On Fri, 30 Oct 2009 13:17:41 -0400, "Ed Huntress"
wrote:



As I said, I'm aware of the physics, Don. But the questions, which have gone
all around the barn, are about whether it's worth it. And the answer is that
it's much more worthwhile if your periods of turning the furnace off (or the
thermostat 'way down) are quite long.

DOE addressed the question accurately: if you have two periods of reduced
heat per day -- the time you're at work and the time you're sleeping at
night -- you'll typically save 10%. That's a figure that's been tested and
reported for decades.

That's as good as any generalization. Savings vary a lot with how
much the homeowner sets back and for what percentage of the time. The
length of each period is only relevant if it is small or comparable to
the time constant of the enclosure. If the period is short enough
that the space can only cool 5 degrees, then lowering the setpoint
more than that has no effect or benefit.

But the assertion about not saving anything until the house is
stabilized at the lower temp is wrong, and the stuff about cycling
thermal mass requiring net energy is also wrong.

When these studies were done, 10% savings was far from trivial.
Houses are *much* more energy-efficient now than they were 30 years
ago.