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?
Reticulan Museum Of Unnatural History
It really depends on the insulation quality in the house. Consider the
two extremes: a house with perfect insulation and a house with no
insulation. Perfect insulation means that the furnace/boiler will come
on just enough to bring the house up to the desired temperature. It
will not come on again because there will be no heat lost through the
perfect insulation. Turning the furnace off at night won't save anything.
The other extreme, zero insulation, means that the furnace will run
continuously and will not be able to counter the heat loss. The house
will not get up to the desired temperature. So heat will be lost at
the rated capacity of the furnace/boiler. Turning the furnace off at
night will save directly in proportion to the time it is off.
Bottom Line: How much fuel you use is a factor of the AVERAGE temperature you
maintain. If you keep your house at 70 degrees during the day, and let it drop
to 60 at night, it will result in a lower average temperature than if you kept
it at 70 degress all the time.
You will also sleep better if the temp is a little cooler at night.
The Second Law of Thermodynamics stipulates that the heat transfer between
masses is a function of their temperature differential. The higher the
temperature differential, the faster the heat loss. If two bodies have the
no temperature differential, when they have the same temperature, no
temperature loss can take place.
Therefore, if you turn the heat down at night, you lower the temperature
differential and thus, the heat loss.
This law of physics is also known as Entropy, probably the most basic of all
laws of nature.
It it is not a heat pump then it is a good bet that turning it down or
off will reduce total usage. If it is a heat pump, it may increase
electricity usage reduce it or make no difference depending on many
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."
Tell that to the girls in the office. I was often the first one in and
turned the heat up to 70. As each of the four women came in every one of
them would turn it up a few more degrees. If someone turned it down, even
though the heater was still running, they would bitch about it.
I one respect they are right. That is the thermostat only records the
air temperature. So when it gets up to 70º the walls, floor furniture etc.
are still cooler and they will feel cooler than they will later in the day
when the area has stabilized at 70º. So maybe overshooting the initial warm
up a few degrees would help.
Two stage furnaces and many heat pumps will indeed put out hotter air
if the thermostat is set higher. Setting back your thermostat is almost
always a good way to save energy. Set back stats are required by code these
Um... yup. However there might be other factors, such as efficiency vs.
run time of the heating unit, humidity changes, or other factors that
would require other Stipulations. Occasionally, two-syllable factors
will upset, for instance, the Stipulations Of The Second Law. Entropy, huh?
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