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

[AJScott]

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 temperature. ..."

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.

AJS

[AJScott]

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

[Murray Peterson]

AJScott wrote in news:nowhon- [e-mail]:

I don't see any contradictions in there.

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 day.

[AJScott]

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 ;)

AJS

[Ron Hardin]

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.

[Murray Peterson]

AJScott wrote in news: [e-mail]:

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.

[George Eberhardt]

Of course they disagree; their SELLING gas; the more you use the higher their income!

[Bob]

room is

having to

tear (aka

harder,

to

If the thermostat is turned down for periods, the heat loss total will be less. The total run time to supply the needed heat will be less, so "wear and tear" will be less.

Bob

[Chris J...]

That's a point.. I also do know that the electric company, due to considerations of peak loads, does not like cooling to be turned off in the daytime then turned on in the evenings.

[Chris J...]

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?

[Chris J...]

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.

[AJScott]

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

[markherm]

IMHO- I live in Minnesota and experience extreme temps, I turn down the temp on the furnace 10 degrees when at work for the 9 hours I am gone, as the home cools, the differential between the out temp and the in temp is reduced, and the temp falls slower as the temps synchronize with each other. This means less call for heat, right? Also, when I return home, I turn the temp back up, and the furnace will run in a steady state condition for a extended period of time, this is where you will find the best efficency with your furnace/AC! The utilities don't like it when people come home and turn down the air conditioners, this will lead to a large peak demand, and will discourage the practice. I use off-peak electric for my AC, so I do not turn the temp up much when at work, I need to have the house cooled when I return since they could be cycling the unit on/off every

15 minutes. The power charge for this is only 3.5 cents per Kwh. I work on boilers (small, 500-1000 HP on common header) and furnaces for a living.

[Jack]

Duh, the gas company SELLS GAS.

-Jack

[Chris J...]

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.

[no1herenow]

The amount of insulation in the building is not relevant to the discussion of savings due to turning the heat off vs. just lowering the thermostat. The amount of insulation only dictates the total fuel consumption (and how low the temperature will get if the furnace is turned off for a fixed period of time).

I am surprised at the length of this thread... It is not due to someone's thinking, but rather elementary thermodynamics that explains why turning off the furnace (if it results in a lower average indoor house temperature), will ultimately use less fuel than setting the thermostat at a level (that results in a higher average indoor temperature).

[Jeff Wisnia]

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

[Chris J...]

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.

[Tom Miller]

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.