Adiabatic (asymmetrical) walls in Eplus

asked 2016-11-07 08:08:19 -0600

Elbaek
53 ●4

updated 2016-11-08 15:47:11 -0600

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

Hi.

I've been reading up on how Eplus handles the adiabatic boundary condition. Documentation says: "EnergyPlus will apply the same boundary conditions to each side of the construction so that there is no temperature difference across the surface."

I get why this would work well for symmetrical walls, but what if you have a typical concrete slab floor with insulation and wood on top? As far as i can tell, wouldn't that cause potential heat exchange/loss to that construction element?

Hope that was clear. :)

answered 2016-11-08 14:19:17 -0600

Archmage

7185 ●3 ●11

There can be plenty of heat exchange in and out of all kinds of surfaces with "Adiabatic" boundary condition regardless of symmetry in the construction. The same boundary conditions get applied to both faces of the surface, but the surface still can have heat capacity and it is likely to be hotter or colder than the surroundings at times. An asymmetrical construction might not end up with symmetrical surface temperature results at the faces at any given time, but over time it will average out. It is just a temporal artifact of how thermal storage charging and discharging happens.

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Hi Archmage.

I'm aware that adiabatic walls still contribute to thermal mass (as they should). What i'm not convinced entirely of is that you can be sure no heat is 'lost' through these walls when they are not symmetric.

The same boundary conditions on a symmetric wall would create an adiabatic layer directly in the middle of the wall, where heat from each zone would never diffuse through. For assymetrical walls (eg. 100 mm insulation followed by 100 mm concrete) in buildings where temperatures are fluctuating - can you be certain of the creation a truly adiabatic layer?

Thank you, Rasmus