Autosizing Process in EnergyPlus
(with thanks to Mike Witte, GARD Analytics; The overall process is mostly reported in the eio output)
1. First, an ideal loads simulation is performed to determine design heating and cooling loads for each zone. .EIO reports these as Zone Sizing Information “Calc Des Load”
2. The respective heating/cooling sizing factors are then applied to these zone loads. .EIO reports these as Zone Sizing Information “User Des Load”
Note: This is the only place that the sizing factors are used.
3. Using the supply air temps from Sizing:Zone and the zone setpoint(?) temp at peak load determine design heating and cooling supply flow rates. EIO reports these as Zone Sizing Information “Calc Des Air Flow Rate” and “User Des Air Flow Rate”.
Note: User Des Air Flow Rate is based on User Des Load OR uses a hard value from Sizing:System if specified.
4. For each zone, the greater of the heating or cooling supply flow becomes the design supply flow for that zone. EIO reports these as Component Sizing Information, AirTerminal:SingleDuct:Uncontrolled, name, Maximum Air Flow Rate, value
5. For each air handler, the zone air flow rates are summed, or the user can override total system flow in the Sizing:System object
· EIO reports these as System Sizing Information, systemname,
o Calculated Cooling Design Air Flow Rate [m3/s], value
o User Cooling Design Air Flow Rate [m3/s], value
o Calculated Heating Design Air Flow Rate [m3/s], value
o User Heating Design Air Flow Rate [m3/s], value
Note: Heating and cooling design flows are the same if it's a single duct system.
6. The greatest of the two User Design Air Flow Rates for the system will become the system supply flow rate.
· EIO reports these as Component Sizing Information,
o Branch, branchname, Maximum Flow Rate [m3/s], value
o AirLoopHVAC, airloopname, Design Supply Air Flow Rate [m3/s], value
o Component Sizing Information, Fan:OnOff, componentname, Maximum Flow Rate [m3/s], value
7. At this point, the central coils are sized to meet the supply air temperatures specified in Sizing:System. Coil sizes will also be impacted by the mixed air temp at time of design load.
· For DX coils there's the added constraint that a DX coil will not autosize outside the range of cfm/ton that the model accepts (300-450 or something in that range). So, it will figure out a capacity that meets the supply temp, then check if the cfm/ton is within range. If not, the DX capacity will be adjusted up or down to fall within that range.
James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of alexmac_00@xxxxxxxx
Sent: Tuesday, November 26, 2013 7:10 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Sizing of heating and cooling coil
Dear EnergyPlus users,
I am trying to understand how EnergyPlus does the sizing of heating and cooling coil of a ventilation system (air loop). Anyone of you know which parameters are taking into account and which objects are involved?
Thanks for your support,
Alessandro
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