to leaving water temperature at design conditions.
PRM baseline systems 7 & 8 in ApacheHVAC have an 85°F design condenser water loop supply temperature by default, with a 10°F design temperature difference. A formula profile is used to control condenser water loop supply temperature. The following formula resets the condenser water loop supply temperature setpoint based upon outside air wetbulb using the following ramp function:
ramp(twb,60,70,75,85)
Where outdoor weather conditions permit the cooling tower to achieve the condenser water supply temperature setpoint, this formula will drive the cooling tower to maintain a 10°F approach. When the outside air wetbulb temperature is 60°F or lower, the condenser water supply temperature setpoint will be 70°F. Above 75°F outside air wetbulb, the setpoint will be 85°F. When the outside air wetbulb temperature is between 60°F and 75°F, the condenser water supply temperature setpoint will vary proportionally between 70°F and 85°F. This operation is represented in the graph below:
The standard requires the baseline building design condenser water pump power to be 19 W/gpm. This is the default specific pump power in ApacheHVAC for PRM baseline systems 7 and 8.
The standard also requires that each chiller shall be modelled with separate condenser water and chilled water pumps interlocked to operate with the associate chiller. By default, ApacheHVAC includes a single chilled water pump and single condenser water pump. Users should change this setting when multiple chillers exist. This can be done by changing the Primary circuit pump configuration pull-down menu on the Primary circuit sub-tab of the Chilled water loop tab and the Condenser loop pump configuration on the Condenser water loop sub-tab of the Heat rejection tab.
Figure 11: Chilled water loop tab with dedicated pumps selected on the chilled water loop for a PRM Baseline System 7 with two chillers
