ASHRAE 90.1 Appendix G Baseline System Selection

System Selection (Table G3.1.1)

Select baseline system based on building type, heating fuel, and floor area / floor count.

Residential Buildings

(hotels/motels, dormitories, apartments)

Heating Source	System
Fossil fuel / hybrid	System 1: PTAC (any size)
Electric resistance	System 2: PTHP (any size)

Non-Residential Buildings

Heating Source	< 25,000 ft2	25,000-150,000 ft2	> 150,000 ft2
Fossil fuel / hybrid (<=3 floors)	System 3: PSZ-AC	System 3: PSZ-AC	System 5: Pkg VAV w/ Reheat
Fossil fuel / hybrid (>3 floors)	System 3: PSZ-AC	System 5: Pkg VAV w/ Reheat	System 7: VAV w/ Reheat
Electric resistance (<=3 floors)	System 4: PSZ-HP	System 4: PSZ-HP	System 6: Pkg VAV w/ PFP
Electric resistance (>3 floors)	System 4: PSZ-HP	System 6: Pkg VAV w/ PFP	System 8: VAV w/ PFP

Heated-Only Buildings

(warehouses, garages, no mechanical cooling)

Heating Source	System
Fossil fuel / hybrid	System 9: Gas Unit Heater
Electric resistance	System 10: Electric Unit Heater

System Summary

#	Name	Equipment Level	Heating	Cooling	Plant Loops
1	PTAC	Zone	Electric resistance	DX single speed	None
2	PTHP	Zone	DX heat pump + elec backup	DX heat pump	None
3	PSZ-AC	Central (single zone)	Gas furnace or elec	DX single speed	None
4	PSZ-HP	Central (single zone)	DX HP + elec backup	DX heat pump	None
5	Pkg VAV w/ Reheat	Central (multi zone)	HW reheat coils	DX two speed	Hot water loop + boiler
6	Pkg VAV w/ PFP	Central (multi zone)	Electric PFP boxes	DX two speed	None
7	VAV w/ Reheat	Central (multi zone)	HW reheat coils	Chilled water	HW + CHW + condenser loops
8	VAV w/ PFP	Central (multi zone)	Electric PFP boxes	Chilled water	CHW + condenser loops
9	Gas Unit Heater	Zone	Gas unit heater	None	None
10	Electric Unit Heater	Zone	Electric unit heater	None	None

Heating Fuel Logic

The heating_fuel parameter maps to:

• "NaturalGas" → fossil fuel systems (1, 3, 5, 7, 9)
• "Electricity" → electric systems (2, 4, 6, 8, 10)
• "DistrictHeating" → treated as fossil fuel path

Economizer Applicability

• Systems 3-8: Support air-side economizer (economizer=True by default)
• Systems 1-2, 9-10: Zone equipment, no economizer possible
• Climate zone affects economizer control type (not yet parameterized)

Multi-Zone Constraints

• Systems 1-2, 9-10: One unit per zone (zone equipment)
• Systems 3-4: One air loop per zone (PSZ = packaged single-zone)
• Systems 5-8: One air loop serves multiple zones (VAV)

When user provides multiple zones:

• Systems 3-4 create separate air loops per zone
• Systems 5-8 create one shared air loop

Tool Mapping

Action	Tool	Key Parameters
Create baseline system	add_baseline_system	system_type, thermal_zone_names, heating_fuel
List available systems	list_baseline_systems	(none)
System details	get_baseline_system_info	system_type
Modern alternatives	add_doas_system, add_vrf_system, add_radiant_system	thermal_zone_names

Two paths, know which you need: the tools above are generic wiring templates (explicit topology control, no standards equipment efficiencies). Use them to SELECT and WIRE a system type. For comparing systems or any decision-grade EUI/comfort numbers, build each candidate through openstudio-standards instead: create_typical_building(system_type=..., hvac_only=True) — replaces only the HVAC (standards-tuned), preserving loads/schedules/thermostats.

When to Recommend Modern Templates Instead

Scenario	Recommendation
High ventilation requirements (labs, hospitals)	DOAS with fan coils or chilled beams
Many small zones with diverse loads	VRF heat pump system
Comfort-critical spaces, radiant heating/cooling	Radiant floor/ceiling
Net-zero or high-performance target	DOAS + Radiant or DOAS + VRF