See Generic Air Loop Tutorial
The air loop edit dialog allows you to control the way that the air system is sized and in particular in how the design supply and outside air flow rates as well as heating and cooling capacities are calculated.
This is the name of the loop. If the supplied loop name is found to already exist, the software will automatically append an integer to create a unique name.
The rest of the Air loop dialog provides the sizing inputs needed to perform a central forced air system design air flow, heating capacity, and cooling capacity calculation for a system serving one or more zones. The information needed consists of the outside environmental conditions and the design supply air temperatures, outdoor air flow rate, and minimum system air flow ratio.
The outside conditions come from the design days in the input. A system sizing calculation is performed for the design day defined in the input file and the resulting maximum heating and cooling air flow rates and capacities are used in the component sizing calculations.
Supply air conditions are specified by inputting a supply air temperature for cooling, a supply air temperature for heating, and a preheat temperature. The system sizing calculation sums the zone design air flow rates to obtain a system supply air flow rate. The design conditions and the outdoor air flow rate are used to calculate a design mixed air temperature. The temperature plus the design supply air temperatures allows the calculation of system design heating and cooling capacities.
Note: Detailed HVAC autosizing calculations are not to be confused with Heating and Cooling design calculations, which use separate simpler data entry from the HVAC tab. No Detailed HVAC data is used for Heating and Cooling design calculations, which are based on the EnergyPlus Ideal loads system.
The design outdoor air flow rate (in m3/s or ft3/min). Generally this should be the minimum outdoor air flow. It is used for both heating and cooling design calculations. The assumption for cooling is that any outdoor air economizer will be closed. If Autosize is input the Design outdoor air flow rate used in the simulations will be taken from the sum of the zone outdoor air flow rates based on the Sizing option below.
Note that the Design supply air flow rate is defined in the AHU dialog contained within the Air loop.
This ratio is used in calculating the central system heating capacity and is only required when the Fan type of the attached AHU is 2-Variable volume. It should be set so as to define the expected system flow rate when maximum heating demand occurs. Thus if the system is VAV with the zone VAV dampers held at minimum flow when there is a zone heating demand, this ratio should be set to the minimum flow ratio. If the zone VAV dampers are reverse action and can open to full flow to meet heating demand, this ratio should be set to 1.
The value entered must be between 0 and 1.
The Sizing option is set to as follows:
For CAV air loops you can define the way the central system supply air flow rate is sized. The choices are:
The central heating coil is always sized at the conditions at the peak sensible heating load.
For VAV systems you should select either 1‑Sensible or 3-Total.
Important Note: For CAV air loops used to model a dedicated outdoor air system (DOAS), you must set the Type of load to size on to 2‑Ventilation requirement. This is true even if the air is being pre-heated and/or pre-cooled to temper it before delivery.
The method used to calculate the system minimum outdoor air flow. The two choices are:
Note: When the Type of load to size on is set to 2‑Ventilation requirement the System outdoor air method control is set to 1-Zone sum and the control is greyed out.
For an air loop, when a zone requires outdoor air higher than the user specified Zone maximum outdoor air fraction, the zone supply air flow will be capped at the maximum value. This allows the system level outdoor air flow to be reduced while the total supply air flow increases. Valid values are from 0 to 1.0. Default is 1.0 which indicates zones can have 100% outdoor air maintaining backward compatibility. This inputs work for constant volume air systems, single and dual duct VAV systems.
This is the design air temperature exiting the preheat coil (°C or °F).
This is the design humidity ratio exiting the preheat coil (in kg/kg).
This is the design supply air temperature for heating (°C or °F). It can be either the reset temperature for a single duct system or the actual hot duct supply air temperature for dual duct systems. It should be the temperature at the exit of the main heating coil.
The default value for newly added air loops is 16°C corresponding to a reheat-type system where the zone temperature control is provided through local reheat terminal units.
Note: For a centralised system where the AHU is to provide zone heating, you should increase this value to say 35-50°C in order for the heating coil sizing to be performed correctly.
This data is set as follows:
This is the design humidity ratio at the exit of the central heating coil (in kg/kg) . The default is 0.008.
The input must be either:
The default method is 1-Design day, i.e., the program uses the calculated design values.
This is the design system heating air flow rate (m3/s or ft3/min). This input is an alternative to using the program calculated value. This input is used if Heating design air flow method is 2-Flow/System. This value will not be multiplied by any sizing factor or by zone multipliers. If using zone multipliers, this value must be large enough to serve the multiplied zones.
This is the design air temperature exiting the pre-cooling coil (°C or °F).
This is the design humidity ratio exiting the pre-cooling coil (kg/kg).
This is the design supply air temperature for cooling (in °C or °F). This should be the temperature of the air exiting the central cooling coil.
This value should typically be lower than the Central heating design supply air temperature.
This data is set as follows:
This is the design humidity ratio at the exit of the central cooling coil (in kg/kg). The default is 0.008.
The input must be either:
The default method is 1-Design Day, i.e. the program uses the calculated design values.
This is the design system cooling air flow rate (m3/s or ft3/min). This input is an alternative to using the program calculated value. This input is used if Cooling design air flow method is 2-Flow/System. This value will not be multiplied by any sizing factor or by zone multipliers. If using zone multipliers, this value must be large enough to serve the multiplied zones.
Schedule values greater than zero (usually 1 is used) indicate that the system is on. Schedule values less than or equal to zero (usually 0 is used) denote that the system is off. This schedule overrides the air loop fan schedules for determining whether the fans are on.