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Suggested number of timesteps per hour in Open Studio/Energy Plus?

asked 2016-06-03 03:09:44 -0500

DavidFGG's avatar

updated 2016-06-03 04:16:16 -0500

I am modeling a building in Open Studio, and was thinking about shortening the timestep per hour in my simulation. The epw weather file consists of hourly weather data, the default timestep in OS is 6 per hour (10 min), and it seemed logical for me to reduce my time step to 1 per hour. The difference in the results was notable (0.35 % reduction for net site energy, 1.2 % reduction of heating energy and .0.9% increase in cooling energy), but not extreme. My surface heat balance algorithm was Conduction transer function and air heat balance algorithm was Analytical solution. My model runs on ideal air loads.

Can anybody explain to me why the number of timesteps matters, if the outside conditions stay the same for one hour?

Which heat balance algorithms are favourable for the surface and the air heat balance ? From what i read in the ergineering reference document, the analytical solution seems the most stable solution for the air heat balance, but for the surface heat balance algorithm, the options stay wider open.

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A good place to start is the Engineering Reference Guide, The Summary of time-marching solution especially

Julien Marrec's avatar Julien Marrec  ( 2016-06-03 04:16:56 -0500 )edit

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answered 2016-06-03 09:29:11 -0500

Archmage's avatar

The weather data may be hourly but they are then interpolated for use with shorter timesteps. So it is not really the case that the outside conditions stay the same over an hour when using shorter timesteps -- they will vary smoothly between weather data points.

One of the deep technical reasons for the existence of timestep dependence in the zone heat balance method in EnergyPlus is that it when predicting a new zone air temperature is uses the lagged zone air temperature for the temperature difference when calculating surface convection heat transfer. (It could go back and iterate with the new temperature but that would slow it down execution.)

For long timesteps and fast execution you want to use Conduction Transfer Function.

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@DavidFGG: for the record, if an answer has solved your problem make sure you mark it as accepted by clicking on the check mark below the vote up vote down buttons next to the answer: it'll change to green color, and will allow everyone to see that your problem has been properly resolved! Thanks!

Julien Marrec's avatar Julien Marrec  ( 2016-06-03 12:31:49 -0500 )edit

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Asked: 2016-06-03 03:09:44 -0500

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Last updated: Jun 03 '16