Simulate ASHRAE Guideline 36 AHU control sequences in real time. Adjust outdoor air temperature, zone demands, and occupancy mode to see how economizer, supply air reset, cooling/heating coils, and fan speed respond.
This simulator demonstrates ASHRAE Guideline 36 AHU control sequences in real time, allowing engineers and commissioning agents to verify economizer logic, supply air temperature reset, and occupancy mode transitions without accessing a live building automation system. It is used during design review and commissioning training.
ASHRAE Guideline 36 (High-Performance Sequences of Operation for HVAC Systems) defines standardized AHU control sequences that replace proprietary contractor logic. The core AHU sequence includes supply air temperature (SAT) reset, economizer control, cooling and heating coil staging, and fan speed modulation based on occupancy mode.
SAT reset varies the supply air temperature setpoint from a maximum (e.g., 65°F at no cooling demand) down to a minimum (e.g., 55°F at full cooling demand), reducing reheat energy while maintaining comfort. The economizer enables when outdoor air temperature drops below an enable limit (typically 65–70°F), opening the OA damper to provide free cooling and reducing mechanical cooling energy.
ASHRAE Guideline 36 (2021) is the primary reference for high-performance AHU sequences, covering economizer control, SAT reset, supply fan speed control, and morning warmup. California Title 24 and many jurisdictions require compliance with Guideline 36 for large commercial HVAC systems. ASHRAE 90.1 Section 6.5 mandates economizer controls for systems above a minimum cooling capacity threshold (typically 54,000 BTU/hr / 4.5 tons in most climate zones). ASHRAE 62.1 governs minimum outdoor air rates that must be maintained during economizer operation.
SAT reset range must be coordinated with the VAV box minimum airflow setpoints: lowering SAT increases fan energy as boxes open wider to maintain zone temperatures, and raising SAT too much causes zones to overheat on hot days. Economizer high-limit controls must use the correct sensor strategy — dry-bulb temperature, enthalpy, or differential enthalpy — depending on climate zone per ASHRAE 90.1 Table 6.5.1-1. Morning warmup sequences must account for outdoor air damper minimum positions to avoid freezing heating coils in cold climates; the damper should remain closed or at a controlled minimum until space temperature reaches its setpoint.
Select an occupancy mode (Occupied, Unoccupied, or Morning Warmup), then adjust the outdoor air temperature, zone cooling demand, and zone heating demand sliders. The outputs panel shows fan speed, OA damper position, cooling coil, and heating coil positions in real time, along with actual supply air temperature. Use this to verify that economizer lockout, SAT reset, and minimum OA fraction all behave correctly before commissioning the actual sequence in the DDC controller.
SAT reset raises the supply air temperature setpoint during low-load conditions, reducing the temperature differential the cooling coil must achieve. This reduces chiller energy and eliminates reheat in VAV systems. ASHRAE 90.1 and Guideline 36 both require SAT reset on most HVAC systems.
The economizer enables when the outdoor air temperature (or enthalpy, depending on control strategy) is below the high-limit setpoint, typically 65–75°F depending on climate zone and control strategy. It disables when OAT exceeds the limit, during unoccupied mode, or when the system is in morning warmup with cold outdoor air.
Morning warmup is an occupancy transition mode that runs the AHU before occupancy to bring the space temperature from its unoccupied setback to the occupied setpoint. During morning warmup, the OA damper is typically held closed or at a controlled minimum to avoid overloading the heating coil with cold outdoor air.
Traditional sequences were written by individual contractors and varied widely in quality. ASHRAE Guideline 36 provides a single, prescriptive, high-performance sequence that has been validated to reduce energy use by 15–30% compared to typical practice, while improving comfort and fault detectability.
The minimum outdoor air fraction must satisfy ASHRAE 62.1 ventilation requirements for the served zones. Guideline 36 uses the Zone Outdoor Air Fraction (Vpz) method from ASHRAE 62.1 to calculate the minimum OA that the AHU must deliver. This is typically 15–25% of design airflow for office occupancies.
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