Knowledgebase : Output
What report variables are available for heat transfer through walls? The simulation shows the zone temperature is lower than outside temperature, but the heat transfer through walls appear negative.

There are no report variables for simple heat transfer through a wall.

There are many reasons this can occur. The usual reason is that solar and thermal radiation energy can heat up the inside surface above the zone air temperature and cause heat flows in the opposite direction of what the air temperature differences would indicate. The storage and release of heat inside the walls due to heat capacity can also lead to this situation.
This is expected. The difference comes from interpolating hourly weather data for subhourly timesteps in EnergyPlus. In an hourly weather file, the temperatures and other state-point readings are the value at the time the reading was taken. For example, in the USA_IL_Chicago-OHare_TMY2.epw file, the outdoor dry bulb value for July 2, hour 1, is 19.4C. This is the temperature at 1:00 am.

If you set Timestep = 1, then EnergyPlus will report 19.4C for 07/02 01:00 and will use that value for the entire one hour timestep.

If Timestep = 4, then 19.4C is used only for the time step which ends at 01:00. The other timesteps use linearly interpolated values between the hourly weather file values. When you report at the "hourly" frequency in EnergyPlus, you see the average temperature over the hour. If you report at the "timestep" frequency, you will see the values from the weather data file appear at the last timestep of each hour.

In summary:
The standard EnergyPlus weather files (epw) contain hourly data. For state variables such as temperature and humidity, the values in the epw file are the instantaneous measurements at 1:00 am, 2:00 am, and so on. Because EnergyPlus uses zone time steps of less than one hour, these values are linearly interpolated for each time step. Weather data values reported from EnergyPlus at the Timestep frequency will match the weather file one timestep per hour on the hour. Values reported at the Hourly frequency represent the average value over the hour, so they will not match the instantaneous value in the weather file.

The solar radiation values in the weather file are integrated total values for the hour, so the interpolation must be done differently. The total radiation in [Wh/m2] is used as an average rate for the hour in [W/m2], and this average rate is assumed to be the value at the midpoint of the hour. Again, linear interpolation is used to determine the value for each timestep. The reported solar values in the output will match the value in the weather file on the half hour.

Question: Is there a way to select all meters on output report ( file just by adding only one object? Currently, I have to go into IDF Editor, select "Output:Meter:MeterFileOnly" and add an object for each meter I want to be included on the output file. If you have 200 or so meters, and want to selected them all, the process of adding one meter object at a time and selecting one meter name from the drop down at a time is time consuming. I was wondering if there is any better way to generate this meters file.

Answer: Put Output:VariableDictionary,IDF; into your IDF file.
Do a simple run. (1 design day for example)

Open the .mdd file and copy those into your IDF file. (It's automated in IDF editor to be able to copy that).

You may have to change the Output:Meter to Output:MeterFileOnly but that should make it easier.
Q: How do I define output variables for my simulation?

A: In your input file place the following statement:


Then you can cut and paste from the rdd file directly into your idf file. You must first run your simulation to create the rdd file. Output variables found in the rdd file are specific to the simulation and are based on the objects used in your input file.

Output:Variable,*,System Node Temp,hourly; !- HVAC Average [C]

Or change to this to get only the information for a single node:
Output:Variable,"The Name of the Node",System Node Temp,hourly; !- HVAC Average [C]

Where "The Name of the Node" is the specific node name for one or more nodes
Q: I am specifying input for Output:Variables but pull-down menu under variable name field is showing blank with no options. Any help?

A: You must run the simulation before anything will show in the dropdown menu (rdd/mdd files must be present)
How do I report the heat transfer through a specific surface or group of surfaces?

There is no such report at present. There is a long-standing project to create a new report which will try to sort out such component loads, but currently, everything is mixed in the heat balance and the identity of heat sources is quickly lost, especially for the envelope.

There are some advanced users who use "Opaque Surface Inside Face Conduction" output variable to report the net heat flux through a given surface over a long period of time. If the surface has any sort of thermal mass, this variable can be misleading on an hourly basis, because it represents the heat flux at the inside face of the surface which may have no current relatioship with the heat transfer at the outside face of the surface.

Other alternatives are to make the other surfaces adiabatic, or to compare results between two runs one with the surface(s) of interest active and another with the surface(s) adiabatic.


I´m trying to size a boiler with a fan coil system. I would like to know the power of the boiler. Where can i find this data? Which output should i use in order to get it?


There are many report variables in EnergyPlus. The ones available for a specific simulaiton are listed in the report data dictionary (rdd) file. These report variables may be generated automatically if the following is included in the input file.

Regular; !- Key Field

When the object above is included in an input file, the rdd file is available for review AFTER the simulation has completed. If this object is not inculded in the input file, the user may still use report variables, but must select them based on the objects inlcuded in the simulation. The Input Output Reference document describes all report variables available for each EnergyPlus object.

There are several choices on format with this object. You can specify "Regular" as the key field and the rdd will show all report variables along with the variable description as shown below.

HVAC,Average,Boiler Heating Output Rate [W]
HVAC,Sum,Boiler Heating Output Energy [J]
HVAC,Average,Boiler Gas Consumption Rate [W]
HVAC,Sum,Boiler Gas Consumption [J]
HVAC,Average,Boiler Water Inlet Temp [C]
HVAC,Average,Boiler Water Outlet Temp [C]
HVAC,Average,Boiler Water Mass Flow Rate [kg/s]
HVAC,Average,Boiler Parasitic Electric Consumption Rate [W]
HVAC,Sum,Boiler Parasitic Electric Consumption [J]
HVAC,Average,Boiler Part-Load Ratio []

As an alternative, the key field "IDF" may also be used.

IDF; !- Key Field

With this option the rdd will format the report variable so that they may be copied directly into the input file using a text editor.

Output:Variable,*,Boiler Heating Output Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Boiler Heating Output Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Boiler Gas Consumption Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Boiler Gas Consumption,hourly; !- HVAC Sum [J]
Output:Variable,*,Boiler Water Inlet Temp,hourly; !- HVAC Average [C]
Output:Variable,*,Boiler Water Outlet Temp,hourly; !- HVAC Average [C]
Output:Variable,*,Boiler Water Mass Flow Rate,hourly; !- HVAC Average [kg/s]
Output:Variable,*,Boiler Parasitic Electric Consumption Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Boiler Parasitic Electric Consumption,hourly; !- HVAC Sum [J]
Output:Variable,*,Boiler Part-Load Ratio,hourly; !- HVAC Average []

The expanded version of the output variable object is shown below.

*, !- Key Value
Boiler Heating Output Rate, !- Variable Name
hourly, !- Reporting Frequency
MyReportVarSchedule; !- Schedule Name

MyReportVarSchedule, !- Name
On/Off, !- Schedule Type Limits Name
Through: 1/20, !- Field 1
For: AllDays, !- Field 2
Until: 24:00, 0.0, !- Field 4
Through: 12/31, !- Field 5
For: AllDays, !- Field 6
Until: 24:00, 1.0; !- Field 8

On/Off, !- Name
0:1, !- Range
DISCRETE; !- Numeric Type

This allows several options for reporting. First the key value may be an asteric (*) where all report variables of this type are reported (for all boilers). Or the key value could be specified such that only a single output will be generated. For example if the key value was specified as "My Boiler" and a boiler object with the name My Boiler was included in the input, only the Boiler Heating Output Rate for this specific boiler will be in the output file (.csv). The reporting output for all other boilers in the simulation will not be included in the csv file.

The reporting frequency is also another option and may be one of several choices (e.g., Timestep, Hourly, Daily, Monthly, RunPeriod, Environment, Annual or Detailed).

The detailed reporting frequency reports the data for every simulation time step (HVAC variable time steps). This choice is useful for detailed troubleshooting and reporting.
The other choices average or sum the data over the selected interval. Timestep refers to the zone Timestep/Number of Timesteps in hour value and reports the data at regular intervals. Using RunPeriod, Environment, or Annual will have the same affect on the reporting frequency and refer to the length of the simulaiton as specified in the RunPeriod object.

4; !- Number of Timesteps per Hour

1, !- Begin Month
1, !- Begin Day of Month
12, !- End Month
31, !- End Day of Month
Tuesday, !- Day of Week for Start Day
Yes, !- Use Weather File Holidays and Special Days
Yes, !- Use Weather File Daylight Saving Period
No, !- Apply Weekend Holiday Rule
Yes, !- Use Weather File Rain Indicators
Yes; !- Use Weather File Snow Indicators

A schedule may also be used to turn on or off report variable at selected intervals.

Table reports and meters are also available as reporting options. See the Input Output and Engineering Reference manuals for further details.