Why is it less efficient to turn off a home furnace than to leave it on???

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A car is a bad analogy. A furnace doesn't have a throttle. It's either on or off. The thermostat is basically a light switch and not a dimmer.
Bob
edee em wrote:

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edee em wrote:

I don't disagree with this, even if you are mixing apples and oranges.
Actually accelerating slowly to say 30 mph would likely take less fuel (gal per minute) than traveling at 60 mph for the same time. However you would travel much further.
In a car you have three factors. First the efficiency of the engine, and that changes depending on its design, rpm and torque output. Then you have the momentum when accelerating and you have the power needed to maintain the speed at a steady speed
A home is a lot different. Most home heating equipment will work most efficiency when the temperature difference is the greatest, since the heat transfer rates will be best. Straight resistance electric does not change and heat pumps can show just the opposite.
You really can't extrapolate much of anything about fuel saving from a car to a house.

Yes.

Yes.

Let's say hour. The furnace will run full for 30 minutes. Ignoring a few other factors it may keep it there for the next 30 minutes by running only 10 minutes
So far that looks like you are right. However:
Now we turn the thermostat down to 60 and go to bed. It may take the next 1.5 hours to get down to 60 degrees, during which time the furnace does not run at all. so we are now even. however over the next 6.5 hours the furnace may need to run only 7 minutes each hour since it does not need to keep it at 72. That means I save 39 minutes of running time over all.

I suggest you should also keep track of "heating degree days" for your area as computed by the weather people. That will tell you how even the weather was.

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Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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That's only true because of the way people actually drive. For example, let's say that your car requires 50 HP to maintain a cruising speed of 70 MPH. All you need to do is govern your use of the throttle such that your engine always puts out 50 HP, right from a standing start. The car will accelerate up to 70 MPH, and the fuel consumption will be constant over time.

The analogy just doesn't work -- or you need to extend it a bit further to make it work the same as a house furnace. First, we design a vehicle that has an engine with a single output ("floored" whenever it is running). This engine uses a constant amount of fuel every minute that it is running. Now drive this vehicle over a 1 day period in two different ways:
1. Cruise for 24 hours at a constant speed of 70 mph (lets assume that the engine is running continuously to maintain this speed).
2. Cruise for 12 hours at 70 MPH and 12 hours at 50 MPH. To cruise at 50 MPH, the engine will need to be turned off periodically -- otherwise the car would go back up to 70 MPH.
In case 2, the total fuel consumption will be reduced, since the engine won't be running as frequently for 12 hours out of the 24.

I have done this over the last several years -- my fuel bills have been substantially reduced by using setback thermostats on my two furnaces. To really measure the change in a short time period, you need to calculate your fuel bill based on the "heating degree day" data during that billing period.
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Joseph Meehan wrote:

They still do that. It sounds counter intuitive, but the vehicle under power is constantly accelerating (constantly decelerating without power). All manufactures of cars say drive at a steady speed for maximum mpg, but then car engines aren't wrapped in insulation to hold heat in either.

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wrote:

[snip...snip...]
The only way to answer this question is to collect real data on your own circumstances. Hand-waving generates heat but not illumination.
I collected data here from a few thermistors to measure the effects of setting the heat back to 55 F during the day (while at work) and also at night. Effectively, this secures the heating system during those times. Otherwise, it's set to 68 F.
In the coldest weather we generally get around here, once the thermostat got the rooms up to nominal temperature, the heating system was on approximately 2/3 of the time in the morning (coldest time of day) and 1/3 of the time in the evening. Assume 2/3 as the worst case.
The time to return to a comfortable temperature after the setback was about 1 to 1.5 hrs. Lets work with 1.5 hrs as a worst case.
Given that, if the thermostat setting had been a constant 68 then that 1.5 hours of run time would have been achieved during 2.25 hours of normal cycling.
Therefore, if my heating system is off longer than a couple of hours then there's a net savings.
Since the heater controller is a "bang bang" system (either on or off) then it "works" just as hard whether it's on for 10 minutes or 1 hour. Since the extended running period also means fewer startup/shutdown transients, there are overall fewer stresses on the system.
--
Rich Webb Norfolk, VA

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On Sat, 14 Feb 2004 17:12:02 GMT, Rich Webb

I do know that installing a programmable thermostat three years ago cut my gas usage by approx 15-20% . It was previously set at 66, but with the programmable I set it for 60 overnight, 62 in the morning and afternoon, and 66 in the evenings.
I can only make rough guesses as the weather is never identical. This is for a forced-air gas system, and my average gas bill in the cold months is around $120 this year.
A couple of days ago, with daytime highs in the upper 30's and overnight lows in the low single digits, I turned it completely off, and it took 48 hours to drop from 66 to 46. I then turned it on again and the furnace had to run near continuously for a few hours to get back to 66, but there was about a 20% drop in gas usage from the previous (normal) 48 hour period according to the meter. I am however in an odd situation regarding the weather; the average difference here between daytime high and overnight low is over 30 degrees (year round).
These results are about what I'd expect given my limited knowledge of thermodynamics, and are about what I believed prior to my conversations with the gas company.
This of course begs the question; why on earth is the gas company giving out bad advice? I'd assumed they were right and I was wrong when I spoke to them, but my test results, and most of what I see in this thread, contradict that.
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This is the retarded guy back again, pestering people.
Actually, seems to me that the gas company really didn't give you severely bad advice. Your question/scenario you originally was, word for word:
"I've been told by quite a few people that turning a heater (or an air conditioner) off for the day uses more energy due to it having to "work harder" to restore the home's temperature."
Now here a few days later, turns out you're only talking about a 2-4 degree difference, which will certainly save on the fuel bill but is nowhere near the massive "working harder"/fuel consumption stretch ("turning it off for the whole day") that you posed originally. So in a somewhat roundabout way, the gas company doesn't seem to be *entirely* wrong within the context of your original question.
Or maybe retarded guy just confused by all the highly geeky discussion of thermodynamics the past few days that caused retarded guy's eyes to glaze over on numerous occasions.
AJS
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wrote:

Excuse me? If you are calling me "retarded" just because I was confused over an energy efficiency issue, you are way off base!

How do you figure that I was only speaking of a two to four degree differential? I did mention that a programmable thermostat with approximately that daily differential reduced my energy usage, but I also turned the system off for 48 hours as a test. What originally perplexed me was the conundrum that lowering the temp a couple of degrees, then bringing it back up a few hours later would save energy, but lowering it more would not.
My test results, under approximately similar weather conditions, do indicate a savings by turning the system off.

Again with the "retarded guy" thing? Because I'm possibly mistaken on this issue? And I'm "pestering people" by asking a question on a discussion group?????? Big freaking deal! What the hell is your problem?
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wrote:

<snip>>
AJS, please disregard my responses to these lines in my previous post. I replied before reading all your posts in this thread, and I foolishly took your self-depricating humor to be referring to me, and thus an attack on me. I sincerely apologize.
Chris.
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No worries, mate. As it is, I don't get personally offended by forum posts anyway. But I commend your concern toward complete strangers.
AJS
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wrote:

Thanks... Well, when I put my foot in my mouth like I did, I feel a need to do something about it... And the only two choices I could see were either apologize, or get better tasting shoes. :-)
BTW, I tried (about a minute after I hit send) to cancel my original post, too, but apparently either my server, or a lot of other servers, does not accept them.
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Chris J... wrote:

and
but
me,
Turn off or leave on and was told leave on but turn down
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Theoretically, one can calculate the energy consumption of a heating system, (if it's not too complex) but there are always errors due to unknowns, inaccurate extimates, etc.,. The truth (proof) of the matter is often better determined by empirical (experimental) methods. For example, One can monitor the temperatures both inside and outside the building/structure along with the energy consumption for each of the two heating system operating procedures, and compare the results statistically. This is what the energy providers do. I would trust empirical results better than purely hypothetical ones in almost any case. However, think of the classical problem of designing and exploding the first atomic bomb - necessarily based mainly on theoretical predictions. Nobody really knew what to expect for sure. Apparently, it was a big relief to those few in charge to find that the world was still intact after the explosion. The rest of humanity knew nothing about what was going on - the world might have ended that day.
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On 15 Feb 2004 08:58:43 -0800, snipped-for-privacy@aol.com (Alias) wrote:

Well, I gave it my best try, and listed my results in a post I just made. Long story short, a 48 hour shutdown under very similar weather to the previous 48 hours saved me around 20%, and the temp was still falling (around 46 degrees) when I switched back on.

That was due to Edward Teller's concern that the bomb might ignite a hydrogen fusion chain reaction in the atmosphere, essentially turning the atmosphere of the planet into one big hydrogen bomb. He gave it an exceedingly small chance (under 1% as I recall) but with the state of theory at that time there was no certainty.
And, it may be Urban Legend, but there is a story that one of the scientists, referring to this theoretical risk, quipped "If Teller is right, and we blow up the Earth, Teller is going to be insufferable..."
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Final answer:
Try it both ways and check your meter for actual consumption. Obviously with the same temperature, humidity and wind speed
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I'd like to thank everyone who has participated in this thread. I've read every post, but can't reply to them all. Thanks, everyone! Chris
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Chris J... wrote:

Chris, never ask a barber if you need a haircut.
People who make a profit by selling you fuel will almost always tell you it's less efficient to set back the temperature at night or when no one's home. You'll always save energy by reducing the temperature.
There's no need to elaborate. Those falacious statement are believed by the same people who'll also believe someone who tells them that it takes more gasoline to restart your car's engine than to let it idle for five minutes. Not true under any reasonable set of circumstances.
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

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On Sat, 21 Feb 2004 11:35:13 -0500, Jeff Wisnia

ROFL!!!!!!!!!!!!!!!!!!!!!!!! How very, very true...

I feel rather foolish for disbelieving my initial inclinations and believing the gas company on this. I'll definitely be more of a sceptic in the future.
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wrote:

Nah, Chris you just got an untrained person at the gas company. No energy company today would dare to give out that kind of wrong advice as a matter of policy.
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