"Don Foreman" wrote in message
...
On Thu, 29 Oct 2009 12:46:13 -0400, "Ed Huntress"
wrote:


"Bill Noble" wrote in message
...


"Ignoramus10802" wrote in message
...
On 2009-10-29, Pete C. wrote:

Ignoramus10802 wrote:

On 2009-10-29, Stormin Mormon
wrote:
Please forgive me while I troll for a moment.....

Is it energy saving to turn the thermostat down, when
leaving the house? I mean, the furnace has to run to catch
up when I get home. I have a way of looking at the matter.
I'll explain my point of view after the argument is
underway.


Imagine for a minute that you have to leave house for a month.

Would it be energy efficient to turn thermostat down? Of course, as
less heat will be produced for a whole month, with only a few minutes
to catch up.

The same applies to only one day.

i

It's far more complicated than that. Factors such as insulation / heat
loss, type of heating, multi-stage heating, electric backup heat on
heat
pumps, etc. all come into play in determining the away duration and
temp
reduction where savings begin, and in some cases (typically high
efficiency homes) it can require a multi day absence to see any
savings.

This is patently untrue.

I

Correct - whatever the net effect of insulation is, there is a net
negative heat flux from the house to the outside. The flux is
proportional to the temperature difference (the exact equation will
depend
on the radiation, convection and conduction components - radiation alone
is governed by the Stephan-Boltzman equation). The larger the
difference
the greater the flux. Averaged over any period of time, any time spent
with the thermostat set lower will yeild a lower internal temperature,
hence less heat flux. Whether that is enough to show up in your bill is
another question, but from a energy savings point of view, it is
incontestible.

The confounding issue, though, is the thermal mass of the house. That's
why
the DOE explanation says that the savings occur when the temperature
inside
the house has stabilized at the lower temperature.

When you shut off the furnace, the thermal mass of the inside of the house
is what's giving up heat to the outside. That's stored energy that came
from
the furnace heat. When you raise the temperature, you have to restore that
heat to the thermal mass. So with the furnace off and the temperature
inside
of the house dropping, you're losing stored heat. When you turn the
thermostat back on, you have to restore that lost heat, which will also
heat
up the atmosphere inside of the house (which is a very small portion of
the
total inside thermal mass).

That's what I read from their description, anyway, and it comports with
things I've read about it from other sources. There is no (theoretical)
net
gain or loss when the thermal mass is put through the cycle of cooling
down
and heating up. The savings occur when the temperature is reduced and
stabilized.

This all assumes that a house is decently insulated and that the thermal
mass of the house is substantial. Of course, the thermal differential
between the inside and outside temperatures are always at work, suggesting
that there is less heat loss with each degree of reduction of inside
temperature, as you say. But the DOE's reference to actual testing agrees
with the fact that, as soon as you turn the thermostat down, you begin
losing *stored* heat, and when you turn it back up, 100% of that lost heat
must be restored, regardless of actual thermal losses through the walls
and
ceiling.

Savings begins the instant inside temperature is reduced, whether or
not it is stable at a lower setpoint. This is true regardless of how
well the house is insulated, what the thermal mass might be, who the
president is, which party controls congress or whether DOE likes it or
not. Rate of heat loss at any (and every) instant depends upon
temperature gradient from inside to outside.

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.


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

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.

--
Ed Huntress