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Information on design day creation for EnergyPlus / DesignBuilder

asked 2018-12-18 11:28:44 -0600

Aurore's avatar

updated 2018-12-19 02:50:37 -0600

Hi, I have climate data of a period (hourly, 30 years), and I am trying to select summer design days (based on drybuld T, wetbuldT, dewpoint T, etc.) and heating design days. I do NOT want to use the ASHRAE values, however I would like to use a similar technique based on my own dataset.

I looked into my 30 years of hourly data, so in my 262800 hours and I looked up the 99.6% for instance. What I find is that the temperature is about 30°C, but at 5pm. That same day, in my dataset, the maximum temperature is at 31°C at 3pm. My question is, is the design day a fictive day or a real day ? On a first attempt, I would like to use this design day in DesignBuilder, i.e. the yearly summer design day based on dry bulb T to size an AC unit in my building. DB requires the maximum Temperature and minimum Temperature of the day, and the coincident wet bulb Temperature (which I would assume is coincident to the max dry T??)

My questions are :

  • Should I use 30°C for the maximum dry T since it corresponds to the 99.6 %, and its equivalent wet bulb T ?
  • Then what should I use for the minimum dry T ? I have thought about looking into my 262800 hours and looking only at daily maximum Temperatures, but then this reduces my set to 10950 hours (262800/24). 99.6% of 262800 is not the same as 99.6% of 10950, so I think this is wrong but I am not sure...

Any help on this matter would be appreciated, if tried a lot to understand :) Thanks!

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answered 2018-12-19 02:59:33 -0600

updated 2018-12-19 03:08:18 -0600

To answer your first question only about fictive v real day, the I/O ref guide for SizingPeriod:DesignDay is pretty clear:

Using the values in these fields, EnergyPlus “creates” a complete days’ worth of weather data (air temperatures, solar radiation, etc.)

More information is in the I/O ref and if that doesn't satisfy your curiosity, the Engineering reference has a section titled EnergyPlus Design Day Temperature Calculations that goes in a bit more detail (it basically just adds the equation). The bottom line is:

$$T_{current} = T_{Max} - T_{range} \cdot T_{Multiplier}$$


  • $T_{current}$ = Air temperature of current Hour of Day
  • $T_{Max}$ = User supplied Max Dry-bulb Temperature
  • $T_{range}$ = User supplied Daily Temperature Range
  • $T_{Multiplier}$ = Range multiplier as shown on the below graph

image description

You can always choose to not use this default range multiplier and provide your own schedule instead using the Dry-Bulb Temperature Range Modifier Day Schedule Name, as well as defining your own Wetbulb temps using the Daily Wet-Bulb Temperature Range field, but some weird processing will be required.

My two cents is that - bottom line - the SizingPeriod:DesignDay object is meant to facilitate your life by asking only for a few key parameters, but if you want to use your own data to fully define the design day, then it's making your life much harder.

Potential workaround

I think there's something you could try, which is to use the SizingPeriod:WeatherFileDays instead. You could define a 367 days weather file, where you'd create two fictious next-year days that would represent your design day conditions so you can select them in SizingPeriod:WeatherFileDays while setting the RunPeriod object to run only on your actual 365 days.

I have never had to try this, so I give no guarantee that it'll work.

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answered 2018-12-19 09:48:50 -0600

Aurore's avatar

Hi Julien, Thanks for your answer. I have already read this section, it's true it indicates the day is fictive. The default range multiplier is fine, but I still need to input the temperature range. I guess one way to do it would be to use my temperature of 30°C as Tmax and then keep the daily temperature range found this same day to find the Tmin. Or to use another daily temperature range, but then how to know what is "representative" for a design day ? I was wondering if people that have actually tried to recreate a design day used that same methodology or did something else ? Thanks, Aurore

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Asked: 2018-12-18 11:28:44 -0600

Seen: 1,026 times

Last updated: Dec 19 '18