# Budget HVAC System Types 1-11

This page defines the detailed design specifications for Budget HVAC System Types 1-11 required by ASHRAE 90.1 Performance Rating Method (PRM) Appendix G. These system types specify how to model each baseline HVAC system in EnergyPlus based on building type, cooling/heating requirements, and climate zone.

System Selection Table

Table: Budget HVAC System Types 1-11 (per Table 11.5.2-1)

| System No. | System Type                                  | Fan Control              | Cooling Type     | Heating Type                  |
| ---------- | -------------------------------------------- | ------------------------ | ---------------- | ----------------------------- |
| 1          | VAV with parallel fan-powered boxes          | VAV                      | Chilled water    | Electric resistance           |
| 2          | VAV with reheat                              | VAV                      | Chilled water    | Hot-water fossil fuel boiler  |
| 3          | Packaged VAV with parallel fan-powered boxes | VAV                      | Direct expansion | Electric resistance           |
| 4          | Packaged VAV with reheat                     | VAV                      | Direct expansion | Hot-water fossil fuel boiler  |
| 5          | Two-pipe fan coil                            | Single- or two-speed fan | Chilled water    | Electric resistance           |
| 6          | Water-source heat pump                       | Single- or two-speed fan | Direct expansion | Electric heat pump and boiler |
| 7          | Four-pipe fan-coil                           | Single- or two-speed fan | Chilled water    | Hot-water fossil fuel boiler  |
| 8          | Packaged terminal heat pump                  | Single-speed fan         | Direct expansion | Electric heat pump            |
| 9          | Packaged rooftop heat pump                   | Single- or two-speed fan | Direct expansion | Electric heat pump            |
| 10         | Packaged terminal air conditioner            | Single-speed fan         | Direct expansion | Hot-water fossil fuel boiler  |
| 11         | Packaged rooftop air conditioner             | Single- or two-speed fan | Direct expansion | Fossil fuel furnace           |

***

Detailed System Descriptions

System 1: VAV with Parallel Fan-Powered Boxes

Component Configuration:

* Supply Fan: Variable volume fan with VFD (variable frequency drive)
* Cooling: Chilled water loop (central plant)
* Zone Heating: Parallel fan-powered boxes with electric resistance heating
* Parallel Fan Sizing: 50% of zone peak design flow rate
* Parallel Fan Power: 0.74 W/L/s (baseline specification)
* Minimum Volume Setpoint: Ventilation requirement per ASHRAE 62.1 Simplified Procedure
* Supply Air Temperature: Constant at design condition (no reset)

EnergyPlus Objects:

* `AirLoopHVAC` (main air distribution)
* `Fan:VariableVolume` (supply fan with VFD)
* `Coil:Cooling:Water` (chilled water cooling coil)
* `SetpointManager:Scheduled` (constant SAT control)
* `OutdoorAir:Mixer` (outdoor air intake)
* `AirTerminal:SingleDuct:ParallelPIU:Reheat` (parallel fan power box)
* `Fan:OnOff` (parallel fan in each box)
* `Coil:Heating:Electric` (electric resistance terminal heating)
* `PlantLoop` (chilled water loop)

Application:

* Large buildings with central cooling plant
* Multiple thermal zones with individual zone heating control
* Parallel fans maintain circulation during heating
* Electric resistance heating at zone level
* Demand-controlled ventilation capable

Climate Zone Application:

* CZ 0-3A (electric heating with chilled water cooling)

Key Features:

* Chilled water supply: 6.7°C, return: 13°C (fixed by specification)
* No supply air temperature reset (constant design SAT)
* Parallel fan operation at 50% zone flow during heating
* Central cooling plant with floating condenser water control

***

System 2: VAV with Reheat

Component Configuration:

* Supply Fan: Variable volume fan with VFD control
* Cooling: Chilled water loop (central plant)
* Zone Heating: Hot water reheat coils in VAV terminal units
* Central Heating: Fossil fuel boiler (matches proposed design fuel)
* Supply Air Temperature Reset: +2.8°C above design at minimum cooling load
* Minimum Volume Setpoint: Larger of ventilation requirement or code minimum

EnergyPlus Objects:

* `AirLoopHVAC` (air distribution system)
* `Fan:VariableVolume` (supply fan with VFD)
* `Coil:Cooling:Water` (chilled water cooling coil)
* `SetpointManager:Warmest` (SAT reset control with bounds)
* `OutdoorAir:Mixer` (outdoor air intake with economizer)
* `AirTerminal:SingleDuct:VAV:Reheat` (VAV terminal with reheat)
* `Coil:Heating:Water` (zone reheat coils)
* `PlantLoop` (chilled water loop)
* `PlantLoop` (hot water boiler loop)
* `Boiler:HotWater` (natural draft fossil fuel boiler)
* `Pump:VariableSpeed` (hot water and chilled water pumps)

Application:

* Most common baseline system for large buildings
* Multiple thermal zones served by single air loop
* VAV terminals reduce air volume at part load
* Hot water reheat provides zone-level control
* Demand-controlled ventilation capable
* Central chiller with cooling tower

Climate Zone Application:

* CZ 3B-8 (fossil fuel heating via hot water loop)

Key Features:

* Supply air temperature reset: +2.8°C at minimum cooling load
* Variable volume fan with VFD saves energy at part load
* Chilled water: 6.7°C supply, 13°C return
* Hot water: 82°C supply, 54°C return
* Reheat modulation at minimum damper position
* Economizer integration for cooling assistance

***

System 3: Packaged VAV with Parallel Fan-Powered Boxes

Component Configuration:

* Supply Fan: Variable volume fan with VFD control
* Cooling: Direct expansion (DX) cooling coil (packaged unit)
* Zone Heating: Parallel fan-powered boxes with electric resistance heating
* Parallel Fan Sizing: 50% of zone peak design flow rate
* Minimum Volume Setpoint: Ventilation requirement per ASHRAE 62.1

EnergyPlus Objects:

* `AirLoopHVAC` (air distribution system)
* `Fan:VariableVolume` (supply fan with VFD)
* `Coil:Cooling:DX:SingleSpeed` (DX cooling coil)
* `SetpointManager:Scheduled` (constant SAT control)
* `OutdoorAir:Mixer` (outdoor air intake)
* `AirTerminal:SingleDuct:ParallelPIU:Reheat` (parallel fan power box)
* `Fan:OnOff` (parallel fan in each box)
* `Coil:Heating:Electric` (electric resistance terminal heating)

Application:

* Mid-size buildings with packaged rooftop DX unit
* Multiple zones with parallel fan-powered terminal boxes
* VAV fan modulation for energy savings
* Electric resistance zone heating

Climate Zone Application:

* CZ 0-3A (electric heating with DX cooling)

Key Features:

* Packaged DX cooling (no central chiller)
* Parallel fans at 50% zone design flow
* Constant supply air temperature (no reset)
* VAV fan with VFD modulation

***

System 4: Packaged VAV with Reheat

Component Configuration:

* Supply Fan: Variable volume fan with VFD control
* Cooling: Direct expansion (DX) cooling coil (packaged unit)
* Zone Heating: Hot water reheat coils in VAV terminal units
* Central Heating: Fossil fuel boiler
* Supply Air Temperature Reset: +2.8°C above design at minimum cooling load

EnergyPlus Objects:

* `AirLoopHVAC` (air distribution system)
* `Fan:VariableVolume` (supply fan with VFD)
* `Coil:Cooling:DX:SingleSpeed` (DX cooling coil)
* `SetpointManager:Warmest` (SAT reset control with bounds)
* `OutdoorAir:Mixer` (outdoor air intake with economizer)
* `AirTerminal:SingleDuct:VAV:Reheat` (VAV terminal with reheat)
* `Coil:Heating:Water` (zone reheat coils)
* `PlantLoop` (hot water boiler loop)
* `Boiler:HotWater` (natural draft fossil fuel boiler)
* `Pump:VariableSpeed` (hot water pump)

Application:

* Standard mid-size baseline system for most buildings
* Packaged rooftop DX cooling with VAV distribution
* VAV terminals reduce air volume at part load
* Hot water reheat provides zone-level control
* Most common system type in practice

Climate Zone Application:

* CZ 3B-8 (fossil fuel heating via hot water loop)

Key Features:

* Packaged DX cooling (no central chiller)
* Supply air temperature reset: +2.8°C at minimum cooling load
* Hot water: 82°C supply, 54°C return
* VAV fan with VFD saves energy
* Economizer for cooling assistance

***

System 5: Two-Pipe Fan Coil

Component Configuration:

* Supply Fan: Single-speed or two-speed fan (within fan coil unit)
* Cooling: Chilled water loop (central plant)
* Heating: Electric resistance heating in fan coil unit
* Fan Control: On/off or two-speed cycling
* No central heating boiler required

EnergyPlus Objects:

* `PlantLoop` (chilled water loop)
* `Pump:VariableSpeed` (chilled water pump)
* `Chiller:Electric:EIR` (central chiller, per Table 11.5.2-3)
* `CoolingTower:SingleSpeed` or `TwoSpeed` (cooling tower)
* `ZoneHVAC:FourPipeFanCoil` or equivalent zone equipment
* `Fan:OnOff` (fan coil unit fan)
* `Coil:Cooling:Water` (chilled water cooling in fan coil)
* `Coil:Heating:Electric` (electric resistance heating in fan coil)

Application:

* Medium-size buildings with chilled water loop
* Individual zone fan coil units in each space
* Two-pipe system (cooling only connection)
* Electric resistance heating at zone level

Climate Zone Application:

* CZ 0-3A (electric heating with chilled water cooling)

Key Features:

* Central chiller with cooling tower
* Chilled water: 6.7°C supply, 13°C return
* Electric resistance heating in fan coil
* On/off or two-speed fan control
* Floating condenser water control

***

System 6: Water-Source Heat Pump

Component Configuration:

* Supply Fan: Single-speed or two-speed fan (within heat pump unit)
* Primary Heating/Cooling: Water-source heat pumps in each zone
* Heat Rejection Loop: Water loop maintained at 16-32°C
* Backup Heating: Boiler for supplemental heating
* Fluid Cooler: For heat rejection to outdoor air

EnergyPlus Objects:

* `PlantLoop` (heat pump water loop at 16-32°C)
* `HeatPump:WaterToWater:EquationFit:Heating` (heating heat pumps)
* `HeatPump:WaterToWater:EquationFit:Cooling` (cooling heat pumps)
* `Pump:VariableSpeed` (heat pump loop pump with shutoff)
* `FluidCooler:TwoSpeed` or `EvaporativeCooler:IndirectResearch` (fluid cooler)
* `Boiler:HotWater` (supplemental heating, if included)
* `SetpointManager:Scheduled` (loop temperature 16-32°C)
* `ZoneHVAC:WaterToAirHeatPump` (zone-level heat pumps)

Application:

* Buildings with electric heat pump primary heating/cooling
* Closed-loop water-source heat pump systems
* Backup heating for peak winter loads
* Flexible heating and cooling year-round

Climate Zone Application:

* All climate zones (CZ 0-8)

Key Features:

* Heat pump loop maintained between 16-32°C continuously
* Boiler activates when loop temperature drops to 16°C
* Fluid cooler activates when loop temperature rises to 32°C
* Individual zone control via water-source heat pumps
* Pump flow shutoff at each heat pump when compressor OFF

***

System 7: Four-Pipe Fan-Coil

Component Configuration:

* Supply Fan: Single-speed or two-speed fan (within fan coil unit)
* Cooling: Chilled water loop (central plant)
* Heating: Hot water loop (fossil fuel boiler)
* Fan Control: On/off or two-speed cycling
* Both heating and cooling connections available

EnergyPlus Objects:

* `PlantLoop` (chilled water loop)
* `PlantLoop` (hot water loop)
* `Pump:VariableSpeed` (chilled water and hot water pumps)
* `Chiller:Electric:EIR` (central chiller)
* `Boiler:HotWater` (fossil fuel boiler)
* `CoolingTower:SingleSpeed` or `TwoSpeed` (cooling tower)
* `ZoneHVAC:FourPipeFanCoil` (four-pipe fan coil unit)
* `Fan:OnOff` (fan coil unit fan)
* `Coil:Cooling:Water` (chilled water cooling)
* `Coil:Heating:Water` (hot water heating)

Application:

* Medium to large buildings with dual water loops
* Individual zone fan coil units in each space
* Four-pipe system (separate heating and cooling connections)
* Hot water heating and chilled water cooling

Climate Zone Application:

* CZ 3B-8 (fossil fuel heating via hot water loop)

Key Features:

* Central chiller with cooling tower
* Central boiler for heating
* Chilled water: 6.7°C supply, 13°C return
* Hot water: 82°C supply, 54°C return
* Individual zone control via two-pipe connections
* On/off or two-speed fan control

***

System 8: Packaged Terminal Heat Pump

Component Configuration:

* Supply Fan: Single-speed constant volume fan
* Heating/Cooling: Air-source heat pump with DX coil (self-contained)
* Supplemental Heating: Electric resistance coil
* Thermostat Control: Multistage space thermostat
* Self-contained unit (one per zone or room)

EnergyPlus Objects:

* `ZoneHVAC:PackagedTerminalHeatPump` or `AirLoopHVAC:UnitarySystem`
* `Fan:OnOff` (supply fan)
* `Coil:Heating:DX:SingleSpeed` (heat pump heating)
* `Coil:Cooling:DX:SingleSpeed` (heat pump cooling)
* `Coil:Heating:Electric` (supplemental resistance heating)
* `ZoneControl:Thermostat` (multistage thermostat)

Application:

* Residential and small commercial buildings
* One unit per thermal zone or room
* All-electric self-contained systems
* No central plant required

Climate Zone Application:

* CZ 0-3A (electric heating climates)

Key Features:

* Air-source heat pump with integrated cooling/heating
* Auxiliary heat: Last thermostat stage AND outdoor air <4°C
* Single-speed constant volume fan
* Integrated defrost cycle
* 100% efficient electric resistance auxiliary heating

***

System 9: Packaged Rooftop Heat Pump

Component Configuration:

* Supply Fan: Single-speed or two-speed variable speed fan
* Heating/Cooling: Air-source heat pump with DX coil (packaged unit)
* Supplemental Heating: Electric resistance coil
* Economizer: Outdoor air damper (if applicable)
* Serves multiple zones from single rooftop unit

EnergyPlus Objects:

* `AirLoopHVAC` (air loop)
* `Fan:VariableVolume` or `Fan:OnOff` (supply fan)
* `Coil:Heating:DX:SingleSpeed` (heat pump heating)
* `Coil:Cooling:DX:SingleSpeed` (heat pump cooling)
* `Coil:Heating:Electric` (supplemental resistance heating)
* `SetpointManager:Scheduled` (heating and cooling setpoints)
* `OutdoorAir:Mixer` (outdoor air intake, optional economizer)

Application:

* Small to medium non-residential buildings
* Packaged rooftop unit serving multiple zones
* All-electric system (no fossil fuel)
* Economizer capability available

Climate Zone Application:

* CZ 0-3A (electric heating climates)

Key Features:

* Air-source heat pump with variable speed fan
* Packaged self-contained unit
* Auxiliary heat activation: Last stage AND outdoor air <4°C
* Optional economizer for cooling assistance
* Multi-zone distribution from single unit

***

System 10: Packaged Terminal Air Conditioner

Component Configuration:

* Supply Fan: Single-speed constant volume fan
* Cooling: Direct expansion (DX) cooling coil (self-contained)
* Heating: Hot water heating coil connected to building loop
* No central chiller required
* One unit per zone or room

EnergyPlus Objects:

* `ZoneHVAC:PackagedTerminalAirConditioner`
* `Fan:OnOff` (supply fan)
* `Coil:Cooling:DX:SingleSpeed` (DX cooling)
* `Coil:Heating:Water` (hot water heating from building loop)
* `ZoneControl:Thermostat` (zone thermostat)

Application:

* Residential buildings (apartments, hotels, dormitories)
* Small commercial buildings
* One unit per thermal zone
* Hot water heating from building loop

Climate Zone Application:

* CZ 3B-8 (fossil fuel heating via building hot water)

Key Features:

* Packaged DX cooling (no central chiller)
* Hot water connection to building boiler
* Single-speed constant volume fan
* Individual zone control
* Simple thermostat control

***

System 11: Packaged Rooftop Air Conditioner

Component Configuration:

* Supply Fan: Single-speed or two-speed variable speed fan
* Cooling: Direct expansion (DX) cooling coil (packaged unit)
* Heating: Gas-fired furnace (integral to rooftop unit)
* Economizer: Outdoor air damper for cooling assistance
* Serves multiple zones from single rooftop unit

EnergyPlus Objects:

* `AirLoopHVAC` (air loop)
* `Fan:ConstantVolume` or `Fan:VariableVolume` (supply fan)
* `Coil:Cooling:DX:SingleSpeed` (DX cooling coil)
* `Coil:Heating:Gas` or `Coil:Heating:Fuel` (gas furnace)
* `SetpointManager:Scheduled` (heating and cooling setpoints)
* `OutdoorAir:Mixer` (outdoor air intake with economizer)

Application:

* Small to medium non-residential buildings
* Packaged rooftop unit serving multiple zones
* Gas furnace for heating (fossil fuel)
* Economizer for cooling assistance

Climate Zone Application:

* CZ 3B-8 (fossil fuel heating)

Key Features:

* Packaged DX cooling with integral gas furnace
* Economizer integration for cooling load reduction
* Single or variable speed fan available
* Multi-zone distribution from single rooftop unit
* Outdoor air damper control

***

Component Specifications by System

Cooling System Efficiencies

Systems 3, 4, 6, 8, 9, 10, 11 (Direct Expansion):

* Baseline COP: 3.0-3.5 typical for packaged AC
* EIR = 1 / COP
* Performance curves required for part-load modeling

Systems 1, 2, 5, 7 (Chilled Water):

* Scroll chiller (≤352 kW individual): COP 3.2-3.8
* Screw chiller (352-1055 kW individual): COP 3.5-4.0
* Centrifugal chiller (≥1055 kW individual): COP 4.0-5.0
* EIR = 1 / COP at rated conditions

Heating System Efficiencies

Systems 1, 3, 5, 8, 9 (Electric Heating):

* Electric resistance: 100% efficiency (1.0)
* Supplemental heat pump: 2.0-2.5 COP typical

Systems 2, 4, 7, 10, 11 (Fossil Fuel Heating):

* Natural draft boiler: 80% AFUE typical (per ASHRAE 90.1 baseline)
* Hot water: 82°C supply, 54°C return
* Furnace: 78-80% efficiency typical

System 6 (Heat Pump):

* Primary heat pump: 2.0-3.0 COP typical (varies by outdoor temperature)
* Boiler backup: 80% AFUE typical
* Boiler activation: When loop temperature <16°C

Fan Specifications

Systems 1, 2 (VAV with Central Chiller):

* Type: `Fan:VariableVolume` with VFD
* Power: 0.74 W/L/s (baseline specification)
* Minimum flow: Per ventilation requirement
* Modulation: VFD control for duct static pressure

Systems 3, 4 (Packaged VAV with DX):

* Type: `Fan:VariableVolume` with VFD
* Power: Per proposed design if specified
* Minimum flow: 30% typical
* Modulation: VFD for SAT setpoint or duct pressure

Systems 5, 7 (Four-Pipe and Two-Pipe Fan Coil):

* Type: `Fan:OnOff` or two-speed fan
* Speed: Single or two-speed operation
* Control: Thermostat cycling control
* Power: Per fan coil unit design

System 6 (Water-Source Heat Pump):

* Type: `Fan:OnOff` or two-speed fan
* Location: Within each zone heat pump unit
* Control: Cycling with heat pump compressor
* Power: Per unit design

Systems 8, 10 (Packaged Terminal Units):

* Type: `Fan:OnOff` (single-speed)
* Speed: Single-speed constant volume
* Control: On/off cycling with thermostat
* Power: Per unit design

Systems 9, 11 (Packaged Rooftop Units):

* Type: `Fan:ConstantVolume` or `Fan:VariableVolume`
* Speed: Single or variable with VFD
* Control: Thermostat or economizer control
* Power: Per rooftop unit design

EnergyPlus Simulation Inputs

Air Loop Configuration

AirLoopHVAC (Systems 2, 3, 4, 9, 11):

* Outdoor Air Intake (with economizer damper if applicable)
* Supply Fan (constant or variable volume)
* Cooling Coil (DX or water-based)
* Heating Coil (water-based or electric, if applicable)
* Supply Air Nodes and Setpoint Managers
* Thermal Zone Connections

Plant Loop Configuration

PlantLoop (Chilled Water, Systems 1, 2, 5, 7):

* Pump (variable-speed)
* Chiller(s) (count and type per Tables 11.5.2-2 and 11.5.2-3)
* Supply nodes to air loops or fan coil units
* Return nodes from zones
* Floating condenser water control per Table 11.5.2-5

PlantLoop (Hot Water, Systems 2, 4, 6, 7, 10):

* Pump (variable-speed)
* Boiler (fossil fuel or electric)
* Supply nodes to air loops, reheat coils, or fan coils
* Return nodes
* Temperature setpoint and reset control

PlantLoop (Heat Pump Loop, System 6):

* Pump (variable-speed with shutoff)
* Heat pumps (water-source type)
* Fluid cooler for heat rejection
* Boiler for supplemental heating (if included)
* Loop temperature control 16-32°C

Climate Zone Definitions

CZ 0-3A: Electric Heating Zones

* Mild to warm climates with minimal heating demand
* Systems: 1, 3, 5, 6, 8, 9
* Examples: Hawaii, Miami, Los Angeles, Phoenix, San Diego

CZ 3B-8: Fossil Fuel Heating Zones

* Temperate to cold climates with heating demand
* Systems: 2, 4, 7, 10, 11
* Examples: Chicago, New York, Denver, Minneapolis, Seattle, Boston

Key Controls and Setpoints

Thermostat Setpoints (Budget Baseline):

* Heating Setpoint: 21°C (70°F)
* Cooling Setpoint: 24°C (75°F)
* Deadband: Minimum 1.7°C (3°F)

Supply Air Temperature (SAT) Reset:

* Systems 1, 3, 5, 6, 8, 9: Constant design temperature (no reset)
* Systems 2, 4: Design + 2.8°C at minimum cooling load
* Systems 7, 10, 11: Per unit control

System 2 and 4 SAT Reset Detail:

* Base SAT: Design temperature (typically 13°C for chilled water, varies for DX)
* Maximum SAT: Design + 2.8°C (typically 15.8°C)
* Reset logic: Linear or staged based on cooling load
* Control: `SetpointManager:Warmest` with bounds

Hot Water Temperature Reset (Systems 2, 4, 7, 10, 11):

* Design: 82°C at peak heating
* Reset: Outdoor air reset recommended
* Example: 82°C at -15°C outdoor, 55°C at +15°C outdoor
* Control: `SetpointManager:OutdoorAirReset`

Chilled Water Temperature (Systems 1, 2, 5, 7):

* Design supply: 6.7°C (fixed by specification)
* Design return: 13°C (fixed by specification)
* No temperature reset (maintain constant 6.7°C)
* Control: `SetpointManager:Scheduled`

Condenser Water Temperature (Systems 1, 2, 5, 7):

* Floating control per Table 11.5.2-5
* Design approach: 10.02 – (0.24 × WB)
* Design temperature rise: 5.6°C (fixed)
* Control: `SetpointManager:OutdoorAirReset` or scheduled

Economizer Operation (Systems 3, 4, 9, 11):

* Dry Bulb Cutoff: 21°C outdoor (varies by climate per Table 6.5.1.1.3)
* Enthalpy Cutoff: When outdoor air enthalpy exceeds return air
* Integrated Mode: Damper modulation for cooling assistance
* Type: Air (dry bulb) or Fluid (enthalpy) per climate zone

VAV Terminal Minimum Flow (Systems 1, 2, 3, 4):

* Default Minimum: 50% of design air flow (can be reduced to 30% with justification)
* Based on: Ventilation requirements per ASHRAE 62.1 Simplified Procedure
* Calculation: MAX(ASHRAE 62.1 ventilation requirement, applicable code minimum)
* Range: Typically 15-50% depending on zone requirements

***

Document Status: Complete Last Updated: November 2025 Covers: Budget HVAC System Types 1-11 (Table 11.5.2-1) Design Specifications for EnergyPlus


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.betterbuilding.io/user-guide/workflows/energy-efficiency/ashrae-90.1-2019/energy-cost-budget-method/budget-hvac-system-types-1-11.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.