# Re: Calculating "Ideal" Thermal Mass for Residence - Page 2

Posted by LarenCorie on August 27, 2003, 12:45 am

I am taking this to mean "inside the living space"
instead of "inside a house" as stated.  There are
configurations where thermal mass can be located
"in the house" yet in a location where it will not
gain from either the occupants or heating system.

Yes, when it can heat something at night that is cooler than it is.

I realize that you are referring to exposed thermal mass within
the living space, and I am only adding that there are other ways
to configure thermal mass, outside of the space that it may heat.
There are other ways to couple mass to a space besides radiantly.

I know what you mean, but it may not be as clear as you
intend. I believe that you are saying that the mass is really not
needed unless there is some sizable unbalanced input, like gain
from sunlight on a bright days. Of course output will always
equal input in the end.

Yep, that is the purpose of storage mass. I see it as more than "however"
It is the essence of why there is a storage, and the foundation of sizing
and designing it. There is only one reason to have a heat store.  That is
to remove excess heat.  Any heat that is not excess does not need to be
stored. Thermal mass is designed to 'cool' a space, not to heat it.  It
can do that any time that there is excess heat in the space. It matters
not what month it is, or where the building is, or whether that heat will
go back into the living space, or be vented to the night air.  It is still
fundamentally the same.  It also should not store 'heat' below a usable
temperature. This can get a bit more complicated with multiple spaces
that follow very different temperature curves, etc, like a low mass
sunspaces, but the viewpoint that mass serves a cooling function is
still just as appropriate.

Yes....or just absorbing excess heat. It may be doing that from
a space that is not ever intended to be comfortable, like an
area higher than the living space, that function thermally only
as a collector, and where people don't go. Again, I agree, I'm
just adding a little variety to the mix ;O)

If it is right in the living space, but that location can not
be assumed.  The mass may be in a location where it can not
radiate to a place that people will be, so the MRT in the living
space might not be effected by the mass that heats the air that
will heat the living space.

It shouldn't be, and I think most of the controversy is in the
misunderstanding of it. It is a more powerful component than
the ambient temperature by a factor of 1.4 as measured by
the National Bureau of Standards.  It gets complex when wide
variations in surface temperatures are present, but under all
circumstances it seems much more valid to attempt to evaluate it,
than it is to ignore it.  I believe a lot of people think of a building
environment in terms of "weather" The MRT is a factor they were
not raised with. That does not invalidate it. It only leaves it ignored.
I am always pleased when I run across someone, like yourself, who
is aware of it.  It seems that hardly anyone is.

Laren Corie

Posted by Nick Pine on August 29, 2003, 1:41 pm

That's most of it, indoors.

That seems unlikely to happen indoors, unless, say, you live in a diner,
surrounded by polished stainless steel. Kreider and Rabl's "Heating and
Cooling of Buildings" book says

The emissivities of most indoor surfaces are around 0.9, sufficiently high
that one can calculate Tr as if all surfaces were black... Inside buildings
the temp diffs are often small enough that Tr can be approximated by
Tr = sum(n,Fcl-nTn)... The operative temperature [for ASHRAE comfort
zone calcs] is close to the average of Tr and Ta...

Ex. 4.10 Consider a room 3mx3mx3m all but one of whose surfaces are 20 C
while the remaining 3mx3m surface is a window at 11 C. The dry bulb temp
Ta = 11 C. Find the mean radiant temp and the operative temp for a person
in the center...

We need the shape factor F from the center of the room to the window...
For the determination of Tr in buildings, it is usually sufficient to
evaluate the solid angle subtended by the surface in question, as seen
from the center of the body, and to approximate the shape factor by
this solid angle after dividing by 4Pi, the solid angle of the complete
enclosure. In this case, the solid angle is easy to determine because
of the symmetry of the room: seen from the center, the window fills
one-sixth of the total field of view, therefore F = 1/6.

Tr has only two terms because there are only two differrent surface
temps, with corresponding shape factores F and 1-F: Tr = Fx11+(1-F)20
= 18.5 C and Top = (21+18.5)/2 = 19.75 C.

As a part of Top. The ASHRAE comfort zone is a box on a temp/humidity ratio
chart that runs from Top = 19 to 27 C and w = 0.0045 to 0.012, approximately.

The ceiling seems like a nice place to put 100 F mass heated by sunspace air
in wintertime, and a slow fan to control the room air temp. In summertime,
a hydronic ceiling might be cooled by outdoor wet surfaces.

Nick

•
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 Re: Calculating "Ideal" Thermal Mass for Residence Michael Dewolf 08-14-2003