Green / Eco-Friendly

How much do you really save by keeping your house at a lower temp?

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  • Jan 16th, 2017 8:53 pm
[OP]
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Dec 23, 2015
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Newmarket, ON

How much do you really save by keeping your house at a lower temp?

I have a smart thermostat which tells me how many hours the heat was on each day. On a typical winter day it'll run for 5 hours, although it can be higher on very cold days or lower on warm ones. Since the start of the winter, I have been experimenting by keeping my house at all kinds of different temps to see what the best bang for the buck is and to my surprise I found little to no correlation with the temperature and how long it runs.

For example, when I set it at only 60, it still ran just as often (after the initial cooling down period) and for just as many hours as it did when I set it to 70. Last weekend I spent 3 days out of town so on the friday I left, I set my thermostat down to 55. For the rest of the day the heat didn't turn on at all so it only heated for 3 hrs that day, but once the temp fell down to 55, it would turn on just as often as it did normally, it still logged 5 hrs of use that saturday and sunday! When I got home on monday and turned it back up to 70, it ran more than 6 hours for that day.

If I turn the heat down to 55 over night, it won't run for a long time, but any cost savings from that period of non-use are negated by the fact that it'll have to run longer in the morning to warm the house again, than if I had kept it at 65 overnight. So if your day temp is 70, there is no difference in cost in keeping your night temp at 55 vs 65.

My overall conclusion is there are no costs savings to keeping your house at a lower temperature, so just keep it at whatever temp you find the most comfortable. Thoughts?
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[OP]
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Dec 23, 2015
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Then why is that article completley at odds with my own findings? Why does my heating run for 5 hours on a typical day regardless if I set it at 70 or 60?
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Aug 10, 2015
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BeapChastard wrote:
Jan 16th, 2017 11:40 am
I have a smart thermostat which tells me how many hours the heat was on each day. On a typical winter day it'll run for 5 hours, although it can be higher on very cold days or lower on warm ones. Since the start of the winter, I have been experimenting by keeping my house at all kinds of different temps to see what the best bang for the buck is and to my surprise I found little to no correlation with the temperature and how long it runs.

For example, when I set it at only 60, it still ran just as often (after the initial cooling down period) and for just as many hours as it did when I set it to 70. Last weekend I spent 3 days out of town so on the friday I left, I set my thermostat down to 55. For the rest of the day the heat didn't turn on at all so it only heated for 3 hrs that day, but once the temp fell down to 55, it would turn on just as often as it did normally, it still logged 5 hrs of use that saturday and sunday! When I got home on monday and turned it back up to 70, it ran more than 6 hours for that day.

If I turn the heat down to 55 over night, it won't run for a long time, but any cost savings from that period of non-use are negated by the fact that it'll have to run longer in the morning to warm the house again, than if I had kept it at 65 overnight. So if your day temp is 70, there is no difference in cost in keeping your night temp at 55 vs 65.

My overall conclusion is there are no costs savings to keeping your house at a lower temperature, so just keep it at whatever temp you find the most comfortable. Thoughts?
I rarely touch my thermostat. When I do, it's usually just a half degree adjustment, so suit if I am working around the house, or just lounging.
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Feb 11, 2007
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BeapChastard wrote:
Jan 16th, 2017 12:29 pm
Then why is that article completley at odds with my own findings? Why does my heating run for 5 hours on a typical day regardless if I set it at 70 or 60?
Because your study has no controls, and lacks stats. Also the percentage differences for just a few degrees is going to be single digits, which is near impossible to see without a controlled environment.

You'd need to chart inside and outside temps, furnace run time (including stages if multistage gas valve and fan speed).

The big reason you save is because of thermal transfer rates. If the difference between inside and outside is 30'C, then the rate of heat loss will be significantly higher than if the difference is only 15'C.
Think of it like putting an ice cube in 10'C tap water, and another in 100'C boiling water. Which will melt first? Obviously the boiling water.

So when your house is kept cooler, it will lose less heat during that time than if it was warmer. The heat used to warm your house back up to temp in the morning is less than what you saved by keeping it cooler.

You can see the rate of heat loss during the day was about 0.5'C per hour, while heating was about 2.5'C per hour in the afternoon. The rate of loss is dependent on how well insulated your home is, as well as how much heat is added from your lights and other heat sources.
Image

If I set the furnace to zero, the inside temps would look more like this asymptotic curve, since less and less heat would be lost as it cooled.
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[OP]
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Dec 23, 2015
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Newmarket, ON
Because your study has no controls, and lacks stats.
I'm specifically charting the number of hours the furnace runs each day. The Nest app shows this in its history report down to the closest 15 minutes so it's not 100% accurate but it's accurate enough to give me a general picture.

Outside temp obviously plays a big role in how often the furnace has to work, so I have changed my temp on days when the temp is roughly teh same, eg day 1 outside temp is -10 degrees C and I keep my thermostat at 70 degrees F, and day 2 the outside temp is teh same as the previous day but I keep my thermostat at 60, and observe no difference in how often it runs once it reaches its set temp.
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BeapChastard wrote:
Jan 16th, 2017 6:58 pm
I'm specifically charting the number of hours the furnace runs each day. The Nest app shows this in its history report down to the closest 15 minutes so it's not 100% accurate but it's accurate enough to give me a general picture.

Outside temp obviously plays a big role in how often the furnace has to work, so I have changed my temp on days when the temp is roughly teh same, eg day 1 outside temp is -10 degrees C and I keep my thermostat at 70 degrees F, and day 2 the outside temp is teh same as the previous day but I keep my thermostat at 60, and observe no difference in how often it runs once it reaches its set temp.
lol, why you measure outside temp in 'C and inside in 'F ? Are you from the USA? :) Your t-stat should have an option to switch temp to 'C.

+/- 15 minutes each cycle is bigger than any gain you could hope to achieve by lowering a few degrees.

Also, what kind of furnace do you have? Do you have a single stage gas burner? Is your fan multi-speed? Do you have a clean cycle/comfort program that could throw off your results?
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