This question comes up year after year.
Standard efficiency fossil fuel fired furnaces run more efficiently if
they are allowed to run continuously. The more on-off cycles, the lower
The old ASHRAE standard gas furnace was specifically limited to 80%
efficiency when operated continuously. Frequent on-off cycles could lower
efficiencies into the 50-60% range. That means if you run an old ASHRAE
standard furnace continuously, about 20% of the heat went up the chimney;
if you ran it intermittantly, as much as 50% could go up the chimney.
Read the label on the furnace, e.g. my older gas furnaces all were rated
at 125,000 btu input, 100,000 btu output. That is interpreted as
"Sufficient 1000 btu/cubic foot gas was piped to the furnace to generate
125,000 btu/hour, but 25,000 went up the chimney when tested while
operating continuously in a testing lab."
When compared to losses to the outside incurred if leaving the thermostat at
constant setpoint, If you let the house cool while you're gone for the day,
then run the furnace hard to heat it up, you will reduce energy losses to
the outside during the cool period, and reduce energy losses up the
chimmney due to frequent on-off cycles while the furnace is heating the
house back to the original set point.
I have never seen the efficiency curves for air conditioners, heat pumps
or the High Efficiency, Forced Draft fossil fuel furnaces. I am certain
that the pleasant-voiced, soft-spoken, "people-persons" who staff those
customer query centers have never seen any efficiency curves of any kind.
> run more efficiently if they are allowed to run
> continuously. The more on-off cycles, the lower
I might agree with that statement if furnaces were capable
of running continuously while adding heat into a house
at the same rate the heat is leaving that house.
I like the "bucket of water with a small hole in it"
theory. Once the level of water (temp of a house) reaches
a comfortable level, the faucet (furnace) need only
add water (heat) to the bucket (house) at the rate water
(heat) is draining out the small hole in the bottom (leaving
the house via walls, windows, etc).
So when we start with an empty bucket, we need to add
water at a very high rate to get the water level where
we want it. Once the level is where we want it, we need
to add water at a much slower rate (the rate at which water
is leaking out the small hole in the bottom) to maintain
the desired water level.
Most natural-gas-fired furnaces I know of heat a home
at a constant (higher) rate. They don't allow us to
reduce their heat output to a rate consistent with
the rate at which heat is leaving our homes.
To compensate, these furnaces cycle on and off. Over
a long period of time this has the effect of adding
heat at a much lower overall "rate".
Cycling this type of furnace uses more energy since
you must heat up the burner to maximum temperature on
This inefficiency occurs only once in a furnace that
can run continuously and operate at a level where it
is simply keeping pace with the rate heat is leaving
Automotive heaters are a good example of this.
They allow us to control the blower speed, which
lets us control the rate at which we are heating
the vehicle cabin. We can better match the rate heat
is entering the vehicle cabin to the rate at which
heat is leaving the cabin.
The bucket is a good analogy, since the water will run out the hole at a
higher rate when the water level is higher. This is how heat loss works --
more differential between the inside and the outside will result in a
higher heat flow/loss.
Actually, you should think of it more like placing your bucket under your
tap and turning the tap either fully on or fully off. You will need to
cycle the tap on less frequently when the bucket's water level is lower.
This isn't an efficiency loss, since the burner resides inside the house.
Only heat that is lost out the chimney is wasted -- heating the burner
itself is merely part of heating the thermal mass of the house itself.
As a blanket statement, the "few" and the utility are wrong or
misrepresenting the facts. If, after an extended setback period, your
system needs to resort to resistive electric heating, that'll really
crank up the expense. Otherwise, it's just a matter of being able to
recover temp in time, and, for efficiency, the slower the better with
the burner on full-time.
If a whole neighborhood (i.e. lots of houses) have had a major
setback, and will all be in recovery-mode for some time, that can
impact the gas main pressure, or power-distribution system (for
heat-pumps.) IOW, if only you do it, it just cuts into their
bottom-line. If lots of people, they may have to increase investment.
To make it clear to the physical-science-challenged, consider an
analogy: a water-barrel with a tap at the bottom, which is cracked
open. Say that you either keep it filled with a make-up water-supply,
or, in the morning, change the fill controls such that the barrel is
allowed to drop to half-full and maintained there, then filled again
per clock setting in the evening. Can't make it much simpler.
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 billa 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."
On Fri, 13 Feb 2004 21:03:04 GMT, email@example.com (Tom Miller)
That's what i always figured, until I listened to the gas company!
Thanks for posting this!!!!!!!!!!!!!!!!
I think I'll call them up again and ask them why they disagree with
the department of energy!
Maybe I'm giving everyone more occasion to pick on the retarded guy
here, but I think there's something rather contradictory (big surprise
coming from the government, eh?) in that energy department statement.
Let's read that DOE statement very carefully and see if it's just me
being the retarded guy:
"... 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
OK, so you're using up pretty much the exact amount of fuel you saved
setting the thermostat back, right? Call me silly, but that doesn;t
constitute "savings" in homeowner terms. Or maybe the DOE was just
speaking in government-spending terms, but ...
"You save fuel between the time that the temperature stabilizes at the
lower level and the next time heat is needed."
No shit. It's when you get to that "next time heat is needed" point that
the savings wheels start coming off the wagon, no?
"So, the longer your house remains at the lower temperature, the more
energy you save."
No shit, again. And as long as you're willing to freeze your ass off,
you'll save a bundle. It's when you start actually wanting to get warm
that you will -- by the DOE's own admission -- start using up that
savings. Unless maybe you want to freeze your ass off the entire winter.
Not only that, but the DOE seems to fail to answer the original question
posted of exactly *how* turning up the thermostat when you're sick of
saving all this fuel and money and make your furnace actually *produce
heat* wouldn't mean additional wear and tear (aka "working harder") than
had it been left to maintain confortable room temperature.
I'd love to discuss the issue further right now, but the turnip truck
just pulled up to to put me back on it.
Let's assume that you set your thermostat to a lower temperature for an 8
hour period every night. Your house takes 2 hours to cool down to the
lower temperature, so for that time period, you aren't saving anything.
However, the next 6 hours at the lower temp are pure savings.
No. In the example above, you have turned down the thermostat for 8
hours, but it only takes 2 of those hour's savings to reheat the house.
The net savings is 6 hours of lower furnace usage.
No, just for the hours when I am in bed or out of the house.
The furnace doesn't "work harder" -- it produces the same amount of heat
for every minute that it is turned on. Setting back the thermostat results
in less wear and tear, since the furnace is running for fewer minutes every
Ah, yes -- but a lot is in that "it depends" gray area, no? How low of a
"lower temperature" are you talking about, exactly, and going from what
current temp to what lower temp? According to a new post by Chris J, his
house takes two whole days to drop something like 10-15 degrees. So if
he's only losing a fraction of 1 degree of heat per hour, how many hours
will it take him to see a huge amount of "pure savings" according to
your scenario? And if his assertions are correct, it would take his
house more than a day to go from 68 (his daytime stat setting) to 64
(his night stat setting). At any given point in the day, his house will
still be pretty warm -- which really, is pretty much the ideal everyone
strives for anyway: a warm house that doesn;t have to suck up a lot of
fuel to get and stay that way.
True. But in Chris' original post, he spoke of turning the system off
for an entire day. Math makes my brain hurt, so how many hours' savings
are left when reheating a house left to cool for 24 hours, not 8?
True again. But in a 24-hour period of no heat as stated by Chris, how
many of those hours do you plan on spending in bed or out of the house?
Might be able to get away with it for a day, maybe, but beyond that I'd
imagine you'd need to be either severely depressed or have a girlfriend
with an apartment she's spending her own money to heat. ;)
Which I think harkens back to Chris' original question of, if room is
left to cool for 24 hours, wouldn't the furnace by nature of having to
reheat a LOT of degrees at once be subjected to more wear and tear (aka
"work harder," altho we've long established furnaces don't work harder,
just longer) than one that was not turned off at all and left to
maintain the current room/house temp.
And this, for those who may be new here, is why I'm the retarded guy ;)
You get the same savings for each degree the inside of your walls are
cooler, given that it's still cooler outside, regardless of outside
temperature otherwise, or how long it takes your house to cool down.
As the house cools, you save at a higher and higher rate.
As it warms, you save at a lower and lower rate.
The heat you pay for leaves the house at a rate proportional to
the temperate difference between inside and outside. Any time
at all spent at a cooler inside temperature reduces the total
amout of paid heat you have to buy by that much.
One consequence is that if your house takes a long time to cool down,
you have to leave the furnace off for a long time to save much.
You never lose money though.
Exception: if you heat with a heat pump, you save even more than
with straight heat, so long as emergency heat doesn't come on
when you want to heat up the house again, and so long as the
pump doesn't go into defrost mode at the reduced inside temperature.
One thing to be careful about here -- the temperature drop isn't linear.
It will drop faster at first, and then progressively slower as the inside
and outside temperatures get closer to each other.
If it takes more than a day to drop from the daytime temp setting down to
the nighttime setting, then the setback isn't going to help very much at
all. On the other hand, my house takes about 2 hours to drop down to the
lower temp, so I really can expect 6 hours worth of savings.
I am assuming that his 24 hour test was just that -- an experiment. The
only time you would turn the heat off for that long would be if you weren't
planning on being in the house at all (and it wasn't so cold outside as to
cause pipes to freeze).
No -- if you measure the total furnace runtime, you will find it is less,
not more over the entire period. In the case of turning the furnace off
for 24 hours, you should measure how long the furnace needed to run over a
two day period. It will be less time than if you just left the temperature
at the higher setting for those two days.
Let's go back to your "bucket of water with a hole in the bottom" analogy,
since it works well. At a lower water level, water is lost at a slower
rate than it would be if the water level was right at the top. All of the
"cooling off" and "reheating" is just changing the water level in the
bucket. What really counts is how much water leaked out of the hole over a
one day (or one month period). For every minute that I have a lower water
level in the bucket, that is one minute where the water is leaking at a
slower rate -- that is where my savings come from.
This is a very interesting topic. At least *part* of the question is
probably the non-linear facet of heat-transfer problems. Not to
mention delay times and heat-sink questions.
The obvious abuses, turning the heat off when one leaves and cranking
the thermostat up to 90 when you return, are easily explained -- the
furnace doesn't have separate 90-degree and 72-degree flavors of
'hot'. It's pretty much binary -- on/off at whatever its capacity is
until the set point is reached.
The question is whether it takes more energy, in a non-linear world,
to maintain 60 degrees for 8 hrs, and then raise the temperature to
72, or warm a 45-degree space to 72. As a posted notice in my old
workplace had it, "the real world is not parallel, uniform, or without
Not a question that can be answered, at least stated as it is -- there
just isn't enough information. How long to accelerate to that speed? What
weight of vehicle? What drag coefficients (including aerodynamic drag
coeeficient)? How long do we cruise at 70 mph?
A totally different question, but one that might help here.
Not too many years ago Shell Oil sponsored mileage competition. Many
colleges competed. At the time most cars were getting 25 mpg or less they
managed to squeeze out around 50-70. They used all kinds of tricks that
real drivers could not easily use.
One trick was to use a engine designed to run at a single power output.
They were optimized to run at only one setting, no throttle. They would
start the motor, accelerate to a given speed and turn the motor off. When
it slowed to a pre-determined speed, they would repeat the process.
This resulted in higher mileage than a steady speed.
You car will not work this way because of the engine design, but even
today a similar idea is used by some/all hybrids. The gas/diesel engine may
only run at one speed, driving the car and charging the batteries, or some
variation of that idea.
Points taken, but all info I have heard re fuel consumption is that it takes
more fuel to get the car up to speed as opposed to keeping it there. I
think that is why you see higher mpg at highway than you do in city driving.
I offered this as an analogy. My furnace will consume more fuel to get my
house to 72 than it will to keep it there. I don't believe there is any
kind of regulator that would inject gas at slower rates when ramping up, so
it would be in the "floored" position until the thermostat shut it down.
How long would it take to get to 72? I don't know and I don't want to find
out, although, that is probably the best solution to this query: try a
month of turning off the furnace and a month of turning the temp down. Then
you can check the bills at the end of the month. Kind of think the best
months would be Jan and Feb (equally cold).
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