Turn thermostat way down, slightly down, or keep it constant?


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Old 01-08-11, 06:57 AM
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Turn thermostat way down, slightly down, or keep it constant?

I'm not sure if this is the right place for this question, but hopefully it is. I have hot water baseboard oil heat. I cannot for the life of me figure out whether it's more efficient and money saving to turn the thermostat way down when I'm not home/asleep (~20 degrees below comfort level), slightly down (5-7 degrees below comfort level), or keep the temperature constant. The periods for these setting are approximately 8-10 hours. Over days, I would assume it's better to keep it constantly very low. With my timeframe, my biggest confusion is if I'm just delaying the energy usage and not saving anything. What I mean is that unless the ambient temperature is equal to or greater than the temperature I turn the thermostat down (which is very rarely the case in the winter) the house will eventually reach that temperature. So when I come home and have the heat start back up again it's going to have to use energy to heat the house and make up all that heat I let it lose.

As a concrete example, let's say it's a high of 30 degrees one day. I heat the house up to 67 in the morning. I then leave for approximately 10 hours. Which is the best way to save money and end upw with a warm house when I get home: 1.) Turn it down to 47 then have the heat kick back on approximately 2 hours before I get home to start warming the house back up.
2.) Turn it down to 60 degrees then have it come on much more frequently when I'm gone to maintain 60 degrees then have it run full blast approximately an hour before I get home.
3.) Leave it at 67 to kick and off throughout the day to maintain 67.

I can see the logic to all three ways, but don't have enough knowledge to really say what's right. As people who no much better than I how the boiler works and uses energy, what makes the most sense?
 
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Old 01-08-11, 08:59 AM
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Originally Posted by DIYnewbie9 View Post
my biggest confusion is if I'm just delaying the energy usage and not saving anything.
It's particularly confusing because there are two separate things going on when you temporarily turn down the thermostat.

First, a percentage of the fuel that is temporarily saved will have to be burned to recover the indoor temp resulting from the colder physical internals and structure of the house. That doesn't constitute any real savings. (Actually, there will be a little savings because your boiler will run more efficiently when it is firing continuously during the recovery period than it does when firing intermittently to maintain a constant temp.)

The second thing going on is that while the indoor temp is lowered, there will be less heat lost to the outside. This is a real savings in fuel - every Btu that escapes from the house has to be made up by your heating system. You can compute this factor by performing a heat-loss calculation for different indoor temps.

Now, would I jack around with the thermostat like you propose? No. Too much trouble and potential loss of comfort. Saving fuel is great, but I spend nearly as much per year on snow plowing, and more on lawn care, than I do for space heating. And, I think major and frequent changes in indoor temp might take it's toll on wood funiture and on the piano going out of tune.

Of course, if we were going to be away for an extended time, for example a week, I would certainly turn down the thermostat in our absence.
 

Last edited by Mike Speed 30; 01-08-11 at 09:54 AM.
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Old 01-08-11, 10:08 AM
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To paraphrase what Mike said, your setback period needs to be long enough to offset the energy used during recovery.

To put real numbers on that time period is nearly impossible because all settings differ. There are many 'it depends' going on... the actual heat loss of your home, the MASS that needs to be reheated, the amount of heat emitter in the home, the size and efficiency of the boiler, etc etc ...

A few years back I did some experimentation and data collection in an effort to prove/disprove that setback actually saved anything in my home. I tracked degree days and oil usage... collected data into a spread sheet... and if there was any real savings to be had, it was very small, so much that I couldn't really 'see' it in the data... it was 'in the noise'. This is not to say that there weren't savings... just that it couldn't be seen in the data that I took. This could be because the 'resolution' of the data was not fine enough, or other reasons. i.e. maybe the data was noisy enough that I couldn't see a 1 or 2 percent saving. Perhaps if I could average over a period of years... but I decided that my time and comfort was worth more than say $80-$100 per heating season...

So, now I do allow the thermostat to setback 2 degrees during my 'away' and 'sleep' time simply because it doesn't 'hurt' to do so, and sleeping is more comfortable that way.
 
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Old 01-08-11, 11:49 AM
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It also depends on your system. If your system offers a quick recovery, then it is more doable. With my new system, I gave up my setback. It just didn't recover fast enough. I also found that the house was far more comfortable than when it had to recover because all the furniture, the walls, and floor were warmer. I still saved 14% in energy with the new system and constant temp. If you do set back, 20 degrees is too much unless you are going away. I think the recommended amount is something like 10 degrees if you are away for at least 4 hours. I don't recall the exact.
 
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Old 01-08-11, 01:19 PM
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something like 10 degrees if you are away for at least 4 hours
No... that's a crazy lot of setback for 4 hours.
 
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Old 01-08-11, 01:31 PM
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Old 01-08-11, 04:40 PM
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I've tried three programmable thermostats. Hate 'em. I can do a good job just by manually adjusting the thermostat - without the hassle, resets due to power outages, daylight-savings time, batteries, trying to figure out again how to reprogram them, and my wife who wound up hating me too. I'm a recovering energy-saving nut, but I'm pretty much through - DIY demand controllers on my electric air-conditioning unit, computer HVAC control out the wazoo, and everybody trying to advocate or sell the next best boiler, lightbulb, toaster, you name it.

My gas bill for space heating runs about $1000/year, and my electric bill is less than an average of $60/mo - both of which I can afford. My water/sewer bill often is higher than my electric bill. I think all this energy-saving has gotten a bit out of hand, accompanied by the rise of the internet, Al Gore, et. al.

If I could buy a new, super-efficient boiler, for $10,000, that consumed zero fuel, it would take ten years for it to pay back.

Don't get me started. Whoops, I already did!
 
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Old 01-08-11, 06:07 PM
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Originally Posted by Mike Speed 30 View Post
Don't get me started. Whoops, I already did!
Here Here!

As individual consumers I do not think we will save anything. The corporations that sell us these point of views continue to make money. As usage goes down there need to buy more from the market is lessen, so profits go up. I'm not sure about any of you guys/gals but my energy bills keep going up, they never seem to go down. (I'm talking about the rates per unit not the actual total bill)
 
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Old 01-08-11, 06:30 PM
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Forced hot water systems do not benefit from setbacks as much as forced hot air. That energystar advice is really targeted to the 90+% of the country that uses forced hot air heating. Forced air heats up quick (and cools quick!) so setback with recovery to blast around hot air in time to get home can save energy.

Less so with forced hot water. It's a slower-reacting (but generally more comfortable) heat.

But in either case more than ~5-8F setback is not going to save anything.

With some good boiler controls (e.g., outdoor reset), it is possible to save on energy, be very comfortable, and not have any setback at all, as someone mentioned above.
 
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Old 01-08-11, 06:40 PM
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Originally Posted by xiphias View Post
But in either case more than ~5-8F setback is not going to save anything
Incorrect. It will save a lot of fuel, but at the sacrifice of comfort.
 
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Old 01-08-11, 06:52 PM
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Originally Posted by grumpy_guy View Post
Here Here!

... I'm not sure about any of you guys/gals but my energy bills keep going up, they never seem to go down. (I'm talking about the rates per unit not the actual total bill)
Hey grumpy, you guys up there near the north pole need to have your heat on in August! That's why your bills are high!(ha-ha, just kidding) Good point about checking rates per unit. That's something you can easily overlook. Think I'll start keeping better track.
 

Last edited by zoesdad; 01-08-11 at 06:56 PM. Reason: stuttering
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Old 01-08-11, 07:05 PM
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Originally Posted by Mike Speed 30 View Post
Incorrect. It will save a lot of fuel, but at the sacrifice of comfort.
Sorry, was unclear. Should have said 20F setback not going to save any more than 5-8F.

But the original statement is not incorrect. The correct answer, however, does depend on a number of different variables (characteristics of building, specifications/performance of heat source and distribution system, daily weather, etc. etc. etc.) so it is something of an over-generalization.
 
 

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