Q&A: DO YOU SAVE ENERGY BY LOWERING YOUR THERMOSTAT WHEN YOU ARE NOT HOME AND THEN RAISING IT WHEN YOU GET BACK?

This is an answer that I left for another very similar question concerning set back of thermostats. It will definitely save money to set it back since heat loss is proportional to the temp difference between inside and outside……see below

There are highly technical answers to this question that are based on the science of heat transfer, but to simplify things, basically that heat loss is proportional to the difference between the outside temperature and the inside temperature. So if the outside temp is 30, and the inside temp is 70, you will have a 40 degree difference.

Likewise, if the outside temp is 30 and the inside temp 60, the difference will be 30 degrees. 30/40=0.75, or you will have only 75% as much heat loss at 60′F than you would have at 70′F.

This “savings” is true for only as long as the time period these conditions exist whether it be 1 hour or 24 hours and it is a real savings dispite the fact that you have to heat the house back up, which then increases the difference in temperature and increases the rate of heat loss. Whether the furnace is on or off does not figure into the heat transfer equation. Only the difference in temperature does.

So, the smart way to run things is to determine when you want your house temp at 72 degrees F, lets say, 7am and turn off the setback the exact time it takes to heat the house to 72 degrees F. If that is 3 hours as suggested, if you turn off the setback at 4am, the house will be nice and cozy at 7am, and you WILL have saved energy. Even during the reheat period you save energy until the temp gets to 72′F, because at 70degrees F the temp difference to the outside will be less than at 72 degrees F. Temp loss does not depend on whether the furnace is on or off, just the diff in temp between the outside and the inside.

There is one other thing that can be done also. Humidity in the air makes the air feel hotter. That is why on a very humid day in the summer you feel hotter than on a “dry” day. It’s why people talk about “dry heat” during the summer vs humid air.

You can use this to your benefit during the winter also. By humidifying the air you can decrease the temp setting and still be comfortable. In doing so, you will decrease that temp difference between outside and inside and therefore slow the heat loss. If you start getting condensation on windows you have gone too far. Another highly efficient form of heat loss comes into play if you start getting condensation.

Condensation heat transfer is much more efficient than the sort of heat transfer that occurs from the air in your house to the air outside the house, so back off the humidity if you are getting condensation on the windows.

I would also like to stress that I am not saying you will save 25% on your bill. For a home, the heat loss formula is constantly changing simply because the outside temperature changes. The savings would also only be 25% if you kept your house at 60′F full time, so if you are only doing this 4 hours of the day the amount saved would be 25% * 4hr/24hr = 4% savings on the bill. Also, as the temp difference gets higher the percentage of savings decreases, although you are still saving the same number of dollars. (If there were a 100′F difference between the inside and outside at 60′F, such as in the Arctic vs 110 ‘F at 70′F you would save only 10% instead of 25%,and if that were for only 4 hours a day it would equal a net savings on the bill of 1.7%.

Other “heat transfer” variables can also come into play, such as wind velocity which increases turbulance and increases heat loss, rain, snow, direct air leakage, opening and closing of doors, etc.