Re: [EnergyPlus_Support] "Trying to understand "Zone Predicted Sensible Loads...."

Jim

Looks pretty unstable. There are different things that can be affecting the HVAC load solution.

- I would have a look at the different time steps at ConvergenceLimits, and try a shorter time step.

- Think that the load is calculated with the DeltaT of Air Supply Temp and Zone Mean Air Temperature. I would have a look at the air supply temperature and see if it varies.

- Think that the zone air temperature is calculated beforehand with a heat balance equation. Within this equation, many things can also make the final temperature vary if, for instance, your weather file varies: infiltration load, convective air transfer according to surface temperature etc (look at your cloud cover fraction). Do you have infiltration set up to your fridge?
- I would plot the Zone Air System Sensible Heating Energy [J] (also cooling) and use the "detailed" reporting period to see what is the actual heating or cooling provided by the HVAC system, not the "predicted".

Saludos from Hibernia

Luis

2015-05-01 20:09 GMT+01:00 Jim Dirkes jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

Â

Dear forum,
I have used a couple of the "Zone Predicted..." output variables in the past with the assumption that they provide a reasonable indication of the cooling or heating load that is present for a particular timestep.Â I'm now working on a refrigerated warehouse that is maintained at -18C and am doubting my previous understanding of what the variable means.

The "Zone Predicted..." definition seems straightforward, but my energy model results don't seem to agree...
The table below shows results for the summer design day afternoon.
It's 30C (86F) outdoors
TheÂ zone temperature is reasonably close to the setpoint (consistently about 1C colder than setpoint)
TheÂ "Zone Predicted Load..." (in red) shows that most of the time there is a heating load, indicated by a positive value.Â That makes some sense because the zone is colder than setpoint and needs some "heating".
It makes no sense to me that an adjacent timestep has a cooling load (negative value) while the zone temperature is also colder than setpoint.
Will you please help me understand what is going on?

â??â??

The I/O Reference definition:
Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate [W]
This is the predicted sensible load in W required to meet the current zone thermostat cooling setpoint. A positive value indicates a heating load, a negative value indicates a cooling load. This is calculated and reported from the Predict step in theÂ ZoneÂ Predictor-Corrector module. This value is not multiplied by zone or group multipliers
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James V Dirkes II, PE, BEMP, LEED AP
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The Building Performance Team Inc.
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LUIS M BLANES RESTOY
IRUSE
INFORMATICS RESEARCH UNIT FOR SUSTAINABLE ENGINEERING

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