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NREL / EnergyPlus

EnergyPlus™ is a whole building energy simulation program that engineers, architects, and researchers use to model both energy consumption and water use in buildings.
https://energyplus.net
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Add more sizing discussion to Tips and Tricks doc #4280

Open BobHenninger opened 10 years ago

BobHenninger commented 10 years ago

Expand the explanation of sizing to the Tips and Tricks doc. Helpdesk Ticket 4002

mjwitte commented 10 years ago

This is the reply to the user. Similar content would be useful in Tips and Tricks:

Well, this may be the first time someone has complained about too few unmet thermostat hours. I took the HVACTemplate-5ZoneFurnaceDX example file, and changed the sizing:parameters sizing factors to be 0.5 and got over 700 unmet cooling hours, but zero unmet heating hours - in Chicago, this is cooling supply flow dominated, so still plenty of airflow available to meet heating loads. So, is it possible that your system was heating supply flow dominated?

There's very little magic involved in EnergyPlus sizing - think simple, very simple. The overall process is mostly reported in the eio output:

  1. First, do an ideal loads simulation to determine design heating and cooling loads for each zone. eio: Zone Sizing Information, Calc Des Load {W}
  2. Apply the respective heating/cooing sizing factors to these zone loads. eio Zone Sizing Information, User Des Load {W} \ this is the only place that the sizing factors are used **

  3. Using the supply air temps from sizing:zone and the zone temp at peak load determine design heating and cooling supply flow rates eio: Zone Sizing Information, Calc Des Air Flow Rate {m3/s}, User Des Air Flow Rate {m3/s}

    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: Component Sizing Information, AirTerminal:SingleDuct:Uncontrolled, SPACE1-1AIRTERMINAL:SINGLEDUCT:UNCONTROLLED, Maximum Air Flow Rate [m3/s], 0.14558

  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: System Sizing Information, FURNACE DX 1-1, Calculated Cooling Design Air Flow Rate [m3/s], 0.14558 System Sizing Information, FURNACE DX 1-1, User Cooling Design Air Flow Rate [m3/s], 0.14558 System Sizing Information, FURNACE DX 1-1, Calculated Heating Design Air Flow Rate [m3/s], 0.10384 System Sizing Information, FURNACE DX 1-1, User Heating Design Air Flow Rate [m3/s], 0.10384

    Heating and cooling design flows are the same here because 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: Component Sizing Information, Branch, FURNACE DX 1-1 MAIN BRANCH, Maximum Flow Rate [m3/s], 0.14558 Component Sizing Information, AirLoopHVAC, FURNACE DX 1-1, Design Supply Air Flow Rate [m3/s], 0.14558 Component Sizing Information, Fan:OnOff, FURNACE DX 1-1 SUPPLY FAN, Maximum Flow Rate [m3/s], 0.14558

  7. At this point, the central coils are sized to meet the supply air temps specified in sizing:system - clueless at this point what the zone heating/cooling loads may be. 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 it the cfm/ton is within range. If not, the dx capacity will be adjusted up or down to be in that range.

    By comparing eio outputs, you should be able to find the reason why your undersized system still met the cooling loads.