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  #1  
Old 01-10-2003, 09:33 AM
bernse bernse is offline
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Electronic home thermostats. Whats the best temp to set it at?

We bought a new home last year. Before we moved in, I also bought and installed an electronic thermostat. The kind that you can set to change to a certain setting at a certain time of day.

The way I have it set now is that at 6:15AM I have it warm the house up to 20C.

7:30AM, shortly before the Mrs and I leave, it goes down to 16C(this is probably the one I'll be questioning later on).

5:15PM, roughly when we get home, it goes up to 21C.

Lastly, 10PM, it sets to 18C for sleeping at night.

The question I have is this. When we are at work for the day, is 16C too "cold" to be economical? For the most part, the weather this winter hasn't been too cold. So, during the daytime when we were at work the house wouldn't even hit 16 (or at least, I don't think it would). When I would get home it would usually show 17 as the current house temp. However, now that the days are cold again (-15 to -25) the house will indeed cool to that temperature after some hours. Then, when I get home the furnace is on for an hour straight (give or take) to get the temperature back up to 21.

Being a new house, the furnace is quite effiecient (a new Trane) and the windows and insulation are all very good. We also have thick, thermal lined curtains or roman blinds on all the windows.

Is there a way to figure out the most "economical" setting?
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  #2  
Old 01-10-2003, 09:39 AM
Crafter_Man Crafter_Man is online now
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Re: Electronic home thermostats. Whats the best temp to set it at?

Quote:
Originally posted by bernse
Is there a way to figure out the most "economical" setting?
I donít mean to sound facetious, but the most economical setting is ďOFF.Ē

Itís pretty simpleÖ regardless of anything else, when your furnace is "on" youíre using energy, and when your furnace is "off" youíre not using energy. The more it is off, the more money youíll save. But this must (obviously) be weighed against comfort.
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  #3  
Old 01-10-2003, 09:47 AM
bernse bernse is offline
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Fair enough, but since most homes have tropical plants and pets that not a very viable option when it is -30 outside.

So, would it be fair to say "let it get as cold as you are comfortable letting it get when you are out?"

If that is 13C, so be it? Obviously, when the furnace is off, its not burning gas. However, if the furnace is going to have to be on for 2 hours straight to warm up the house, does that outweigh the benefit or is it always better to keep it as low as you can?
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Old 01-10-2003, 09:48 AM
sailor sailor is offline
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>> I donít mean to sound facetious, but the most economical setting is ďOFF.Ē

Pretty much on target. Other considerations: You probably want to start the heating between 1/2 and one hour before you get home and turn it off the same amount of time before you leave. It depends on the thermal inertia of the system. Other than that, the lower the setting, the more you save.
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  #5  
Old 01-10-2003, 09:51 AM
NYR407 NYR407 is offline
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Not sure I agree Crafter. It may require more energy to reheat the entire house than it is to maintain a low tempertature. Someone else may be able to back me up or tell me how wrong I am.

Bernse, I will have to get my Celcius converter to see what temperatures you are talking about. I am used to Fahrenheit.
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  #6  
Old 01-10-2003, 09:52 AM
pipper pipper is offline
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IANAHISH (I am not a home improvement show host) but...

Furnace/boilers usually take a little while to get up to their most efficient operating temperature. A general rule of thumb is that a slighty underpowered furnace that has to run longer is ultimately more efficient than an overpowered furnace that only runs for a short period (and never reaches its peak operational efficiency). So I would think that your strategy of letting the house cool down and then having the furnace be on for a long time is more efficient than having the the furnace cycle on and off to 'hold' a more moderate temperature.
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  #7  
Old 01-10-2003, 09:53 AM
sailor sailor is offline
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>> Fair enough, but since most homes have tropical plants and pets that not a very viable option when it is -30 outside.
>> So, would it be fair to say "let it get as cold as you are comfortable letting it get when you are out?"

I am not sure I understand. This is a question about what plants and pets need? I can't answer that. But, at any time the lower the setting the more you save. How low you want to set it is up to you.

>> Obviously, when the furnace is off, its not burning gas. However, if the furnace is going to have to be on for 2 hours straight to warm up the house, does that outweigh the benefit or is it always better to keep it as low as you can?

The answer is YES, popular misconceptions not withstanding. The furnace may be ON for two hours but it is heat you are using. If it had been on while you were away, the heat would have been lost. You are *always* better off turning it down or off while you are away.
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  #8  
Old 01-10-2003, 09:55 AM
sailor sailor is offline
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Quote:
Originally posted by NYR407
Not sure I agree Crafter. It may require more energy to reheat the entire house than it is to maintain a low tempertature. Someone else may be able to back me up or tell me how wrong I am.
You are wrong. Any engineer can tell you that. And we have had many similar threads about this.
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Old 01-10-2003, 10:01 AM
bernse bernse is offline
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Quote:
I am not sure I understand. This is a question about what plants and pets need? I can't answer that
What I meant is this. If it is -35 outside, since I have plants and animals inside, I'm not comfortable just setting the furnace to "off" all day. While my house is pretty effecient, I'm not entirely comfortable letting it settle to what it would with no furnace heat during a frigid winter day. However, I may be comfortable with say, 15C as I am sure that cat and plants aren't going to feeze at that temp.
Quote:
The answer is YES, popular misconceptions not withstanding. The furnace may be ON for two hours but it is heat you are using. If it had been on while you were away, the heat would have been lost. You are *always* better off turning it down or off while you are away.
Thanks for the nice summation, Sailor. Make sense to me. Spud the cat may have to get used to having a "chilly" home during the daytime.
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  #10  
Old 01-10-2003, 10:01 AM
CookingWithGas CookingWithGas is online now
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I have the same situation and same question as bernse. But I have not seen an authoritative answer, notwithstanding sailor's uncited response. This can be boiled down to a simple question.

Let me rephrase.

Does it take more energy to maintain 20C for 9 hours or to raise the temp from 16C to 20C?

Given that I set the thermostat at 21C (69.8F) while I'm home in the morning, and 20C when I return home, what is the optimum temperature setting for the 9 hours while I'm gone?

If I leave it at 20C all day, then the furnace will have to do work intermittently during the day to maintain that temperature but then have no extra work do when I return home.

If I turn it off during the day, and the temp drops to say, 16C (60.8F) during the day, then the furnace does no work all day but has to raise the temp by 4C when I get home.
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  #11  
Old 01-10-2003, 10:04 AM
NYR407 NYR407 is offline
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Quote:
Originally posted by sailor
You are wrong. Any engineer can tell you that. And we have had many similar threads about this.
Ok fair enough. I was just thinking along the lines of how highway driving is more fuel efficent than city driving.

Obviously I am wrong.
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  #12  
Old 01-10-2003, 10:08 AM
sailor sailor is offline
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Quote:
Originally posted by CookingWithGas
I have the same situation and same question as bernse. But I have not seen an authoritative answer, notwithstanding sailor's uncited response. This can be boiled down to a simple question.

Let me rephrase.

Does it take more energy to maintain 20C for 9 hours or to raise the temp from 16C to 20C?

Given that I set the thermostat at 21C (69.8F) while I'm home in the morning, and 20C when I return home, what is the optimum temperature setting for the 9 hours while I'm gone?

If I leave it at 20C all day, then the furnace will have to do work intermittently during the day to maintain that temperature but then have no extra work do when I return home.

If I turn it off during the day, and the temp drops to say, 16C (60.8F) during the day, then the furnace does no work all day but has to raise the temp by 4C when I get home.
Do the calculation and you will see I am right as has been established in many prior threads. To me it is quite obvious but I have an engineering background. Just do the calculation and, if you come up with anything else, I'll show you where the mistake is.

The mathematical model is similar to the following experiment which you can do if you dislike math: take a platic cup and make a hole in the bottom so water can drip out. Now pour water in to keep a certain level. water dripping out is heat lost. Water poured in is heat added. Now tell me if by keeping the level higher for a while you save anythi water overall. The answer is NO. But you can try it. I prefer doing the math *_*
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Old 01-10-2003, 10:12 AM
Popup Popup is offline
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Let me try to explain, CookingWithGas.

Try an anology. Imagine that you have a bath tub with a small hole in the bottom. If the tub is full, it's leaking, and the more water you have in the bath, the faster it leaks.
Imagine further that you want the bath to be full at a certain time. It should be obvious that the best is to let the tub drain, and then to fill it up when you need it, instead of constantly topping it up.

--
On preview I see that sailor beat me with a similar explanation, but I'll post anyway...
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  #14  
Old 01-10-2003, 10:27 AM
CookingWithGas CookingWithGas is online now
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I understand quite well [b]sailor's[b] and Popup's explanations but I think they may simplify the situation and I would be most interested to any empirical data on the subject.

Let me rephrase your explanations back in terms of an idealized house thought experiment. A house where you turn the heat off is going to have a temperature drop due to heat loss to its environment. (Let's assume that eventually the temp inside matches the temp outside and we have steady state with no further heat loss.) Then when we reheat, running the furnace to restore the temperature adds an amount of heat equal to that which was lost during the drop. Alternatively, if we heat the house during the day, the rate of heat loss will be greater because the temp difference between house and its environment is greater. So you would conclude that you're better off turning the heat off entirely during the day.

But all this assumes a closed system.

Here's another point that may be splitting hairs but it makes for interesting discussion. A house is not a closed system. As the air heats, it expands, and houses are not airtight. The increasing pressure pushes the warmed air out of cracks and crevices. As the air cools the opposite happens, and cold air is drawn into the house from the outside. So we cannot assume that heat loss is confined to convection and radiation on the outside of the house.

Therefore I would suggest that a house at a steady state temperature is losing less heat to pressure increase than a house that starts out being cool then is heated up.

How does the expansion and contraction of the air in the house affect this thought experiment? Doesn't the loss of heat during a reheating offset some of the advantage of the reduced heat loss to convection and radiation during the day?
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  #15  
Old 01-10-2003, 10:28 AM
pipper pipper is offline
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Not a mathematical formula, but from the a document produced by the DOE. link

It basically says that the amount of energy required to reheat a home is equal to the amount of energy saved by not running the furnace while the house cools off. The amount of energy 'saved' only comes from the period of time when the temperature has leveled off at the low point.

So what this seems to imply is that you wouldn't save any energy if the house never gets a chance to settle at it's lowest temperature.

As an example, the house starts off at 70 degrees to wake up. Daytime setback on the thermostat is for 62 degrees, so the house gradually cools but only reaches a temp of 64 degrees at 5 pm before the thermostat kicks it back up to 70 for the evening. In this scenario, the article would imply there is no fuel savings because the temperature was always rising or falling, and never leveled off at a low temp.

So this raises the question, is it better to set the thermostat back only slightly, so the period of time where the temperature is held low (and where the saving supposedly occurs) is lengthened?

This doesn't seem to make sense. Is the article wrong?
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Old 01-10-2003, 10:51 AM
handy handy is offline
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I only heat the rooms I use. Why are you suggesting heating the whole house, isn't that expensive?
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  #17  
Old 01-10-2003, 10:59 AM
sailor sailor is offline
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Quote:
Originally posted by CookingWithGas
I understand quite well sailor's and Popup's explanations but I think they may simplify the situation and I would be most interested to any empirical data on the subject.
Look, this has been discussed several times in the past. We recently had a thread about leaving an electric water heater on.

I have a background in engineering and I have calculated plenty of similar things (heat sinks, heat exchangers etc). The math involved is fairly simple. Can *you* do it? Then do it and you'll find out. If you can't do it, then just take the word of those who can. The math model is similar to charging a capacitor across a resistor and many other similar things. If you look in the book is it is lesson 1 of differential calculus.
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Old 01-10-2003, 11:08 AM
scr4 scr4 is offline
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Quote:
Originally posted by pipper
The amount of energy 'saved' only comes from the period of time when the temperature has leveled off at the low point.

So what this seems to imply is that you wouldn't save any energy if the house never gets a chance to settle at it's lowest temperature.
It's over-simplified. Even if the temperature never reaches outside temperature, the lowered temperature will lead to reduced heat loss. Lower temperature is always more efficient. CookingWithGas's theory about expanding/contracting air is true, but a temperature drop of 10 degrees C will only cause a 3% drop in volume. I'm willing to bet that the heat loss through this is negligible.

(Central heating is one thing I really miss about living in the US. Even if my Tokyo apartement had it, I'd never be able to afford the fuel here. I use a kerosene space heater for a couple of hours a day.)
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Old 01-10-2003, 11:09 AM
bernse bernse is offline
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Quote:
Originally posted by handy
I only heat the rooms I use. Why are you suggesting heating the whole house, isn't that expensive?
That's a little awkward to do with central heating, Handy. Generally speaking, you have 1 furnace and ductwork connecting it to every room of the house. When the furnace is on, hot air travels in the ducting and hence heats every room. That is more or less Central Heating by definition.
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  #20  
Old 01-10-2003, 11:20 AM
sailor sailor is offline
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From the Department of Energy source cited by pipper:
Quote:
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 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 you save.
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Old 01-10-2003, 11:37 AM
pipper pipper is offline
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Quote:
You save fuel between the time that the temperature stabilizes at the lower level and the next time heat is needed.
Sailor - So is the example I gave out earlier accurate? That no fuel savings would occur if the temperature was always either falling or rising and a 'lower limit' holding temperature was never reached?

I ask because this is the situation I have currently in my house. I set back the temp from 70 to 62 while at work. However over the holidays I observed the temperature would only fall to roughly 64 degrees before the evening heating kicked in (now I assume that when winter finally arrives in Chicago - like today!- the rate of decline will increase and the house will probably hit 62 degrees during the daytime).
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Old 01-10-2003, 11:44 AM
CookingWithGas CookingWithGas is online now
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Quote:
Originally posted by sailor
Look, this has been discussed several times in the past. . . . I have a background in engineering and I have calculated plenty of similar things (heat sinks, heat exchangers etc). The math involved is fairly simple. Can *you* do it? Then do it and you'll find out. If you can't do it, then just take the word of those who can.
No one's questioning your background or the veracity of your information. But usually "trust me" is not a thread-ender on this board. People are interested in more than a bottom-line answer--the discussion and underlying information is why people use this board. I'm terribly sorry if I've missed other similar discussions.
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Old 01-10-2003, 11:52 AM
sailor sailor is offline
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Quote:
Originally posted by pipper
Sailor - So is the example I gave out earlier accurate? That no fuel savings would occur if the temperature was always either falling or rising and a 'lower limit' holding temperature was never reached?

I ask because this is the situation I have currently in my house. I set back the temp from 70 to 62 while at work. However over the holidays I observed the temperature would only fall to roughly 64 degrees before the evening heating kicked in (now I assume that when winter finally arrives in Chicago - like today!- the rate of decline will increase and the house will probably hit 62 degrees during the daytime).
No, you can forget about the furnace. The heat loss is directly proportional to three things:

1- Thermal conductivity (or the reverse: insulation) which is constant
2- Temperature difference between inside and outside
3- Time.

Whether the temperature is falling or rising is irrelevant, what counts is the temperature inside: the higher the temperature the higher the amount of heat lost which will have to be replaced. So, you do save by letting the temperature drop, even if it dos not drop much.
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Old 01-10-2003, 11:52 AM
ZenBeam ZenBeam is offline
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Quote:
As an example, the house starts off at 70 degrees to wake up. Daytime setback on the thermostat is for 62 degrees, so the house gradually cools but only reaches a temp of 64 degrees at 5 pm before the thermostat kicks it back up to 70 for the evening. In this scenario, the article would imply there is no fuel savings because the temperature was always rising or falling, and never leveled off at a low temp.
Obviously something is wrong here, because if you had instead set the house at 66 degrees, your interpretation says that you would get a fuel savings. You are saving energy whenever the house is below 70 degrees, because less heat will leave the house, since the amount of heat leaving is proportional to the temperature difference.

As to how obvious it is that lower temperature = energy savings, it's not of necessity the case that lower is always better. If the furnace somehow became less efficient when run non-stop for a long time, you'd have to take that into account. (I suspect the furnace actually becomes more efficient, based on pipper's reasoning.)

In the similar case of setting the temperature higher when air conditioning during the summer, I've heard setting too low can be bad, because when an air conditioner runs non-stop for a long time, it can ice up, reducing the efficiency. Whether or not that actually occurs for any given air conditioner, it's certainly plausible, and can't be discounted as easily.
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Old 01-10-2003, 11:59 AM
sailor sailor is offline
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ZenBeam, I could find a plausible scenario where driving your car at 80MPh would save gas compared to driving at 50 MPh. I suppose I could find a plausible scenario where burning $100 bills to heat yourself would make sense too. But that's not realistic advice in general.
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Old 01-10-2003, 12:33 PM
Drum God Drum God is offline
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Couldn't this be settled by a simple experiment. Find a couple of days where the weather is forcast to be the same -- same cloud cover, daytime temp, etc. On day one, leave the thermo at a comfortable temp all day and record how much electricity (and/or gas)went through the meter. On the second day, lower the daytime thermo and see if a different amount of fuel passed through the meter.

In warmer regions, does the same rule about heat-loss (or gain) apply when air-conditioning the house? Is it better to warm the house during the day, then bring it back to cool after work?
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Old 01-10-2003, 01:19 PM
Una Persson Una Persson is offline
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Well, I'm an engineer, and I can do the first part (I think) - determining how much energy is lost by a house maintained at a constant temperature. What I can't do right away is to determine the loss from a house where the house temperature falls naturally. I would have to make some more assumptions about the heat capacity of the entire house. Maybe I can come back later, or someone else can take it the next step.

Assume: A 30x40 house with 12-foot exterior walls, flat roof.

Total area: 2*(30x12) + 2*(40*12) + 30*40 = 2880 square feet

Assume: No slab loss.

Assume: R-19 insulation all-around. This is 19 ft2*F/Btu/hr thermal resistance.

Assume: Measuring from 7:00 am to 5:00 pm - 10 hours total.

Assume: Exterior temperature starts at 20 F, rises to 40 F by 3:00, falls to 30 F by 5:00. Smooth linear rise and fall.

Map: Exterior temperatures

7:00am - 20F
8:00am - 22.5F
9:00am - 25F
10:00am - 27.5F
11:00am - 30F
12:00pm - 32.5F
1:00pm - 35F
2:00pm - 37.5F
3:00pm - 40F
4:00pm - 35F
5:00pm - 30F

Equation to determine heat loss: Q/R

Q = Area*(Tinside - Toutside)
R = 19 ft2*F/Btu/hr

Loss is given in terms of Btu/hr.

Assumption 1: House interior maintained at constant 70F

7-8am: Energy = 2880 ft2*(70-21.25)/19 = 7389.5 Btu
8-9am: Energy = 2880*(70-23.75)/19 = 7010 Btu
9-10am: Energy = 2880*(70-26.25)/19 = 6332 Btu
10-11am: Energy = 2880*(70-28.75)/19 = 6253 Btu
11-12pm: Energy = 2880*(70-31.25)/19 = 5874 Btu
12-1pm: Energy = 2880*(70-33.75)/19 = 5495 Btu
1-2pm: Energy = 2880*(70-36.25)/19 = 5116 Btu
2-3pm: Energy = 2880*(70-38.75)/19 = 4737 Btu
3-4pm: Energy = 2880*(70-37.5)/19 = 4926 Btu
4-5pm: Energy = 2880*(70-32.5)/19 = 5684 Btu

Total Energy Required: 58,816.5 Btu

Look OK? Now...I would do the example where the house temperature is allowed to fall, if we can determine a specific heat capacity of the house and its belongings, to determine how fast the interior temperature drops.
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Old 01-10-2003, 01:21 PM
sailor sailor is offline
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Quote:
Originally posted by Drum God
Couldn't this be settled by a simple experiment. Find a couple of days where the weather is forcast to be the same -- same cloud cover, daytime temp, etc. On day one, leave the thermo at a comfortable temp all day and record how much electricity (and/or gas)went through the meter. On the second day, lower the daytime thermo and see if a different amount of fuel passed through the meter.
You can experiment and the results will confirm what I said or the experiment was done wrong. We are talking about something pretty basic here. There are plenty of people out there doing experiments and getting results which contradict the most basic laws of physics. That does not mean their experiments are correct.
Quote:
Originally posted by Drum God
In warmer regions, does the same rule about heat-loss (or gain) apply when air-conditioning the house? Is it better to warm the house during the day, then bring it back to cool after work?
Yes, the same rule applies. The warmer the house at any given moment, the more you save. There is no "it costs *more* to cool it later. It also applies to water heaters and any other similar instance.
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Old 01-10-2003, 01:26 PM
pipper pipper is offline
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Optimal settings aside, I will say that I just purchased a new Honeywell programmable thermostat that I think is excellent. It has a modifiable feedback loop that gives it a type of "learning ability". Over the course of a few days it recognizes how quickly your house increases/decreases temperature and adjusts itself to turn the furnace on/off at the proper time. So you no longer have guess that turning the thermostat back up at 5:30 should result in it being warm by 6:00- the unit learns this on its own. It constantly monitors the rate of temperature change which allows it to gradually accomodate for decreasing/increasing outdoor temperatures. Probably not as big a deal for forced air systems that make a house feel warmer as soon as they kick in, but it certainly has been a lot nicer for our slower radiator system (it also seems to regulate a constant temperature better and doesn't overshoot the setting like the old themostat).
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Old 01-10-2003, 01:37 PM
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What if you used a heat pump instead of gas? Would it make sense to keep the house warmer in the daytime (when it is cheaper to pump heat into the house because the ambient temperature is relatively high) and count on retention of that daytime cheap heat to keep you cozy all night or would it be better to use the same strategy as you would for a gas furnace where you would do most of your heating at night.

(Actually, the real question I want the answer to is what strategy to use in the summer with air conditioning.)
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Old 01-10-2003, 01:42 PM
pipper pipper is offline
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Actually you don't want to use a set-back strategy with a heat pump. Most heat pumps have an inefficient secondary heating element (usually electrical) that helps boost the temps quickly. If you set the themostat down, when it comes time to reheat the house you'll be doing it with the secondary element which ends up being more costly than having let the heat pump maintain a constant temp.
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Old 01-10-2003, 01:50 PM
sailor sailor is offline
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Anthracite, I believe you are making it more complex than it needs to be. A house loses heat directly in proportion to the temperature difference with the outside and the conductivity. It is a simple differential equation similar to the discharge of a capacitor across a resistor. (I am not going to attempt posting formulas with sub and superindices here).

The furnace would be represented by a current source so, the mathematical model is extremely simple and there's no need to specify specific values of temperatures or insulation. It is easy to design an electronic circuit which has the same behavior and then you can just observe it.

An extra complication is when you have hydronic heating as this inserts an extra delay in the system which really complicates it. That was my case. When you set the thermostat up the furnace starts, The water in the radiators gets very hot before the room temperature gets up to the set point. Then, even though the furnace shuts off, the water continues to transmit heat out. The mathematical model is a bit more complex but not much. These are simple problems of feedback and control systems.
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Old 01-10-2003, 01:54 PM
Una Persson Una Persson is offline
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OK, I made some simplifying assumptions. I hope they are not wrong.

Assumption 2: House interior allowed to cool to 50 F until 4:00pm. Then is increased to 70F over one hour. Assume (nearly) constant drop of interior temps over the day.

OK...let's assume that 1/10 of the volume of the house is wood, and 9/10 air.

Total house volume is therefore 30*40*12 = 14,400 ft3

Wood, at 36 lbm/ft3, makes up 1/30 of that, or 480 ft3. This equates to 17,280 lbm wood.

Air, at 0.063 lbm/ft3, makes up 29/30 of that, or 13,920 ft3. This equates to 878 lbm air.

At specific heat capacities of 0.4 Btu/lbm*R for wood and 0.24 Btu/lbm*R for air, we have a house heat capacity of:

17,280 lbm * 0.4 Btu/lbm*R + 878 lbm * 0.24 Btu/lbm*R = 7123 Btu/R.

Map: Exterior temperatures / Interior Temperatures

Since I'm not going to do the differential equation, I'll guesstimate. I'll assume that the temperature drop of the house is assessed at the end of each hour.

7-8am: Energy = 2880 ft2*(70-21.25)/19 = 7389.5 Btu

New house temperature: 70 - (7389.5 / 7123) = 68.96 F

8-9am: Energy = 2880*(68.96-23.75)/19 = 6853 Btu

New house temperature: 68.96 - (6853 / 7123) = 68 F

9-10am: Energy = 2880*(68-26.25)/19 = 6328 Btu

New house temperature: 68 - (6328 / 7123) = 67.1 F

10-11am: Energy = 2880*(67.1-28.75)/19 = 5813 Btu

New house temperature: 67.1 - (5813 / 7123) = 66.3 F

11-12pm: Energy = 2880*(66.3-31.25)/19 = 5310 Btu

New house temperature: 66.3 - (5310 / 7123) = 65.6 F

12-1pm: Energy = 2880*(65.6-33.75)/19 = 4821 Btu

New house temperature: 65.6 - (4821 / 7123) = 64.9 F

1-2pm: Energy = 2880*(64.9-36.25)/19 = 4346 Btu

New house temperature: 64.9 - (4346 / 7123) = 64.3 F

2-3pm: Energy = 2880*(64.3-38.75)/19 = 3871 Btu

New house temperature: 64.3 - (3871 / 7123) = 63.75 F

3-4pm: Energy = 2880*(63.75-37.5)/19 = 3979 Btu

New house temperature: 63.75 - (3979 / 7123) = 63.2 F

Now: It's 4:00pm. We need to heat the house to 70 F. This means we need to regain the heat we lost, and account for continued heat losses over the hour. First - the heat losses over the hour of heating:

4-5pm: Energy = 2880*(((70+63.2)/2) - 32.5)/19 = 5168 Btu

Now, add to that the heat required to raise the house from 63.2 F to 70 F:

(70-63.2)*7123 = 48,436 Btu

Adding the two gives us:

48,436 + 5168 = 53,604 Btu

Which is less heat required than the previous example.

Please check for math and logic errors. I'm in a hurry, and may have screwed up.
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  #34  
Old 01-10-2003, 02:07 PM
sailor sailor is offline
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Quote:
Originally posted by pipper
Actually you don't want to use a set-back strategy with a heat pump. Most heat pumps have an inefficient secondary heating element (usually electrical) that helps boost the temps quickly. If you set the themostat down, when it comes time to reheat the house you'll be doing it with the secondary element which ends up being more costly than having let the heat pump maintain a constant temp.
I would like to see some confirmation of this. AFAIK the the purpose of the resistor heating element is not to gain any heating speed (which the system does not know if you want or need anyway) but to heat when the temperature outside is so low that the pump cannot function efficiently. In anycase, if enough time elapses, it would still be worth while. If you give me the specs of any heat pump system which does this, I can do the calculations, but I have never seen this.
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Old 01-10-2003, 02:28 PM
pipper pipper is offline
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Here you go: Full link


Quote:
The explanation is simple: When you bump your thermostat up again at the beginning of the workday, your system will call on its back-up heat to get the temperature up quickly. Because back-up heat doesn't use the heat pump's more efficient technology, the extra cost incurred by using it each day is often more than you would spend by leaving your thermostat at the same setting throughout the heating season.

The optimal solution is an automatic, programmable thermostat designed for heat pumps; this limits back-up heat use by raising temperatures in small, incremental boosts. To explore this option, contact your heating and air-conditioning contractor.
I frankly don't know all that much about heat pumps and I've just heard this as a rule of thumb. I have never had the opportunity to deal with a heat pump to heat a building.
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  #36  
Old 01-10-2003, 02:34 PM
toadspittle toadspittle is offline
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For those too lazy to search:

"Where should I set the day thermostat temp for max efficiency?"

http://boards.straightdope.com/sdmb/...ght=thermostat
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  #37  
Old 01-10-2003, 02:57 PM
sailor sailor is offline
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pipper, that seems to be a power company. Since I once had to write my own power company, PEPCO, for some similar mistaken information in one of ther informative flyers, I tend to be suspicious. I would like to see some reliable information on some actual equipment and see when exactly does the heating element come on. Even then a simple switch would enable you to turn it off. As I say, I am not convinced until I see it and even then it would not mean it is a widespread system.
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  #38  
Old 01-10-2003, 03:01 PM
sailor sailor is offline
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I see that site says:
Quote:
The optimal solution is an automatic, programmable thermostat designed for heat pumps; this limits back-up heat use by raising temperatures in small, incremental boosts.
So, even in that case it makes sense to set back the temperature. You just need to account for the heat pump's isiosincracies.
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  #39  
Old 01-10-2003, 03:59 PM
Una Persson Una Persson is offline
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Let me see if I understand this - trying to work out examples of both cases by hand is being "too lazy"?

My numbers may assume a lot and be simplistic, or even have a small error or three, but at least I tried to show step-by-step how one might look at it so it would try to help people out here.
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  #40  
Old 01-10-2003, 04:07 PM
sailor sailor is offline
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Anthracite, I can't see how you interpret toadspittle's post as referring to you. He did a search and posted the link. I hate doing searches because the server is so slow and the search function pretty bad so, I think his post was a positive contribution and I do not think it referred to you in any way.
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  #41  
Old 01-10-2003, 04:25 PM
Crafter_Man Crafter_Man is online now
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Iím not about to whip out the calculator to address this issue, since itís more about common sense (IMO) than anything else.

Can we all agree that, if you leave the house for a few hours, it makes sense to allow the set-point temperature to drop? And that by doing so youíre saving money?

While Iím at it, Iíd like to throw in a few 2nd and 3rd order variables that might come into play:

1. The longer a furnace runs, the hotter its combustion chamber gets. This increases its efficiency. Therefore, the larger the controllerís hysteresis (i.e. bandwidth) is at steady-state, the more efficient it becomes.

2. The R-value of insulation is often a function of temperature & RH, thus the heat loss may be a function of not only the temperature differential, but the absolute temperature.
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  #42  
Old 01-10-2003, 04:27 PM
ZenBeam ZenBeam is offline
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Sailor, this site provides at least some corroboration:
Quote:
Do not set back your heat pump thermostat, unless you have an "intelligent recovery" programmable model. The resistance heat will come on when you turn it back up, resulting in much higher energy consumption. Currently the only models that are "intelligent recovery" are the Honeywell Chronotherm III and Chronotherm IV
The fact that they point out two thermostat models that prevent the problem adds a little believability.

Quote:
I just purchased a new Honeywell programmable thermostat that I think is excellent.
I bought one of those from Honeywell about four years ago. Mine lies to us. If it's within a degree of the set temperature, it changes its readout to claim that's what the temperature actually is. You can see this (if yours does the same thing) by changing the set temperature up or down a degree, and see if the room temperature changes right away. On top of this, it also seems to read a degree or two warmer than other thermometers in the same room.
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  #43  
Old 01-10-2003, 05:03 PM
Crafter_Man Crafter_Man is online now
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ZenBeam Thanks for the info. There may be idiosyncrasies with certain energy sources (e.g. heat pumps) which may require the homeowner to keep the temperature above a threshold in order to obtain high efficiency, but I believe Sailorís comments are still valid in a general sense.
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  #44  
Old 01-10-2003, 05:12 PM
sailor sailor is offline
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ZenBeam, yes, I understand the point but I believe such a system should only be correctly installed with the right type of thermostat and so, the answer is still that you save energy by setting back the temperature setting.

Crafter_Man, your two points are irrelevant for all practical purposes.
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  #45  
Old 01-10-2003, 05:19 PM
Una Persson Una Persson is offline
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Quote:
Originally posted by sailor
Anthracite, I can't see how you interpret toadspittle's post as referring to you. He did a search and posted the link. I hate doing searches because the server is so slow and the search function pretty bad so, I think his post was a positive contribution and I do not think it referred to you in any way.
Well I was hoping I misinterpreted it, which is why I was asking.
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