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).

Imagine a bucket with a pinhole leak near the bottom. Let's consider
the level of the water in the bucket as analogous to temperature in an
insulated enclosure. The diameter of the bucket determines the mass of
water needed to reach a given level. It's a valid analogy because
temperature is a measure of thermal potential, water height determines
pressure pushing water out the pinhole leak. Water leak rate is qty
of water per unit time, heat leak rate is qty of heat energy (Joules,
BTU, etc) per unit time.

Let's rig a little toilet-valve arrangement to maintain the level
except that it will have hysteresis: it will click on when water is
below a certain level and refil the bucket until the water rises some
incremental amount whereupon it will click off. Refill will stop and
the level will gradually go down because of the pinhole leak. How
long it takes to go down depends on the size of the leak (insulation)
and the diameter of the bucket (thermal mass).

I hope it's apparent that if the leakrate is 1 gallon per minute then
the average replenishment rate must also be 1 gallon per minute. If
it's less, the level will recede, if it's more the level will rise.

If we now lower the height of the toilet valve, there will be no
influx until the bucket leaks down to that level. If we then raise
the valve, the valve will stay open until the water has risen to that
level. But the average long-term water consumption will still be the
leakrate.

Note that this is true regardless of the diameter of the bucket.

It is true that both leak rates depend on potential (depth or
temperature difference) so differential equations are required to
express them correctly. This does not change the fact that what goes
in must equal what goes out long term if the long-term average
temperature is constant over several cycles of moving the setpoint up
and down.










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.