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# time steps in energy plus?

Hi

I am using ConductionFiniteDifference method as the HeatBalanceAlgorithm. when I assign 1min, 2min or3min time steps, the results are different. But when I go higher (like 15min or 20min), the results are metered and reported in 3min time step. what is the limitation that E+ doesn't produce the results in bigger time steps. (When I use the TransferFunction method, there is no limitation in timesteps)

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I am not sure about the exact answer, but I can tell you this: ConductionFiniteDifference march through time, solving one timestep after another, relying on the solution found for the previous and/or following instant. This 'and/or' will depend on the solution scheme used. (I guess this is controlled with difference scheme in the HeatBalanceSettings:ConductionFiniteDifference object in EPlus)

For example, you start at t=0, solve an equation and find the values for t=1. Then, use t=1 to find values of t=2.

These methods usually involve a critical timestep that assures convergence (if timestep is bigger than that, your solution will explode in plusHUGE or minusHUGE numbers). Fulfilling this criteria, however, does not assure accuracy, so it is a common practice to divide the critical timestep by some constant, and use that as the timestep in your solution. This constant, I think, has something to do with the space discretization constant in the HeatBalanceSettings:ConductionFiniteDifference object in EPlus.

CRITICAL TIMESTEP IS SMALLER IF YOUR MATERIALS ARE LIGHT (it is actually a relationship between thermal mass and conductivity --> thermal diffusivity)

Finally, (and here I am guessing) if you have some "light" materials (i.e. plywood, gypsum boards) the critical timestep might be smaller than what you asked for EVEN IF THE MATERIAL IS NOT SUBDIVIDED, thus forcing EnergyPlus to ignore the input option and making it 3 minutes or so.

I would love to know if I am correct.

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