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# How EnergyPlus determine the boundary condition before warmup convergence?

Hi all,

According to what I read from E+ engineering reference, the warmup process in E+ basically compares the four parameters: maximum zone air temperature, minimum zone air temperature, maximum heating load, and maximum cooling load for each individual thermal zone. So if the difference between days is less than the tolerance value, convergence is reached.

I understood that part, but still do not quite understand how the boundary conditions were chosen by E+ Since the warmup convergence itself has to go through the definition of boundary conditions such as indoor surface temperature.

Or can I just assume that:

In terms of specifying the indoor surface temperature directly as a boundary condition, EnergyPlus doesn’t typically require us to input this value explicitly. Instead, the indoor surface temperatures are calculated as part of the simulation process based on the interactions between the weather file, material properties, HVAC settings, and internal load. Since Honeybee usually assign a default value if we do not put anything.

Hopefully, somebody can clarify this. For example in Honeybee (using E+ engine), I only put weather file and material properties, and somehow E+ spits out values for indoor surface temperature, outdoor surface temperature, indoor air temperature, indoor operative temperature, etc. While in the heat transfer textbook, the heat balance equation might need the indoor surface temperature to actually calculate the outdoor surface temperature. So it baffles me. How can E+ spits out value with lots of unknown?

Thank you

Regards, Ricardo

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## 1 Answer

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There is a function in E+, ResetNodeData, that is called at the beginning of each new environment. A new environment is a sizing period day, a weather file period, etc. This function resets all node data to a prescribed condition. Tyically 0 for temperature and humidity ratio. The warm up period then converges on a solution, as you describe, for the first day of the simulation. The initial conditions and history of each node in the simulation is then known. The simulation then proceeds normally for the remaining days in that environment.

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## Comments

Thank you for the insight!

When you said "typically 0 for temperature and humidity ratio", does it happen to every node? For example, as the main input, we have external meteorological conditions from the weather file, and in the initial condition set by E+, the indoor temperature and indoor surface temperature are 0? Then through each iteration, the indoor air temperature rises, and subsequently the indoor surface temperature also rises? Seems like I misunderstood something since I read that the initial indoor temperature should not be 0 (because it is unrealistic).

( 2023-08-21 17:47:09 -0500 )edit

I think it's all nodes. Then during warmup, all nodes will come to some steady-state over the day, which is when convergence is reached, and begin the simulation from there. It's not as if the zone temperature will begin the simulation at 0 C, it will begin warmup at 0 C and over a few days will reach what is likely the thermostat set point temperature for that day.

( 2023-08-22 19:03:06 -0500 )edit

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Asked: 2023-08-21 07:08:21 -0500

Seen: 103 times

Last updated: Aug 21 '23