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Energyplus design cooling load is zero

asked 2015-07-20 16:44:40 -0500

apetersen gravatar image

Hello,

I saw someone else asked this question, but they didn't have a thermostat in their zones and that isn't my issue. I have a supermarket with 13 RTUs. One has a design heating load of 0 and 8 have a design cooling load of 0. This surprises me because there is lighting, people, and equipment load in all 9 RTUs that are specifying no load. EnergyPlus still allows me to run the model, so I am able to see that there actually is no heating or cooling energy showing up annually for any of those systems. I tried manually inputting the supply CFM and cooling capacity, but the warning still shows up that there is no cooling load for those rooftop units. Has anyone run into this before?

Thank You, Alisa

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Do you have lots of refrigerated cases in your model? They can provide lots of space cooling. Also, check your design day conditions to make sure they are appropriate for your climate.

aparker gravatar image aparker  ( 2015-07-20 17:09:04 -0500 )edit

Do people/equipment/lights/zone control thermostats/cooling and heating SP schedules use SummerDesignDay and WinterDesignDay keywords?

Chandan Sharma gravatar image Chandan Sharma  ( 2015-07-21 02:15:43 -0500 )edit

I do have a lot of refrigerated cases in my model, but a lot of the rooftop units that have 0 cooling load do not have any refrigerated case loads, so I don't think that is the issue. Do the SummerDesignDay and WinterDesignDay key words somehow get recognized by EnergyPlus? The lighting for sure has a summer design day schedule of 1 at all time and winter design day has a design day schedule of 0 at all times, which I think makes sense.

apetersen gravatar image apetersen  ( 2015-07-21 09:08:51 -0500 )edit

Without looking into the input file, few things to first check would be internal loads during summer and winter design days. This warning is generally followed by the message to check Sizing:Zone and ZoneControl:Thermostat inputs. I pulled the RefBldgSuperMarketNew2004_Chicago.idf and commented out SummerDesignDay from the internal loads objects (Equipment/occupancy) and ran it for Chicago weather. It threw the same warning that calculated design cooling load for zone zone is zero. It may not be the same issue in your model but reporting what I found in case that helps.

Chandan Sharma gravatar image Chandan Sharma  ( 2015-07-21 13:20:02 -0500 )edit
1

If the schedules do not specify all day types within a Through period, including SummerDesignDay and WinterDesignDay, then design days will use the AllOtherDays part of the schedule.

rraustad gravatar image rraustad  ( 2015-07-23 14:43:57 -0500 )edit

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answered 2015-08-26 11:28:55 -0500

apetersen gravatar image

Thank you everyone for your help. I determined the issue with my model. The walk-in cooler and freezers had incorrect inputs for the U value of the walls and doors of the walk-in. I believe this incorrect U value carried through from my OpenStudio model. Essentially the U value was very high (~400) instead of 0.04 like it should have been, so it was causing an almost infinite cooling load. This was tricky to see because the input comes in as 4+02, but should have been 4+-02 which is difficult to realize right away. I do think there is an error in how OpenStudio translates this input, so I will be aware of this in the future. Thanks again for everyone's feedback!

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answered 2015-07-24 21:18:53 -0500

Alisa, there are many causes for a no load condition.

I recommend:

1) turning off the display cases to see what the zone loads are:

 Output:Variable,*,Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate,hourly;
 Output:Variable,*,Zone Predicted Sensible Load toHeating Setpoint Heat Transfer Rate,hourly;

2) if no load conditions remain (heating), check infiltration or zone mixing inputs 3) Turn the cases back on and compare the zone loads to the expected display case impact on the zone load using:

 Output:Variable,*,Refrigerated Case Zone Sensible Cooling Rate,hourly;
 Output:Variable,*,Refrigerated Case Zone Sensible Heating Rate,hourly;
 Output:Variable,*,Refrigerated Case Return Air Sensible Cooling Rate,hourly;
 Output:Variable,*,Refrigerated Case Return Air Sensible Heating Rate,hourly;
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Asked: 2015-07-20 16:44:40 -0500

Seen: 1,516 times

Last updated: Aug 26 '15