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Ashrae sizing: Zone Sensible Cooling and ZoneHVAC:IdealLoadsAirSystem

asked 2018-11-02 12:33:38 -0500

Gio's avatar

Hi, I'm studying ASHRAE 2009 cooling sizing (RESIDENTIAL COOLING AND HEATING LOAD CALCULATIONS Chapter 17) and in my model I'm using an IdealLoad system.

Can I get the sizing results from EnergyPlus? I see two tables that could be useful (in the html file): Zone Sensible Cooling and ZoneHVAC:IdealLoadsAirSystem tables. Is one of these the ASHRAE sizing result?

Referring to the html tables, if I read the table Zone Sensible Cooling it misses the ventilation losses. On the other hand, the ZoneHVAC:IdealLoadsAirSystem gives me very high values (much more than the Zone Sensible Cooling plus the ventilation loss) and I can't find an E+ document where the sizing method is described.

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answered 2018-11-06 17:44:39 -0500

See Engineering Reference section "Zone Design Load Calculation" for a description of the zone sizing methodology. ASHRAE sizing starts with a pure zone sensible load, then adds ventilation, and then can determine equipment sizes. The design loads reported in the HVAC Sizing Summary table output are equivalent to the pure zone load with no ventilation. One must be careful with the assumptions that go into this (internal gain and thermostat schedules for the sizing day types, and more).

The difference in the ideal loads results compared to the peak zone sensible load is likely due to latent cooling. Either turn off dehumidication in the ideal loads object, or report the ideal loads components to break out the components. Output:Variable "Zone Ideal Loads Zone Sensible Cooling Rate" is equivalent to the load reported in the HVAC Sizing Summary table. If you run a simulation for the sizing periods, you should see the same number show up. "Zone Ideal Loads Supply Air Sensible Cooling Rate" includes the ventilation load. And there are similar latent and total outputs as well.

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I think the dehumidification is included (not controlled with a setpoint) depending on the fields "Minimum Cooling Supply Air Temperature" and "Minimum Cooling Supply Air Humidity Ratio" in the ZoneHVAC:IdealLoadsAirSystem object. I wonder if the results of the html table "ZoneHVAC:IdealLoadsAirSystem" are equal to the one of the "Zone Ideal Loads Supply Air Sensible Cooling Rate" output variable. Unfortunately I need a table, not a variable.

Gio's avatar Gio  ( 2018-12-12 02:31:03 -0500 )edit

You refer to an html table for "ZoneHVAC:IdealLoadAirSystem". I don't know what table that is. Please provide a link to the idf file or send it to the helpdesk at

MJWitte's avatar MJWitte  ( 2018-12-14 13:48:11 -0500 )edit

Ok thank you. To get the table you can use the example file "HVACTemplate-5ZonePurchAir". However, if you want to get the "Design Size Maximum Total Cooling Capacity " in the table you have to change "Cooling Limit" to "Limit Capacity" and the "Maximum Total Cooling Capacity" to "autosize" in the "HVACTemplate:Zone:IdealLoadsAirSystem" object.

Gio's avatar Gio  ( 2018-12-15 06:04:10 -0500 )edit

Ok, so you're referring to the "Component Sizing Summary" report. Starting with that base file, a couple of things need to be changed to bring the Design Size Maximum Total Cooling Capacity closer to the maximum of the "Zone Ideal Loads Supply Air Total Cooling Rate". 1. Change the sizing factors to 1.0 in Sizing:Parameters. 2. Change SimulationControl to "Run Simulation for Sizing Periods"=Yes.

MJWitte's avatar MJWitte  ( 2018-12-20 14:23:55 -0500 )edit

Ok, thank you very much

Gio's avatar Gio  ( 2018-12-24 01:24:01 -0500 )edit

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Asked: 2018-11-02 12:33:38 -0500

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Last updated: Nov 06 '18