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?
Thanks
GOOGLE: thermostat + setback + savings
Jim
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.
rusty
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 variables.
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.
Thank you, Tom, that is exactly what I was looking for. OBTW, they have changed the URL to:
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 days.
Joseph
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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