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Change Initial Condition

asked 2015-03-10 10:12:20 -0500

vibra gravatar image

updated 2015-03-10 10:28:29 -0500

I want to change the initial condition of the simulation. (like the indoor temperature at the very beginning) But I found this from the documentation:

EnergyPlus “runs” the first day of the environment (starting with a set of hard-coded initial conditions) until the loads/temperature convergence tolerance values are satisfied or until it reaches “maximum number of warmup days."

Does it mean there is no chance I can modify the initial condition?

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answered 2015-03-12 09:08:48 -0500

Archmage gravatar image

Yes, you have no chance to directly modify the initial condition. E+ starts out with all the surfaces at 23C and runs warmup days until temperature results for the first day converge. You can turn on report during warmup if you want to see how this progresses.

What you could do as a workaround is start the simulation many days in advance of the "beginning" you want to study and setup schedules and custom weather to do a long soak of the building under the conditions you think would produce initial condition you would like to have. I have heard other tools that don't have warmup days are used in this way.

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@Archmage, Thanks for your advice.To make the building arrive at the conditions I desired, I made a weather file with constant data, put the building in it. But I found even though I make all the weather data constant, the solar parameter was still changing according to time (High at noon Low at midnight). How can I make it constant.

vibra gravatar image vibra  ( 2015-03-18 13:37:26 -0500 )edit

Again, sorry, but you have no chance of stopping the sun. This is on the wishlist I believe. If a diurnal steady periodic condition is not okay, then I think you have to go with no solar during the soak.

Archmage gravatar image Archmage  ( 2015-03-18 17:07:49 -0500 )edit

Hi, can I ask where do you find that E+ start out with all the surface at 23C?

zhangzhizza gravatar image zhangzhizza  ( 2015-07-08 09:25:43 -0500 )edit

I don't see it anywhere in the user documentation but this is set in source code in a routine called InitThermalAndFluxHistories in the file HeatBalanceSurfaceManager.cc. see https://github.com/NREL/EnergyPlus/bl...

Archmage gravatar image Archmage  ( 2015-07-14 06:20:06 -0500 )edit
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answered 2015-03-10 10:42:52 -0500

updated 2015-03-10 11:00:17 -0500

You can make the setpoint schedules whatever you want for that first day, so in that way you can control what it converges on, but if you make a drastic change in the setpoints after day 1 it defeats what the warmup days are trying to do, so you may get unexpected results.

One main reason you can't just tell EnergyPlus what initial condition to start with and skip the warmup days is because you don't set surface temperatures; they are calculated based on the environmental conditions.

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Thank you for your answer. I ask this question because I want to analyze if the model is sensitive to the initial condition. (If a tiny change of the initial condition will impact the result a lot).

vibra gravatar image vibra  ( 2015-03-10 11:24:44 -0500 )edit

@vibra, make sure you look at Archmage's answer above. I was mistaken on how warmup days work. The day one setpoints won't affect the warmup.

David Goldwasser gravatar image David Goldwasser  ( 2015-03-13 08:50:40 -0500 )edit

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Asked: 2015-03-10 10:12:20 -0500

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Last updated: Mar 12 '15