Heat balance: missing gain/loss

asked 2023-01-10 12:37:00 -0600

savage's avatar

I am trying to understand all the zone heat gain/loss components and I appear to be missing something related to the windows. I have a simple box model with one exterior face and five adiabatic faces, with an ideal air loads system, and to really make it simple, I have zero outdoor air, no infiltration, and am looking at the winter design day, where there is no sun, the environmental conditions are constant, and there are no internal loads. I find a small discrepancy in the supply air heating/cooling energy and the following gains/losses:

  • Zone opaque surface inside faces total conduction
  • Zone windows total

I have verified that the internal loads (people, lights, electric equipment) and outdoor air gains/losses are zero. The slight difference is proportional to the window area and there is no discrepancy if there is no window. The difference is consistent across all hours of the winter design day, so it does not appear to be related to thermal masses or other dynamical effects. Changing the simulation tolerance does not alter the discrepancy. Maybe there is some radiation traversal that is not accounted for by "Zone Windows Total Heat Gain/Loss Energy"?

In the grand scheme of things, the discrepancy is small once internal loads and ventilation are included, but it is clear I am missing something in my understanding of the zone heat balance that I am trying to fill in.

I am not yet able to attach files, but a test model can be found here (very low U-values to really make the discrepancy apparent).

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