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# Thermal Load too low in the OpenStudio / EnergyPlus

The first step to project HVAC System is to calculate the Thermal Load. I'm using the function Ideal Air Loads for this, is it all right? The problem is that my thermal loads in "HVAC Sizing Summary" are normally too low. The value "User Design Load per Area" is common 80 W/m2, while in my region (ASHRAE Climate Zone 2) with the typical construction here, a common value for this is 160 W/m2. Somebody has the same question? Is it wrong? I´ve already done several models with several constructions and the Thermal Load is always too low.

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@Pithon - please tag your question with the appropriate software so that others can find it. Also suggest correcting idelairloads tag to ideal-air-loads. Thanks!

( 2014-12-07 17:21:51 -0600 )edit

Thank's Matthew! I'm going to correct this!

( 2014-12-08 17:28:31 -0600 )edit

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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.

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Dear Parker,

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
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.

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Pithon,

Thank you for this post. I too noticed a low calculated cooling load here. However, you actually did articulate a possible culprit.

There are differences in solar modeling between Trace & E+, but this paper back in 2010 states the "difference from the mean for peak cooling load" was < 3% when using ASHRAE 140. In fact, E+ was found to have higher gains than Trace 700?

I believe you are on to something here.

( 2014-12-30 10:43:41 -0600 )edit

Additionally, I would look at the difference between WindowMaterial:SimpleGlazingSystem & the more detailed models. I am not saying this is the culprit, but you do have a simple glazing material specified for the construction set in OpenStudio.

( 2014-12-30 11:01:48 -0600 )edit

A difference in fenestration solar load of 10x seems to indicate something is fundamentally different about the two models, or the values being used to compare "fenestration solar load" may not mean the same thing in the two programs, possibly one is reporting gains and the other one loads? Looking forward to seeing the results from your simplified comparison.

( 2015-01-08 12:20:08 -0600 )edit

I got similar results from Pithon in one of my simulations. We both have similar climates in our location: hot and humid all the year. The openstudio application does not have by default a way to specify ground temperatures. In my case I was getting a huge heat loss through the floor slab (39%) which obviously does not make sense. It was not obvious to me until a saw the "Zone Component Load Summary Report". This report is not a default in eplus but it is a much better troubleshooting tool to see what's happening in peak loads calculations.

( 2015-04-03 10:36:10 -0600 )edit

i have the same question. I did my calculations in both softwars, One of my thermal zones has 3 times more cooling load in trace than OS. I compared the zone component load summary and found that the fenestration solar is more 3 times in Trace. I dont know if energy plus is taking in to account the shading from the same building and trace dont or what. (I exported the gbXML to trace, used same U values, SHGC people etc.)

( 2015-07-20 19:38:11 -0600 )edit

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.

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Geraldo, Have you arrived to final answer to this question? I’ve noticed in several E+ models that the HVAC systems have a much lower auto-sized supply air volume (which is based on the cooling load) than the systems sized by Trace. I'm trying to understand why I have such a big discrepancy. Thanks!

( 2017-08-30 19:55:32 -0600 )edit

I saved the following from March 2016:

The E+ Helpdesk passed your files to me since I am the principal developer for Trane Trace700. Here are some general comments: 1. Trace700 usually will run all 12 cooling design months and so it may happen that a zone does not peak in same month as the summer design OADB and this may be due to a higher solar component in the off month. So I always recommend, whether running Trace or E+, to run all 12 months for cooling design.

1. Trace allows the user to choose from several cooling load methodologies, some going back to the 1970's (such as TETD and CLTD/CLF) which are generally quite conservative compared to more recent methods. The most accurate of the Trace load methods is the RTS Heat Balance method which uses the ASHRAE Loads Toolkit algorithms to calculate both the hourly loads and the "room load" components. This method nearly always generates lower cooling loads than the older ASHRAE load methods. For solar calculations, the RTS method is the only Trace load method which automatically accounts for solar that reflects off the floor then back out through the windows. (Though in your case, this loss was quite small.)

2. You chose the correct E+ summary report to compare against Trace but I'll repeat for other users' benefit: When comparing the load design output from Trace700, the closest report in EnergyPlus is the Zone Component Load Summary report which is automatically generated by the keyword Output:Table:SummaryReports,AllSummaryAndSizingPeriod. These reports are meant to display both the instantaneous and delayed load components so the user has an idea which part of the building envelope or interior loads contributed to the HVAC sizing requirements under Ideal Load conditions. The E+ "Sensible Heat Gain Summary" summary report is also very useful for displaying realtime simulation heat gains but is not meant for to show load sizing components.

3. The window "90.1 Window Zone 3 Metal All Other" referenced throughout these datasets uses the simplified glass model for both Trace and E+. However, the E+ "simplified" model (WindowMaterial:SimpleGlazingSystem) can produce different results than the Trace700 "Std DS Glass" model. Just because the SC and SHGC can be made to match, each have different methods for calculating the SHGC incidence angle modifier. Perhaps, I'll compare these two methods at a later time.

4. It is important to note how E+ and Trace report the glass solar and glass conduction. For example, in Trace, the solar absorbed by the window is added to the solar heat gain, not the window conduction (which is simply UATD in Trace) whereas in EnergyPlus, the the opposite occurs, i.e. solar absorbed by the window is part of the conduction load component. So the correct procedure is to compare E+ (Fenestration Solar + Conduction) vs Trace (Glass Solar + Conduction).

With that as background, I looked at the E+ IDF and Trace file you sent in. For comparison purposes I only looked at ...

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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.

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