Air Handling Unit
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An Air Handling Unit (AHU) contains the following equipment, some of which are optional.
Supply Air Fan
Return Air Fan
Outside Air System
Heating Coil
Cooling Coil
Setpoint Manager
The AHU connects to Zone HVAC units like Air Terminals, Fan Coil Units, VRF Terminals, and Water-Air Heat Pumps.
Specify the Air Loop Type by choosing Cooling And/Or Heating for an AHU with cooling and/or heating coils. This option manages supply air flow for zone temperature control while meeting minimum ventilation needs. Alternatively, select Dedicated Outside Air System for ventilation-focused supply, accommodating up to 100% outdoor air for cooling/heating.
Specify the Sizing Type of Load by choosing from Sensible, Latent, Total, or Ventilation Requirements. Sensible, Latent, and Total consider zone design airflow rates to meet sensible and latent loads. Latent sizing prioritizes zones sized for latent loads; otherwise, sensible loads are used. If zone latent loads surpass sensible loads, latent sizing is applied. Opt for Ventilation Requirements to determine the central system supply air flow rate based on the system's ventilation needs.
Specify the Cooling Design Supply Air Temperature for heating in degrees Celsius. This value sets the design temperature at the exit of the main heating coil. Note its application in sizing zone equipment (e.g., reheat coil) for systems with central heating coils. If there is no central heating coil, this value is not utilised.
Specify the design humidity ratio in kilograms of water per kilogram of dry air at the exit of the central cooling coil. The default is 0.008 (kg Water/kg Dry Air).
Specify the Heating Design Supply Air Temperature in degrees Celsius. This sets the design temperature at the exit of the primary heating coil. Note its application in sizing zone equipment (e.g., reheat coil) for systems with central heating coils. If there is no central heating coil, this value is not utilised.
Specify the Heating Design Supply Air Humidity Ratio at the exit of the central heating coil, in kilograms of water per kilogram of dry air. This value, used for sizing zone equipment (e.g., reheat coil) in systems with central heating coils, defaults to 0.008 (kg Water/kg Dry Air). Adjust as needed, considering system requirements for optimal performance.
Design Day: Use the System Sizing input data for a design day simulation to calculate the system design cooling supply air volume flow rate.
Flow/System: Utilize the Cooling Design Air Flow Rate input as the system design cooling supply air volume flow rate.
Flow Per Floor Area: Calculate the cooling supply air volume flow rate based on the zone floor area served by the air loop and the user-specified Flow Per Floor Area value.
Fraction Of Autosized Cooling Airflow: Determine the cooling supply air volume flow rate from the user-specified fraction and the simulation's autosized design cooling supply air volume flow rate.
Flow Per Cooling Capacity: Calculate the supply air volume using the user-specified flow per cooling capacity and the design cooling capacity determined by the simulation.
Specify the central cooling coil control method based on system needs VAV for variable air volume cooling output, Bypass if capacity control involves bypassing mixed air, VT when cooling output is regulated by varying coil exit temperature with constant flow, and On-Off for cooling control through air flow cycling.
Specify from Autosize or Manual.
Specify the Setpoint Manager Type: Warmest Temperature Flow or Constant. The Warmest Temperature Flow Setpoint Manager adjusts the cooling supply air temperature based on the warmest zone's demand. Choose between Temperature First, seeking the highest setpoint temperature satisfying all zone cooling loads at minimum flow, or Flow First, finding the lowest flow rate at the maximum setpoint temperature. Temperature First minimizes fan energy but may increase reheat coil or overcooling, impacting chiller energy. Flow First minimizes reheat coil energy and chiller consumption, potentially increasing fan energy. Consider system dynamics and energy priorities when making this selection.
Specify the minimum supply air temperature for Setpoint Manager control. This value sets the lower limit for the cooling supply air temperature as part of the control strategy.
Specify the maximum supply air temperature for Setpoint Manager control. This value establishes the control strategy's upper limit for the cooling supply air temperature.
Specify the flow strategy for the Warmest Temperature Flow setpoint manager. Opt for Temperature First for lower fan and higher chiller energy, or Flow First for higher fan and lower chiller energy. Balance the trade-off based on system priorities and efficiency considerations.
Specify the minimum turndown ratio, representing the minimum ratio of the actual air flow rate to the maximum air flow rate. This applies to the supply fan in the absence of VAV terminal boxes or to all VAV boxes if present. Ensure consistent application across VAV boxes if applicable.
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