# Thermal mass effects of interior walls

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Hello,

Is it possible to model the effects of thermal mass of interior
walls? if so, how is that done?

Peter Schonherr

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Joined: 2013-03-07
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Hello Peter,

Create layered walls with the materials you intend to simulate and then set the floor weight = 0 to trigger custom weighting factors. You may have to fiddle with the interior convection/radiation coefficient to get a meaningful ?connection? to the room air node.

Jeff

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Jeff S. Haberl, Ph.D.,P.E.inactive,FASHRAE,FIBPSA,......jhaberl at tamu.edu
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Joined: 2011-10-02
Reputation: 200

Classification: UNCLASSIFIED
Caveats: NONE

General question:

Do any energy models take into account the thermal mass of stuff?

Then extreme case I know about is a garage. When my garage was empty and I turned on the heater it heated up fairly quickly. When I brought in my truck, tools, shelves, etc. (a few tons of steel) it took a lot lot longer to get warm.

This is an extreme case, but the same thing would happen in every type of building on some level.

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Joined: 2010-10-07
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Jeff,

In E+ there's also an "InternalMass" objec. It is listed under the group
"Thermal Zones and Surfaces". It only asks for a construction, a zone name,
and its surface (m? / ft?): might be easier.

*Internal Massis used to specify the construction/material parameters and
area of items within the space that are important to heat transfer
calculations but not necessarily important geometrically. (For example,
furniture within the space ? particularly for large spaces).*

John,

Both e+ and equest can have internal mass objects, that you can add/or
customize.

E+ having no defaults on the subject, it won't take any internal mass into
account unless you tell it to.

eQuest has by default some internal mass: Floor-weight and Furniture. For
Furniture - default is FURN-WEIGHT = 2 lb/ft? of floor area - compare that
to what you think you have in your garage and you'll see the difference :))

I think an even bigger problem is to use external walls (especially in
equest where it's so easy...) defined only by R-value as opposed to
layer-by-layer, thus with no thermal mass...

I'm pretty sure as a general rule we tend to underestimate the overall
thermal mass.
On the subject, I remembered Joe Huang (Joe, I hope you don't mind me
quoting you) making a comment about DOE2 making "lighter" models than real
anyway so you have to "boost up the internal mass considerably beyond
what's there to get the floating temperature profiles to match measured
temperatures" (see his post here

)

Best,
Julien

a ?crit :
I turned on the heater it heated up fairly quickly. When I brought in my
truck, tools, shelves, etc. (a few tons of steel) it took a lot lot longer
to get warm.
building on some level.

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Joined: 2013-01-09
Reputation: 0

Julien,

I don't mind at all being quoted or cited, but since I don't read everything that's
posted, just the ones that intrigue me, you might want to cc me via e-mail to get my
attention. It's true that I had said in an earlier post that I've found DOE-2 models to
be "lighter" than measurements, it was not just by a bit, but by a lot. I didn't do any
serious investigation into the discrepancy, but I attributed it to the models assuming
uniform air conditions within a space, surface air film coefficients based on smooth
planar surfaces, and uniform distribution of the internal thermal mass within the space.
As scientists or engineers, there's a preference for "getting the inputs right", i.e.,
having the furniture-fraction correspond to the actual amount of furniture in the space,
but it may be that either the algorithm or the inputs need to be tweaked in order to get a
indoor air temperatures closer to what's actually observed.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com