On Sat, 26 Mar 2005 15:34:08 -0800, RMUH wrote:

| One of my customers asked me whether she should believe a furnace
| installer who told her it is more efficient to leave the heat on all
| night than to turn it off.
|
| Clearly, the heat loss from the building is less if the temperature
| inside is lower, as it would be when the heat is off.
|
| However I have heard it stated so often and by people who ought(!) to
| know, that I wonder if there is some reason not to turn the heat off.
| Anybody really know exactly why this is or is not true?


From the US Department of Energy:

"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.

"Another misconception is that the higher you raise a thermostat, the
more heat the furnace will put out, or that the house will warm up
faster if the thermostat is raised higher. Furnaces put out the same
amount of heat no matter how high the thermostat is set— The variable
is how long it must stay on to reach the set temperature.

"In the winter, significant savings can be obtained by manually or
automatically reducing your thermostat's temperature setting for as
little as four hours per day. These savings can be attributed to a
building's heat loss in the winter, which depends greatly on the
difference between the inside and outside temperatures. For example,
if you set the temperature back on your thermostat for an entire
night, your energy savings will be substantial. By turning your
thermostat back 10° to 15° for 8 hours, you can save about 5% to 15% a
year on your heating bill, a savings of as much as 1% for each degree
if the setback period is eight hours long. The percentage of savings
from setback is greater for buildings in milder climates than for
those in more severe climates. In the summer, you can achieve similar
savings by keeping the indoor temperature a bit higher when you're
away than you do when you're at home."

http://www.eere.energy.gov/erec/factsheets/thermo.html