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Posted by Morris Dovey on November 10, 2009, 8:54 pm
Ecnerwal wrote:

Actually, there may be. An insulated concrete slab floor can store a
considerable amount of heat - and while the same mass of water can store
more heat, it requires plumbing, pumps, control system, etc that the
slab does not. The entire water system is vulnerable to freeze damage in
the event of a prolonged power outage of failure of any of its
components - and the slab has no vulnerabilities.

Back on 2/21/2009 I posted a program here to do heat storage
calculations for an insulated concrete slab floor in a workshop building
with 10' ceilings [subject: Re: Physics help please - heat storage
(summary)] that calculated the thickness of slab needed to store enough
heat energy to warm the air in the space above from _absolute zero_ to
an arbitrary room temperature was 0.081923 inch (assuming that the
concrete had previously been warmed to that arbitrary room temperature).

What that means is that a 6" slab at room temperature contains enough
heat energy to warm the room air from -273C to room temperature 73
times (48 times for a 4" slab).

Now the dynamics aren't likely to work out quite that cleanly, but
neither is it likely that there'll be a need to start with -273C air in
the structure. :)

The reason for going through that exercise was that a shop building here
in Iowa had been built with a 6" slab and 10' ceilings (and two 8'x6'
passive solar air-heating panels) and the lowest temperature the owner
recorded over two winters was 65F/18C.

That was in spite of the fact that the owner opened a 10'x9' door every
morning and every evening to move his pickup truck out/in. He did not
check the temperature immediately after opening the door, and the 65F
temperature occurred only one time (in the morning).

No water, no pumps, no plumbing, no control system, and no freeze - not
even in overcast/subzero weather.

If it were me, and I could get the job done without incorporating
"breakable" elements, I would - and it's possible to buy a pretty decent
amount of concrete for the cost of labor, materials, and maintenance for
a water system.

 > Still, for simplicity of getting a regular construction crew to

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Robert Scott on November 10, 2009, 9:59 pm

The amount of heat it takes to heat up a volume of air is nearly irrelevant.
The real challenge in any practical heating system is to keep up with the heat
losses to the outside.  What practical use can you make of the fact that a 6"
slab of concrete has 86 times as much heat storage as the air in the room for
the same delta T?  If you waited for that heat to be 90% equalized, it would
probably take several days.  And if you resort to active means to extract the
heat from the concrete faster (using fans to speed up heat exchange) you may
find that it takes more electrical energy to move the air than you are gaining
from the heat in the concrete.

The real advantage of using concrete is that it might be free.  That is, you
needed to have a floor anyway, so you are not paying extra for a heat storage

Are you sure that the results might not be nearly as good if the floor were
wood?  All building materials, including the contents of the building,
contribute to evening out temperature fluctuations.

Bob Scott
Ypsilanti, Michigan

Posted by Morris Dovey on November 10, 2009, 11:29 pm
 Robert Scott wrote:

That's true (and I think it's important).

Also true. Good insulation and lack of leakage are important.

I can use that as a measure of heat energy available for (re)heating the
air and surfaces in the room.

Possibly, but this is not the result experienced. Mid-winter, at my
urging, the owner did turn on his ceiling fan and dialed the speed down
to just enough to blow ceiling air down to floor level. After that he
reported warmer morning temperatures and a general improvement in
comfort throughout the shop (night /and/ day). As far as we could
determine, the electrical consumption was negligible, and thereafter he
left the fan running 24/7 (as far as I know it may still be running).

Well, yes - that was one of the points I wanted to make, along with the
point that, for all practical purposes, it has no failure modes and no
operating overhead. It's immaterial that it's not as good a storage
medium as water if the storage it provides is /sufficient/.

I'm limited to saying that I'm reasonably sure, because the wood floor
option wasn't tested. I'm inclined to believe, though, that a layer of
wood on top of the concrete would constitute a heat barrier exactly
where I wouldn't want one.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Josepi on November 11, 2009, 1:54 am
 While I agree with the simplicity you refer to, and the concept, getting the
heat into the concrete slab can be a problem when any heat captured is
typically above the concrete slab and does not like to be absorbed well.

Posted by Morris Dovey on November 11, 2009, 3:22 am
 Josepi wrote:

I do seem to have a bit of difficulty communicating this point :)

In my response to appsol and in my response to Rob Scott I extolled the
virtues of using a slow-turning ceiling fan to move the heat to the floor.

If you take a look at the bottom three photos at


you can see the real, non-hypothetical, actually-installed fan in operation.

I didn't mention, and probably should have done, that during the
daylight hours the panels continuously draw in the coolest air near the
floor, so that even during power outages the panels work somewhat to
bring warm air to lower 'altitudes'.

It may appear as if it could be a problem, but the actuality is that it
hasn't been a problem at any time during the past two winters.

I kinda like the simplicity myself. ;-)

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

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