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Old 02-19-2011, 04:06 PM   #1
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Lowering The Thermostat


Will I be able to save heating dollars if the thermostat set temperature is 69 degrees F but is programed four days a week at 60 degrees for nine hours per day while away at work?
A friend has stated that to bring the house temperature back to 69 degrees will offset a lot of the savings.
This is contrary to what I have seen in advertizements.

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Old 02-19-2011, 05:09 PM   #2
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Lowering The Thermostat


You will save money for sure. Why would programmable thermostats be so popular if they didn't work?

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Old 02-19-2011, 05:59 PM   #3
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Lowering The Thermostat


Your friend is right, when the furnace has to bring the house up 9 degrees on an almost daily basis your savings go down the toilet. I would kick it down to 64 or 65 from 69 if it were me
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Old 02-19-2011, 06:43 PM   #4
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Originally Posted by rjordan393 View Post
Will I be able to save heating dollars if the thermostat set temperature is 69 degrees F but is programed four days a week at 60 degrees for nine hours per day while away at work?
A friend has stated that to bring the house temperature back to 69 degrees will offset a lot of the savings.
This is contrary to what I have seen in advertizements.
If you have a heat pump with electric aux heat, you won't save anything when the aux heat comes on to recover.

If you have a gas, oil, or electric strip heat only. Then yes, you will save a lot. But be prepared for it to take some time to recover, when them house has actually drop 9 degrees.

I turn my heat off when I'm gone for the day(which can sometimes be 14 to 18 hours) and I save a tremendous amount on my heating bill.
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Old 02-19-2011, 07:12 PM   #5
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Lowering The Thermostat


My stat is set at 61 during the day and 65 at night.

The temp rarely drops down to 61 during the day though. It's typically around 63 in the house when I get home, unless it's overcast and very cold all day.

My home is around 800 square feet and my gas bill for heat and cooking fuel last year was under 500 dollars for the year.
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Old 02-19-2011, 07:55 PM   #6
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Thanks,
The reason I asked is that on January 17, my oil company delivered 116 gallons and today ( Feb. 19th) he delivered 150 gallons.
It was three weeks ago, that I installed a new thermostat. I was wondering about the high consumption during this period.
But this period also had the lowest temperatures of the season so far. So it appears that weather played a big part in the increased consumption.
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Old 02-19-2011, 08:48 PM   #7
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Originally Posted by beenthere View Post
If you have a heat pump with electric aux heat, you won't save anything when the aux heat comes on to recover.

If you have a gas, oil, or electric strip heat only. Then yes, you will save a lot. But be prepared for it to take some time to recover, when them house has actually drop 9 degrees.

I turn my heat off when I'm gone for the day(which can sometimes be 14 to 18 hours) and I save a tremendous amount on my heating bill.
So a 10 degree drop would save more energy with a gas or electric furnace than a 5 degree drop?
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Old 02-19-2011, 10:06 PM   #8
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Lowering The Thermostat


It is myth as far as I know that keeping your heat on during the day can save energy by not letting the temp drop too much during that time. For it to be true you'd have to argue that the insulation in your home or your furnace were more efficient around 70 degrees vs around 50 degrees somehow. It was pointed out by someone that homes with heat pumps can maintain temp extremely efficiently if the temp change needed is limited to small changes of a degree - so that scenario seems the only reasonable exception of people who might want to leave the heat(pump) on all day. I'm not a pro, just $.02 - I turn mine down to 60 daytime and like someone else mentioned often it never gets down that far while I'm gone.
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Old 02-20-2011, 06:29 AM   #9
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Nobody has mentioned the amount of heating cycles that occur when temperatures are maintained at 69 degrees when outdoor temperatures during the day are about 28 to 32 degrees vs turning the thermostat down to 60. Seems to me that should be considered. My oil furnace takes about 1 hour to 1 hour and 15 minutes to recover and its 21 years old.
So correct me if I am wrong; are most furnaces designed to cycle no more then 3 times an hour? I know my furnace cycles at least 2 times an hour ( in cold weather) and it might be as high as 2-1/2 per hour. I'll have to check to be sure.
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Old 02-20-2011, 06:39 AM   #10
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Originally Posted by unicursalhex View Post
So a 10 degree drop would save more energy with a gas or electric furnace than a 5 degree drop?
Yes, as the house temp drops, it loses less heat per hour.

A house at 60 degrees, won't require the heat to run as often or as long to maintain 60, as it will to maintain 65.

Take a house that looses 65,000 BTUs an hour when its zero outside and 70 inside. At 65 inside its only loosing 60,000 BTUs an hour, at 55 inside its only loosing 55,000 BTUs an hour.

4 hours at 70 is a total of 260,000 BTUs
4 hours at 65 is a total of 240,000 BTUs
4 hours at 60 is a total of 220,000 BTUs

Each 5 degree set back saves 20,000 BTUs of heat loss in 4 hours. On a 80% efficient gas furnace that comes out to .25 therms of gas not used(keep in kind I'm using an outside temp of zero degrees). In 20 days time(4 weeks of working 5 days a week) that comes out to 5 therms of gas not used for every 5 degrees of set back(Its actually more, because heat loss is not linear when you include stack effect). And when you use temp set back during the day. You also have solar gain helping to heat the house(provided its not raining, sleeting, snowing, or over cast) and reducing how often the furnace runs, and run time when the furnace does run.

The amount of time for recovery will vary with weather recovery is done while the sun is still out or not, and of course the size of the furnace.

Since the house is loosing 10,000 BTUs less at 60 then it does at 70. When it comes out of set back, the furnace will run until that 10,000 BTUs of heat is put back into the house. Since the savings of fuel used between maintain 70 to 60 is .5 therm in four hours, and it will take .125 therm to recover that 10 degrees, the total savings for gas for 4 hours in 1 day is .375 therms. In 20 days it 7.5 therms. This is just for a 4 hour period. An 8 hour period would of course be doubled.

Again, none of this includes stack effect savings. So in real world, the savings are greater then I posted. Since the greater the difference is between inside and outside temp, the more infiltration the house will have.

If at 70 degrees inside and 0 outside, the house has an infiltration rate of .7 an hour. And its 2000 sq ft with 8 foot high walls. 14,112 BTUs of its heat loss is from infiltration.

But, if at an inside temp of 60 with an outside temp of 0 its infiltration reduces to .6 an hour, the infiltration heat loss drops to 12,096. Or, reduces the homes hourly heat loss by an additional 2,016 BTUs an hour.

At 65 indoor temp, you could expect the infiltration rate to be .65 an hour, which would mean that its infiltration rate would be 1,008 BTUs an hour more then at 60 degrees indoor temp.

If an 80% furnace is in the basement and uses basement air for combustion, the fewer and shorter run times of it also reduces the amount of cold air being drawn into the basement, and lowers hourly heat loss.

Temp set back saves a small amount of fuel a day, but adds up over a years/heating season's time. If your heating bill to maintain 70 indoor temp is 600 bucks, you might only save 60 bucks for the year. On the other hand, if your heating bill for the year is 1900 bucks, a 190 dollar savings for the year makes temp set back more then worth while.

My turning the heat off, and being willing to wait hours for my place to warm back up saves me over a thousand bucks a year. But my place has been as cold as 39 degrees when I got home, and took 6 hours to recover. I didn't care, i took my shower and went to bed and covered up.

For the record, I heat with oil.
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Old 02-20-2011, 07:18 AM   #11
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Lowering The Thermostat


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Originally Posted by rjordan393 View Post
Nobody has mentioned the amount of heating cycles that occur when temperatures are maintained at 69 degrees when outdoor temperatures during the day are about 28 to 32 degrees vs turning the thermostat down to 60. Seems to me that should be considered. My oil furnace takes about 1 hour to 1 hour and 15 minutes to recover and its 21 years old.
So correct me if I am wrong; are most furnaces designed to cycle no more then 3 times an hour? I know my furnace cycles at least 2 times an hour ( in cold weather) and it might be as high as 2-1/2 per hour. I'll have to check to be sure.
Furnaces aren't designed with any set cycle per hour consideration.
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Old 02-20-2011, 11:14 PM   #12
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Originally Posted by beenthere View Post
Yes, as the house temp drops, it loses less heat per hour.

A house at 60 degrees, won't require the heat to run as often or as long to maintain 60, as it will to maintain 65.

Take a house that looses 65,000 BTUs an hour when its zero outside and 70 inside. At 65 inside its only loosing 60,000 BTUs an hour, at 55 inside its only loosing 55,000 BTUs an hour.

4 hours at 70 is a total of 260,000 BTUs
4 hours at 65 is a total of 240,000 BTUs
4 hours at 60 is a total of 220,000 BTUs

Each 5 degree set back saves 20,000 BTUs of heat loss in 4 hours. On a 80% efficient gas furnace that comes out to .25 therms of gas not used(keep in kind I'm using an outside temp of zero degrees). In 20 days time(4 weeks of working 5 days a week) that comes out to 5 therms of gas not used for every 5 degrees of set back(Its actually more, because heat loss is not linear when you include stack effect). And when you use temp set back during the day. You also have solar gain helping to heat the house(provided its not raining, sleeting, snowing, or over cast) and reducing how often the furnace runs, and run time when the furnace does run.

The amount of time for recovery will vary with weather recovery is done while the sun is still out or not, and of course the size of the furnace.

Since the house is loosing 10,000 BTUs less at 60 then it does at 70. When it comes out of set back, the furnace will run until that 10,000 BTUs of heat is put back into the house. Since the savings of fuel used between maintain 70 to 60 is .5 therm in four hours, and it will take .125 therm to recover that 10 degrees, the total savings for gas for 4 hours in 1 day is .375 therms. In 20 days it 7.5 therms. This is just for a 4 hour period. An 8 hour period would of course be doubled.

Again, none of this includes stack effect savings. So in real world, the savings are greater then I posted. Since the greater the difference is between inside and outside temp, the more infiltration the house will have.

If at 70 degrees inside and 0 outside, the house has an infiltration rate of .7 an hour. And its 2000 sq ft with 8 foot high walls. 14,112 BTUs of its heat loss is from infiltration.

But, if at an inside temp of 60 with an outside temp of 0 its infiltration reduces to .6 an hour, the infiltration heat loss drops to 12,096. Or, reduces the homes hourly heat loss by an additional 2,016 BTUs an hour.

At 65 indoor temp, you could expect the infiltration rate to be .65 an hour, which would mean that its infiltration rate would be 1,008 BTUs an hour more then at 60 degrees indoor temp.

If an 80% furnace is in the basement and uses basement air for combustion, the fewer and shorter run times of it also reduces the amount of cold air being drawn into the basement, and lowers hourly heat loss.

Temp set back saves a small amount of fuel a day, but adds up over a years/heating season's time. If your heating bill to maintain 70 indoor temp is 600 bucks, you might only save 60 bucks for the year. On the other hand, if your heating bill for the year is 1900 bucks, a 190 dollar savings for the year makes temp set back more then worth while.

My turning the heat off, and being willing to wait hours for my place to warm back up saves me over a thousand bucks a year. But my place has been as cold as 39 degrees when I got home, and took 6 hours to recover. I didn't care, i took my shower and went to bed and covered up.

For the record, I heat with oil.
Enlightening. I was always under the impression that it takes more energy for the furnace to bring the house up 10 degrees then it does to keep it a bit warmer. I guess it really depends of how tight and well insulated the house is. I know with the manual J sized furnaces I install they have much longer run times and take longer to recoup than the oversized junk that the previous contractor installed. Im sure with oil it doesn't take that long to bring the house back up with such high discharge temps.
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Old 02-20-2011, 11:38 PM   #13
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Originally Posted by unicursalhex View Post
Enlightening. I was always under the impression that it takes more energy for the furnace to bring the house up 10 degrees then it does to keep it a bit warmer. I guess it really depends of how tight and well insulated the house is. I know with the manual J sized furnaces I install they have much longer run times and take longer to recoup than the oversized junk that the previous contractor installed. Im sure with oil it doesn't take that long to bring the house back up with such high discharge temps.
We heat with an oil fired boiler steam single-pipe radiators controlled by a programable thermostat. On a normal winter day (temps in the 40s no wind) our heat only comes on just a few times a day.
- It takes around 45 minutes for the heat to recover from overnight temps (63) to (68) for a warm morning.

- We set it at 65 during the day to keep it comfortable for me working around the house. It might fall to this by 2 or 3 pm.

- At 5PM the heat comes on for 25 minutes to raise temps up to 69 until 10PM

When the wind is blowing the heat cycles more often in our drafty brick house. And when the temps fall well below freezing the heat might come on overnight to maintain 63.

Works well for us and saves us a good deal of oil. It hasn't been a bad heating February so far. We've had two days this month that it was warmer outside than inside (time to air out the house).
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Old 02-21-2011, 04:23 AM   #14
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Originally Posted by unicursalhex View Post
Enlightening. I was always under the impression that it takes more energy for the furnace to bring the house up 10 degrees then it does to keep it a bit warmer. I guess it really depends of how tight and well insulated the house is. I know with the manual J sized furnaces I install they have much longer run times and take longer to recoup than the oversized junk that the previous contractor installed. Im sure with oil it doesn't take that long to bring the house back up with such high discharge temps.
Oil has a high discharge temp because most of them are either over sized for the house and duct system, or the duct system is under sized for the furnace. Temp rise is a function of CFM per BTU.

I run my oil furnace at a temp rise of 55 degrees. So my register temps are never over 125 when the house is at normal/occupied indoor temp. Which is what many gas furnaces run.

Many people think it takes more to recover then what you save. Its a common false hood that will be around for decades to come.

Tight well insulated houses(actually tight) will see very little savings.

A properly sized furnace will have long run times, but still won't use more fuel then what was saved. Simply because the house can't lose more heat at a lower indoor temp then what it does at a higher indoor temp.

Many many years ago, I thought it used more to recover also. Then I did some testing of my own. Since I found it hard to believe that so many people could be claiming to save money by using set backs. If it indeed used more fuel to recover. Found out they were right and I was wrong. And so were the people that taught me it used more fuel.
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Old 02-21-2011, 11:04 PM   #15
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Oil has a high discharge temp because most of them are either over sized for the house and duct system, or the duct system is under sized for the furnace. Temp rise is a function of CFM per BTU.

I run my oil furnace at a temp rise of 55 degrees. So my register temps are never over 125 when the house is at normal/occupied indoor temp. Which is what many gas furnaces run.

Many people think it takes more to recover then what you save. Its a common false hood that will be around for decades to come.

Tight well insulated houses(actually tight) will see very little savings.

A properly sized furnace will have long run times, but still won't use more fuel then what was saved. Simply because the house can't lose more heat at a lower indoor temp then what it does at a higher indoor temp.

Many many years ago, I thought it used more to recover also. Then I did some testing of my own. Since I found it hard to believe that so many people could be claiming to save money by using set backs. If it indeed used more fuel to recover. Found out they were right and I was wrong. And so were the people that taught me it used more fuel.
Nice, I love learning new sh!t! I bet its nominal and most people will choose comfort over a crispy hundred dollar bill at the end of the year but its good to know these things, thanks for the info

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