Ed Huntress wrote:

"Lewis Hartswick" wrote in message
...
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.

That is another one of those, " It Depends".
In this case on how long you will be gone.
...lew...

You start saving energy as soon at the house temperature stabilizes at the
lower temperature. Except for very short times, when you let the temperature
drop and then immediately ramp it up again, you always save energy by
lowering your house temperature.

Here's what the DOE says about it:

"A common misconception associated with thermostats is that a furnace works
harder than normal to warm the space back to a comfortable temperature after
the thermostat has been set back, resulting in little or no savings. This
misconception has been dispelled by years of research and numerous studies.
The fuel required to reheat a building to a comfortable temperature is
roughly equal to the fuel saved as the building drops to the lower
temperature. You save fuel between the time that the temperature stabilizes
at the lower level and the next time heat is needed. So, the longer your
house remains at the lower temperature, the more energy you save."

--
Ed Huntress

What you have there *is* a misconception in that it does not account for
multi stage / mixed technology heating systems which are not that
uncommon. A good example is a high efficiency heat pump with backup heat
strips. Depending on the controls, such a heat pump may engage the
backup heaters when it is unable to produce an acceptable rate of temp
rise with just the heat pump, and this switches the effective efficiency
from 300%+ to 100%, making it more costly to bring the temp back up to
normal than it would have been to maintain it at normal. This situation
was documented on a high efficiency model home.