Question-and-Answer Resource for the Building Energy Modeling Community
Get s tarted with the Help page
Ask Your Question

Revision history [back]

click to hide/show revision 1
initial version

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 Office 2 since that was the one zone which peaked in May for both Trace and E+. I changed the Trace dataset's Cooling Load Methodology from TETD to RTS Heat Balance, though this change reduced the total solar by only a small amount. Here are the results for Office 2:


   Peaks at 5/21 17:00
   Outside Dry Bulb Temperature  = 19.88 C
   Zone Dry Bulb Temperature =  23.99 C
   Fenestration Conduction = 1122.9 W
   Fenestration Solar = 1356.45 W
   Fenestration Conduction + Solar = 1122.9 + 1356.45 = 2,479 W

Trace per RTS (Heat Balance):

   Peaks at  5/21 16:00
   Outside Dry Bulb Temperature  = 19 C
   Zone Dry Bulb Temperature =  24 C
   Glass Conduction = -600 W
   Glass Solar = 3480 W
   Glass Conduction + Solar = 3480 - 600 = 2,880 W

Which results in a 2,476 (E+) vs 2,880 (Trace) which seems reasonably close.