In article ,
"Ed Huntress" wrote:

"Joseph Gwinn" wrote in message
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In article ,
"Ed Huntress" wrote:

"ATP*" wrote in message
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"Ed Huntress" wrote in message
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"Don Foreman" wrote in message
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On Thu, 29 Oct 2009 16:22:30 -0400, "Ed Huntress"
wrote:



The point is that what you're talking about is insignificant in most
homes.
That's what the DOE statement is all about: cycling of the heat mass
overwhelms the effect of the insulation, until the temperature is
reduced
and stabilized for a while. Otherwise, all you're doing is cycling
the
heat
retention of the thermal mass, with relatively much less actual
savings
as
the temperature drops in the house.

Furnaces cycle anyway, and often at about the same cyclic rate almost
independent of temperature or heat load. To meet higher heat load
(higher indoor temp or lower outdoor temp) they just have a higher
percentage of "on" time each cycle. Even the venerable Honeywell
"round" (T-87), which has been around for over 50 years, did and does
a surprisingly good job of this by virtue of it's anticipator --
which almost nobody outside of their engineering org really
completely
understood.


Whether or not the savings on the energy bill is noticable is
another
question depending on how much the setback was for how long.

I wonder how much the DOE spent on a study to address what would be
an
easy test question in thermodynamics 101.

Probably enough to know that you learned your thermodynamics with a
calculator and a vacuum jar, rather than a house. g

They'd be wrong about that too. During my 33 years with Honeywell I
worked with engineers and scientists worldwide,including those from
their homes and buildings divisions.

I was invited by DOE just last week to serve as a technical reviewer
for buildings programs. (I'm not going to do it.) I was also on the
Technical Advisory Board for DOE's Brookhaven National Labs back in
the late 90's.

I've seen those studies for years, Don, starting with a book I read
in
the
'70s, titled something like _Low-cost, Energy-efficient Shelter_.
What
the
DOE says is widely known.

It is indeed, and it's known in the technical community to be wrong.

I've lost track of what you're saying is "right" or "wrong." It's a
fact
that your savings are much lower than many would expect when you
simply
let the temperature drop to some particular temperature and then start
heating the house up again, as you would if you turned the furnace off
when you left in the morning and then turned it back on when you got
home.

It's another fact that the savings become significant when the lowest
inside temperature prevails for most of the cycle time, whether it's
because the house has reached outside temperature or because you
simply
set the thermostat down to some temperature above outside temperature.

Physics was my best subject, too, Don. I'm aware of the physics
involved.
There even was a time when I could do the equations without batting an
eye. Now, I let my thumbs rule. d8-) The question concerns whether the
saving is enough to make it worthwhile, not whether you can calculate
the
exponential decay and the integrated temperature differential and show
some numerical value for the savings. Again, it's much less than most
people expect.

--
Ed Huntress

The misconception is that the heating plant will have to "work harder"
to
catch up, leading some people to believe that a setback actually wastes
energy. Except in the specialized case of a heat pump on the verge of
electric resistance operation, that is not the case. Whether it is
"worth
it" or not is another question. In a leaky building it definitely is.

Yeah, that's one of the misconceptions the DOE is addressing with their
notes to consumers. The other one, which I've heard a lot more, actually,
is
that shutting off your furnace for half of a day saves a half-day's worth
of
the energy you'd use if you kept the house at your usual temperature.
That,
too, is a big misconception.

Well, when this issue was hot some years ago, I measured the thermal
time constant of my then house, which was built in 1896. The test is
simple:

In the winter, heat house to 90 F, turn the boiler off, and record the
declining temperature periodically. The time constant (to 1/e of temp
difference between 90 and outside ambient temperature) was an hour or
two, if memory serves.

So, for my then house, it made sense to reduce the temperature if one
would be away for more than about 5 hours.

It turned out that the temperature decline curve was well described by a
simple exponential curve, plotting as a straight line on log-linear
paper. This implies that the thermal mass of the wall and ceiling
plaster and to a lesser degree the wood floors dominated, and these
swamped all the other thermal storage mechanisms.

Joe Gwinn

Veddy interesting. You must have a very determined curiosity. Although your
house may be different from a typical house built more recently, it does
give an idea about how long one has to reduce the temperature to make it
worthwhile.

Well, I couldn't make head or tails of the then debate, and the
measurement is pretty easy to make. The measurement times need not be
evenly spaced, so long as one records the actual time each temperature
is taken. The general pattern is to measure just before turning the
heat off, after 10 minutes, after 20 minutes, after 40 minutes, after 80
minutes, and so on, each time measuring from the prior measurement time.
This works because the rate of temperature change is fastest at first
but soon slows. So long as one knows when the measurements were made,
it doesn't really matter how well one follows the schedule.

Or, one can buy one of those USB temperature monitor/recorder units and
use it to collect the data.

Excel is adequate to reduce the data, however generated.

So one can know exactly how one's house behaves, bringing the discussion
out of the theoretical into the practical.

Joe Gwinn