Thermal time constant and energy consumption

I am using Energy Plus to evaluate two different houses and, going through the outputs, it came to my mind that the time constant during indoor temperature thermal decays (in the night for instance) of the house could be related directly to the energy consumption for space heating. Therefore I wrote a code in Matlab to compute the thermal time constant from the 3 minutes data of indoor temperature provided by Energy Plus.

However, the time constant is not consistent between winter and shoulder months. I know it is very simplistic, but there should be some kind of consistency in it, right? What do you think I might be missing?

Thanks in advance for the help

If I am following correctly, you need to be sure that there is only one driving force for the time under consideration. No internal gains, no infiltration, and no solar. Also, all non-adiabatic surfaces need to have the same outside boundary condition. For example, a floor slab which has a ground boundary condition that is different from the outdoor air temperature would cause a difference.

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MJWitte

answered 2015-11-10 10:27:52 -0500

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I was looking at thermal decays only during the night. This should prevent the influences of internal gains and solar, shouldn't it? How much do you think the infiltration and the different boundary conditions could matter? Which do you think the order of magnitude could be?

W (2015-11-17 14:35:50 -0500) edit

Infiltration could be huge, especially since it has a small impact in shoulder months and the largest impact in winter. For the floor boundary condition, the floor area is typically one of the larger surfaces and will likely have less variation in temperature between shoulder and winter than the exterior walls and roof will see. The best approach is to make the floor adiabatic and turn off infiltration and see how things change.