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loadmodeling.tool
Air Systems
Air System Inputs
Air System Inputs

Settings and customization of air system in loadmodeling.tool

Patrick Pease avatar
Written by Patrick Pease
Updated over a week ago

Air Systems allow engineers to create and customize the wide range of air handling systems used in HVAC design. This includes Air Handling Units (AHU), Dedicated Outdoor Air Systems (DOAS), Energy Recovery Ventilators (ERV), fan only systems and more. There is a wide range of settings and controls for these design elements which are summarized here. More details on inputs can be found by following the relevant links.

User Interface Guide

Air System Details

  • Name: This is the title for the system. It must be unique.

    • Note: Ideal Air Loads are auto assigned when projects do not have systems. Therefor Ideal Air Loads cannot be used to name a created system.

  • Energy Code: This field describes the energy code associated with the system, if applicable.

  • Air System Type: This is the general type of the air system, used for easy organization

  • Description: Notes about the air system

  • Creator: This is the owner of the air system, it will be auto assigned when created

  • Source: The Firm name who created the original air system

  • Template: Reference to the original template used to generate the air system

  • Number of Zones: The total number of zones assigned to this air system

  • Area Served: The total area of the project served by the air system

Air Flow Parameters

  • Min Outdoor Rate [CFM (IP) | L/S (SI)]: The minimum outdoor air requirement for the system. Can be Autosized

  • Max Outdoor Rate [CFM (IP) | L/S (SI)]: The maximum outdoor air requirement for the system. Can be Autosized

  • Min Supply Air Rate [CFM (IP) | L/S (SI)]: The minimum supply air requirement for the system. Can be Autosized

  • Max Supply Air Rate [CFM (IP) | L/S (SI)]: the maximum supply air requirement for the system. Can be Autosized

  • Return Air Flow Fraction: based on the supply air flow rate, this fraction determines the return air flow. This is useful for controlling pressurization of the system or if setting to 0, indicating a DOAS system. Allowable values are 0 to 2.

  • Zone Max Outdoor Air Fraction: for VAV systems this fraction sets the maximum outdoor air % allowed at a zone. For zones that require higher outdoor air than the fraction, the supply air flow is increased. Allowable values are 0 to 1. Setting to 1 allows zones to go 100% outdoor air.

    See Air Flow Parameter Details for more information and explanation of autosizing methods.

Fan Parameters

  • Supply Fan Type: Types available are variable, constant or -- for no fan

  • Relief Fan Type: Types available are variable, constant or -- for no fan

  • Supply Fan Location: Options are Blow-through and Draw-through

  • Relief Fan Location: Options are Return and Exhaust

  • Supply Fan Motor Fraction in Air Steam: The fraction to which the fan motor is exposed to the conditioned air, thus transferring heat to it. Range allowed is 0 (motor fully out of air stream) to 1 (motor fully in air stream).

  • Relief Fan Motor Fraction in Air Steam: The fraction to which the fan motor is exposed to the conditioned air, thus transferring heat to it. Range allowed is 0 (motor fully out of air stream) to 1 (motor fully in air stream).

    See Fan Details for definitions of the locations and their impact to sizing

Primary Cooling Coil Configuration

  • Coil Type: Options are Hydronic, DX (Direct Expansion), and -- for none

  • Entering Air Dry Bulb and Wet Bulb [°F (IP) | °C (SI)]: The entering air conditions of the coil. The default entering temperature is read from the Summer Design Day

  • Leaving Air Dry Bulb and Wet Bulb [°F (IP) | °C (SI)]: The leaving air conditions of the coil.

  • Entering/Leaving Fluid Temp [°F (IP) | °C (SI)]: The entering and leaving fluid temperatures of the coil. This field is required for Hydronic type coils only.

  • Rated Capacity [Btu/h (IP) | Watts (SI)]: Default is Autosized, however the value can be manually assigned.

  • Cooling Source: The Mechanical Plant acting as the cooling source for this coil. The default is 'ideal Cooling Loop' which allows the coil to be sized without defining a Mechanical Plant. This field is required for Hydronic type coils only.

The Mixed Air Temperature for cooling coils can be autosized. This instructs the simulation to account for return air mixing and/or energy recovery devices impact on the entering coil temperature for the cooling coil.

Set Mixed Air Temperature to Autosize

Primary Heating Coil Configuration

  • Coil Type: Options are Hydronic, Electric, Gas, DX (Direct Expansion), and -- for none

  • Entering/Leaving Air Temp [°F (IP) | °C (SI)]: The entering and leaving air temperatures of the coil. The default Inlet temperature is read from the Winter Design Day

  • Entering/Leaving Fluid Temp [°F (IP) | °C (SI)]: The entering and leaving fluid temperatures of the coil. By default these are assigned per the Heating Source Mechanical Plant.

  • Rated Capacity [Btu/h (IP) | Watts (SI): Default is Autosized, however the value can be manually assigned.

  • Heating Source: The Mechanical Plant acting as the cooling source for this coil. The default is 'ideal Heating Loop' which allows the coil to be sized without defining a Mechanical Plant. This field is required for Hydronic type coils only.

Economizer Parameters

  • Control Type: Options are Fixed Enthalpy, Fixed Dry Bulb or -- (none)

  • Enthalpy Max Limit [Btu/lb (IP) | J/Kg (SI)]: The numeric value for enthalpy limit

  • Dry Bulb Max Limit [°F (IP) | °C (SI)]: The numeric value for dry bulb limit

  • Min Outside Air Fraction: The min allowable fraction of supply air that can be outside air. Assigned to an 'other' type schedule

  • Max Outside Air Fraction: the max allowable fraction of supply air that can be outside air. Assigned to an 'other' type schedule

    See Modeling DOAS for how to apply these settings to make any Central system 100% outside air.

Energy Recovery

  • Type: Options are Plate, Rotary or -- for none

  • Sensible Heating %: Effectiveness of sensible heat exchange at 100% capacity and ARI standard conditions for heating

  • Latent Heating %: Effectiveness of latent heat exchange at 100% capacity and ARI standard conditions for heating

  • Sensible Cooling %: Effectiveness of sensible heat exchange at 100% capacity and ARI standard conditions for cooling

  • Latent Cooling %: Effectiveness of latent heat exchange at 100% capacity and ARI standard conditions for cooling

  • Frost Control: Options are Exhaust Air Recirculation, Exhaust Only, Minimum Exhaust Temperature or -- for none.

  • Economizer Bypass: Yes or no. When yes, the recovery device is bypassed or off at times the air side economizer is active

  • Nominal Power [Watts]: The electrical power required to operate the energy recovery device. This can be for controls, and/or motors associated with the device

    See Air Side Energy Recovery for more information

Zone Sizing

  • Cooling Air Distribution Effectiveness: Numeric value for the cooling air distribution effectiveness as defined by ASHRAE Standard 62.1

  • Heating Air Distribution Effectiveness: Numeric value for the heating air distribution effectiveness as defined by ASHRAE Standard 62.1

  • Zone Supply Air Temperatures:

    • Cooling Dry Bulb [°F (IP) | °C (SI)]: Supply Air Temperature used for zone air flow sizing in cooling

    • Cooling Wet Bulb [°F (IP) | °C (SI)]: Supply Air Temperature used for zone air flow sizing in cooling

    • Heating Dry Bulb [°F (IP) | °C (SI)]: Supply Air Temperature used for zone air flow sizing in heating

    • Heating Wet Bulb [°F (IP) | °C (SI)]: Supply Air Temperature used for zone air flow sizing in heating

System Sizing Settings

  • Loading Type for System Sizing: Controls what peak the system sizing calculation solves for. Options are Total, Ventilation Requirement or Sensible

  • System Sizing Option: Controls if the system will be sized for coincident (block) or peak loads. Options are Coincident and Non-Coincident

  • Outdoor Air Sizing Method: Controls if the outdoor air will be sized via the ASHRAE Standard 62.1 Ventilation Rate Procedure or by the simple Sum of Zones methods.

  • Cooling Capacity Control Method: Options are:

    • On Off - cooling capacity controlled by cycling air flow

    • VT - Variable temperature, cooling capacity controlled by constant airflow, variable air temperature

    • Bypass - cooling capacity controlled by by passing variable airflow around cooling coil

    • VAV - Cooling capacity controlled by varying air flow

  • Sizing Cooling for 100% OA?: Option to force cooling coil to be sized for 100% OA

  • Sizing Heating for 100% OA?: Option to force heating coil to be sized for 100% OA

Control Parameters

  • System Availability Schedule: Schedule of operation for the system, this can be any Other type schedule within the project.

  • System Night Cycle: Options are Cycle on full system if heating or cooling required, Follow the HVAC operation schedule, Cycle on zone terminal units if heating or cooling required, or -- for none. The night cycle defines system behavior when at setback.

  • Demand Control Ventilation: On/off toggle for system level DCV

Zone Terminal Units

Video Tutorial

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