[Equest-users] Error - Wall is too thick or too dense

Joe Huang yjhuang at whiteboxtechnologies.com
Tue Jul 29 12:00:53 PDT 2014


Prasad, others,

When DOE-2 (the base program under eQUEST) reports a wall as too thick or too dense, what 
it's really saying is that it failed to find a Common Ratio between the three response 
factors at the end of 100 time steps (hours).  Response Factors are what DOE-2 uses to 
model heat flows, where the three response factors (X,Y,Z) give the fraction of the heat 
flow occurring each hour after the initial pulse.  For the residual heat flows beyond 100 
hours, DOE-2 uses the Common Ratio since all three response factors are decaying (so to 
speak) at the same rate.

When the Common Ratio is still changing, then DOE-2 says that the wall is "too thick or 
too dense".  Such situations are caused not just by the thickness or density of the wall, 
but also when there are anomalies in the thermal characteristics of the layers.  For 
example, I've found that for a foundation layer (which is where I've most often 
encountered this problem),  DOE-2 will accept a single layer of dirt 5.25 ft thick, but 
once a layer of insulation is added, the dirt layer can only be 2 ft thick or less.

In the case cited below, what is the order of the 3 layers?  Is it as written - plaster, 
concrete, then air ? - or is it plaster, air, and then concrete?
In the latter case, it's the air layer that's probably limiting the allowable thickness of 
the concrete.  How are you modeling the air gap?  If it's as a mass layer and not a 
resistance layer, you might want to play around with its thermal characteristics to reduce 
the discontinuity, and see if that helps.  What I mean is that as long as the resistance 
of the air layer is correct, adding some thermal mass to it (or "transferring it" from the 
concrete :-)) might help.  Although this might sound wrong, but it's probably better than 
limiting the thickness or mass of the concrete just to avoid the dreadful "too dense or 
too thick" error.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 7/29/2014 5:51 AM, Prasad S Wani wrote:
>
> Dear All,
>
> I am modelling a multilayer wall consisting of materials -- Cement Plaster, Medium 
> weight concrete and Air.
>
> The total wall thickness is ~ 1.2 ft. However, a simulation error is obtained -- "Wall 
> is too thick or too dense".
>
> Error persisted even when total wall thickness was increased by increasing _air gap 
> thickness_.
>
> However, error did not occur when air gap thickness was reduced.
>
> It appears that reducing air gap reduces density of the wall (density varies directly 
> with air gap).
>
> Kindly inform whether there is a method in eQUEST by which such simulation error (Wall 
> is too thick or too dense) can be nullified.
>
> Thanks in advance.
>
> Regards
>
> Prasad
>
>
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