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

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And another fly in the ointment could be whether the heating system is water or forced air, I think. Not perhaps actually true, but that's what I'm inclined to think. IMO water heat does a faster and better job of warming a cold room -- and sustaining that warmth level longer -- than forced air.
But then again, people with forced-air systems don't end up paying what I did in copper pipe and some built-in redundancies that make my boiler area setup looking like something straight out of the engine room of a WW2 U-boat submarine <g>.
AJS
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wrote:

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.
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Chris J... wrote:

For the most part, you have it correct. However different systems have different problems.
For an electric resistance heat system without any time of day rate changes, you will save money turning it off during any part of the day (assuming you don't suffer damage to your home or contents due to the temperature changes). However you may want to upsize the total heating capacity to make sure you are able to bring the temperature up quickly.
For combustion type systems, there can be some additional issues because of possible efficiency issues, but in general I would have to guess they are a wash, some plus some minus.
On the other side we have the A/C. It also acts the same with one difference. If you decide to open the windows during the evening/night and let the house cool off, then close up and turn on the air in the morning as it get hotter, it is likely to cost you more, unless you live in a very dry area because it takes a great deal of energy to remove all the moisture that came in with the cool most night air.
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Yeah, the OP is right. _Most_ times setback will save money, maybe not that much, but it is basic thermodynamics, and the amount of energy required _is_ less. All things being equal (are they ever? ;-) this will save money.
Perhaps the only time you really really have to think twice about setback is with heat pumps.
Everything is fine until the point at which you try to bring the temperature back up. You flip it to 70F, and since the temperature differential between current temp and setpoint is so high, the backup heat kicks in.
So you've saved a little money on "cheap and efficient heat pump" heat, but consume lots of "expensive backup heat" to bring it back up to temperature.
This is _particularly_ nasty with electric strip backup. With gas backup, you _may_ still save something, depending on cost of energy computations.
With heat pumps you need to get set back thermostats that bring the temperature up _gradually_, so the difference between set point and current temperature is at most a few degrees at any given moment.
And note well when you manage the setpoint like this so that only the HP runs, it will take a _long_ time for the temperature to come up.
HPs are cheap to run, but they don't produce a lot of heat fast...
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On 14 Feb 2004 05:55:05 GMT, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

This is accurate and makes an excellent point.
To sum it up accurately, "Any set back, for any length of time, will save you money - -providing there is no triggering of expensive emergency heat by a heat pump," Now, what you pay for that savings is lack of comfort for the time you are waiting for the system to bring back the desired temperature. So it boils down to a tradeoff between savings and comfort.
PJ
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The temperature of anything (in this case the interior of your home) is a measure of the energy contained in that body. Heat flows from an object of higher temperature (and energy) to one of lower temperature. The rate at which heat flows is a function of the temperature difference of those two objects. As your home cools it looses lease heat per hour than it did when it was warmer because it's temperature is closer to the outside temperature. When equilibrium is reached (i.e. the interior and exterior is at the same temperature there is no heat loss because there is no temperature difference.)
When you return to a cool house the process of heating it requires that you deliver energy to it. How much energy? The amount that was lost when it cooled.
Since less heat was lost because the house was cooler you have saved the amount of energy that would have been required to keep the house at a higher temperature.
The kicker here is that if you want to warm your house rapidly upon returning there will undoubtedly be inefficiencies because not all materials increase in temperature at the same rate (they have differing "specific heats" or thermal capacities and rates of thermal transfer.) Much more energy is needed to raise the temperature of a cubic foot of water than a cubic foot of air. A cubic foot of wood will be in between those two extremes.
I believe in allowing my home to drop to as low as 50 F when I am away (this usually takes several days though) because I am very well insulated. I heat a little over 6,000 sq. ft and heat water all year 'round with less than 1,000 gallons of oil even though the outside temperatures this winter have hovered around -10 F.
If the temperature falls below 50 F my experience is that some damage will be observed. Paint may well separate from walls and ceilings, household plants may be damaged, etc.
RB
Chris J... wrote:

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I have always used " if you can vary the temp by more than 10 degrees (F) for 8 hours or more then you can save ~5%. Could so many manufactures be wrong in producing set back thermostats? I use a set back stat and it does seem to help. I like it cooler when I sleep. I have connected my stat to a simple x-10 modem and when I land at the airport I call home and turn on the water heater and a/c. I use a 10 degree off set for cooling and 15 for heating when I am traveling. Temp rarely goes above 80 or below 60. When it does the unit turns on and returns the home to the set point. Obviously exact conditions vary and your results may be different. http://www.mge.com/business/saving/setback.htm
http://www.energy.ca.gov/releases/1999_releases/features/1999-feature-08.html
http://www.cmhc-schl.gc.ca/en/imquaf/himu/wacon/waensatip/waensatip_042.cfm
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This is Turtle.
Here is the old thoughts on leaving the heat / cooling on for different lengths of time. This will be for Gas / propane / electric strip hot air furnaces and freon type cooling systems. The gas / oil boilers / hot water systems may not apply for I don't know.
Leave off for less than 4 hours -- It cost you money to do so. Leave off for 4 hours -- You break even and does not save anything. Leave off for 5 to 8 hours -- There is some savings but small but you do save something. Leave off for 8 or 8+ hours -- There is a good savings. Leave off all together 24+ hours -- saves the hell out of the fuel.
I really don't know about the hot water systems as to this thinking.
TURTLE
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Chris J... writes:

No. The process is analogous to keeping a bucket (house) filled with water (heat) at some level with a faucet (furnace) when there's a slow leak at the bottom (heat leaving the house). If you don't care about the level for some time, then it ALWAYS uses less water to let the bucket drain, and refill later, compared to keeping it filled. The confusion comes because you typically trade a lot of short heating cycles for one long one.
Some people won't believe it, because it takes differential equations to really understand this quantitatively and prove it, which is beyond most.
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That's my opinion also.

They don't want you to seriously reduce your consumption. It would cost them. Could that have something to do with their answer?

Not me. I don't believe it.
Bob
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These folks are known as "wrong".

You are correct.

They are idiots.

You are forgetting the power of Urban Legends to propogate and aquire adherents.
John
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This guy has the correct answer.
All the above answers that says that it doesn't save money are wrong and they are idiots just like this guy says.
PJ
On Fri, 13 Feb 2004 16:30:50 GMT, snipped-for-privacy@westnet.poe.com wrote:

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On Fri, 13 Feb 2004 16:30:50 GMT, snipped-for-privacy@westnet.poe.com wrote:

That's the way I used to argue it; I just couldn't comprehend how it could be otherwise due to heat loss over time being a function of temperature differential.

And I was one for listening to them. I just assumed they knew a lot more about the thermodynamics at work in this situation than I did. So, I accepted that I was wrong, but it's been nagging at me for quite a while because I just could not comprehend how leaving the heat up was more efficient. That's why I finally decided to post my question.

Ahhh... That makes sense; the person I spoke to was unaware of the real position of the Utility, and had just "heard" what he passed on to me and claimed it was real? That makes more sense than an energy company not understanding thermodynamics.
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Chris J... wrote:

Your mistake is assuming that the person you talked to on the phone must know what they were talking about.
Best regards, Bob
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wrote:

Jumping in rather late.
My big question is why doesn't some utility company, govt lab, university or consumer protection group do a proper study on this subject and publish the results in graph form that everyone can understand at a glance.
Graphs to illustrate heat loss or savings from different thermostat settings, different time cycles and some other parameters versus time should settle many arguments and satisfy numeric challenged people like me.
The best enegry saver is to properly insulate your home. I had seen a graph that plots heat loss against R values. The slope rises sharply upward and then flattens abruptly after R30. This is a clear illustration that your heat losses drop with increased R value insulation up to R30. Installing more insulation beyond R30 is not worth the extra cost in construction from wider studs and walls, batts, etc. Therefore product advertisers touting extra-extra R values beyond R 30 as the next best thing to sliced bread are just urging to to spend money that you will not recover in energy savings.
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Nelson, L.W. 1973. "Reducing Fuel Consumption with Night Setback," ASHRAE Journal, Feb., Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers..
Pilati, D.A. 1975. The Energy Conservation Potential of Winter Thermostat Setback and Energy Savings, ORNL-NSF-EP-80, Oak Ridge, TN: Oak Ridge National Laboratory.
Quentzel, D., 1976. "Night-time Thermostat Setback: Fuel Savings in Residential Heating," ASHRAE Journal, March, Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning.
http://epics.ecn.purdue.edu/hfh/web_project/HEO/Options/thermostat.htm
www.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/results.html

There are too many variables to allow a simple graph to be drawn (at least in the "general case"). A simple graph would show the typical 1% savings for every 1 degree setback over an eight hour period.

I assume that this was the heat loss for an entire house, not just the insulation? R-value doesn't "flatten"; it's a perfectly linear relationship. However, your entire house cannot be insulated perfectly, since things like the wall studs will transmit heat, and extra insulation just won't have any effect on these heat losses.

I believe that your best "bang for the buck" would come from reducing air leaks, and using a setback thermostat.
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On Sat, 14 Feb 2004 18:10:30 GMT, Murray Peterson

There was one very beautiful chart published by FORTUNE Magazine about the rise and fall of the PC makers over the years. The vertical axis showed their relative rankings. The new and very informative innovation was to thicken each company's chart line to refelct the volume of its sales as the company rose and shrunk. The same plots showed clearly their relative market sizes over the same period. One look and each PC company's history was immediately evident and agreed with the impressions all of us had formed from reading the news over the years.
My visualization of a useful chart would be to plot room temperature against time (24 hours over several weeks), with the thickness of the plot line to represent the BTU required to maintain whatever the thermostat setting is. Then have different thermostat setbacks.
I haven't figured out how to relate these plots to the outside temperature conditions. But then this is what we have experts for - to make studies and present them kin an easily readable form.

I am numerically challenged so please make allowance for my description. The flat part of the chart plot was the heat loss (as a reciprocal percentage?) curve against the R value. So the heat loss curve would rise steeply and linearly before it flattens off at just over 80%.
The interpretation is that below R30 every bit of extra insulation will significantly improve heat retention for the house, to equal big energy savings. Thereafter adding thicker insulation (higher R value) will not improve this ~80% efficiency significantly and doubling the R value may perhaps help you achieve 85% efficiency. It would of course never reach 100%. It convinced me to stay with 2 x 6 studs for my outside walls and R30 batts when I built my house. I was also convinced that triple glazed insulated windows was not worth the money.

I went through a course that enabled people to be their own general contractor. I really learned a lot. It helped me to plan my HVAC so that it is centrally located and had the shortest straight runs thereby avoiding trunking losses and enabling easy heat balance throughout the house.
That said the course instructor's caution was to insulate the house well but not over seal it. This was during the 70/80s energy crisis when retro-insulating old houses was the rage. (Mine was a new build.) We need the house to be slightly leaky to get fresh air in. There were many reported examples then of people who overdid this sealing in their old homes and died of suffocation.
I set my thermostat at 68 degF. I tried the electronic programmable thermostat and hated that thingy because its so hard to alter the setting when I want to lower or raise the temperature on demand.
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You should have a second look at thermostats -- many of the new ones are extremely easy to use. Mine (a Honeywell) has two buttons to increase/decrease the temperature -- as soon as you do so, it overrides the temperature setting for that time period.
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makes loota sance , i leave in winter turn down me heet an me bills still hiher than summer ---go figure... i cant Help me.Hwelp me, help pleeese
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The problem is that the amount of savings depends on dozens of different factors - a graph simple enough for everyone to understand would represent only a tiny fraction of "real systems". Degree days, heating system type, energy costs, etc. etc. etc.
Whenever you're looking into something like this, add "CMHC" into the google search.
The search "cmhc setback thermostat savings" finds _lots_ of solid data.
"CMHC" is a Canadian government agency "Canadian Mortgage and Housing Commission" and is one of the very best (and unbiased) sources of information regarding this sort of thing.
Their "Keeping the heat in" booklet is about the very best homeowner material regarding energy conservation there is.
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