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

[Prasad S Wani]

Dear All,

 

Regarding the error "wall too thick or too dense" encountered in eQUEST
while modelling a multilayer wall consisting of materials - Cement Plaster,
Medium weight concrete and Air. 

 

Whether any other software tool (e.g. Energy Plus) or engine (other
thanDOE-2) that can solve thicker wall problems without giving such errors ?

 

Thank You

 

Regards

Prasad

 

 

 

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On
Behalf Of Joe Huang
Sent: Wednesday, July 30, 2014 12:31 AM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Error - Wall is too thick or too dense

 

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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