The difference in solar gains between Trace 700 and EnergyPlus is typically due to the sizing run specifications and not the thermal engines.

The zone thermal loads in EnergyPlus for the sizing runs are based on the design days that you have provide in the IDF. Trace 700 uses monthly design days, whereas the default provided in the EnergyPlus weather files are annual (summer and winter) design days. Most people think that Trace 700 is using summer and winter design days, but it actually has an algorithm to generate monthly design day sequences. The results between each software will be significantly different unless you specify the same design day information for autosizing.

Annual design days are inadequate for sizing most thermal zones, so I would only using the monthly values. This is especially true in the case of solar-driven loads which often occur in the fall and winter.

I have not found the monthly design days packaged with the software, but you can find them in ddy format here: ASHRAE Design Day Data

You may also find differences between the software based on the load calculation methodologies. My understanding is that the Radiant Time Series method found in Trace can have issues in cases that are highly glazed due to the use of the sol-air temperature in calculating heat gains.

Dear Parker,

Thank you for your analysis.

The outdoor air in this model really is empty and the thermal load was a little low because of that, but I have done several other models and I always think the load is too low. I´ve compared the same model simulations in Trace 700 and EnergyPlus and the results in Trace are always higher, in some cases up to 3 times.

When analyzing the sources of the loads in the results of both softwares, I noticed that the loads, due to solar radiation, are the most divergent. In some cases, the fenestration solar load is up to 10 times higher in Trace. I find that very strange. My opinion is that the Trace results seem more suitable, because in some cases the glass area is very large and the loads of EnergyPlus do not seem to follow the increase in glasses. The loads due to ventilation and infiltration are also very different.

I don´t think the input data is wrong. I think it could be some software configuration on how to account for the load due to radiation. Perhaps the following fields are not with the defaults well configured in OpenStudio:


In Timesteps Averaging Window: blanck field
Number of Timesteps Per Hour: 6
Maximum Number of Days Warmup: 25
Minimum Number of Days Warmup: 6
Loads Convergence Tolerance Value: 0:04
Temperature Tolerance Value Convergence: 00:04
Solar Distribution: FullInteriorAndExterior
Tolerance For Time Cooling Setpoint Not Met: 0.2
Maximum HVAC Iterations: 20
Minimum Plant Iterations: 2
Maximum Plant Iteration: 8
Minimum System TimeStep: blank field

I will develop a simplified model in Trace 700 and EnegyPlus - OpenStudio and publish the files and results in a new topic for further discussion.

I believe it would be of great importance to have an opinion about possible differences between both softwares, since they are two well-known tools. However, I think the results should be similar, which I have not seen in my experiments.

In OpenStudio, choosing the Ideal Loads System results in this IDF input for each zone:

ZoneHVAC:IdealLoadsAirSystem,
  Thermal Zone: P1- Sl. Empresa 1 Ideal Loads Air System,  !- Name
  ,!- Availability Schedule Name
  Thermal Zone: P1- Sl. Empresa 1 Ideal Loads Supply Inlet,  !- Zone Supply Air Node Name
  ,!- Zone Exhaust Air Node Name
  50,  !- Maximum Heating Supply Air Temperature [C]
  13,  !- Minimum Cooling Supply Air Temperature [C]
  0.0156,  !- Maximum Heating Supply Air Humidity Ratio [kg-H20/kg-air]
  0.0077,  !- Minimum Cooling Supply Air Humidity Ratio [kg-H20/kg-air]
  NoLimit, !- Heating Limit
  ,!- Maximum Heating Air Flow Rate {m3/s}
  ,!- Maximum Sensible Heating Capacity {m3/s}
  NoLimit, !- Cooling Limit
  ,!- Maximum Cooling Air Flow Rate {m3/s}
  ,!- Maximum Total Cooling Capacity {m3/s}
  ,!- Heating Availability Schedule Name
  ,!- Cooling Availability Schedule Name
  ConstantSensibleHeatRatio,   !- Dehumidification Control Type
  0.7, !- Cooling Sensible Heat Ratio
  None,!- Humidification Control Type
  ,!- Design Specification Outdoor Air Object Name
  ,!- Outdoor Air Inlet Node Name
  None,!- Demand Controlled Ventilation Type
  NoEconomizer,!- Outdoor Air Economizer Type
  None,!- Heat Recovery Type
  0.7, !- Sensible Heat Recovery Effectiveness
  0.65;!- Latent Heat Recovery Effectiveness

Notice that the Design Specification Outdoor Air Object Name field is empty. Per the Ideal Loads System documentation, "If this field is blank, this system will have no outdoor air, and all outdoor air control and heat recovery options will be ignored.." Outdoor Air is typically a large part of HVAC loads, so not having OA in your model might be the cause of the unexpectedly low loads you are seeing.

I still have differences in the cooling loads. Usually, the ground cools down the space a lot so I prefer to use adiabatic boundary conditions for the floor. Glass solar loads are a lot lower in Openstudio/e+ than other software.

Last in the Openstudio result in the Plant Loops Detail the Chiller cooling capacity is very different from the "chiller cooling coil total cooling rate" Output Variable.