Originally Posted by Kyamo
Are you sure? Do you have any data on that?
This is the way I think about it. Assume these two example houses have the same daytime temperature. If you leave your thermostat at the same temperature all day and all night, at night you are paying to replace exactly the amount of heat that gets lost to the outside overnight. If you turn your heat down at night, then warm up again in the morning, you are still paying to warm it up and replace exactly what was lost overnight, just distributed differently (using a little through the night and a lot in the morning). However, the house that was cool overnight should lose LESS heat overnight than the house that was warm at night. Why? Because the larger the difference in temperature between two things, the more heat is transferred from the hotter object to the cooler one. Another way to think of it is if tiny amounts of air escape your house through cracks, poor insulation, etc, losing x amount of 20 degree air is worse than losing the same amount of 18 degree air.
Where am I wrong? (If I am)
Think of it this way. If your house cools off at night when you turn down the heat, the furnace has to run very hard to bring it back up to a warmer temperature during the day (especially if you live in a place where the daytime temps are not all that much warmer than the nighttime temps). If you keep if at the same temp it runs less, the fan kicks on less too. Read the article I posted. It's ok to set your thermostat a couple degrees cooler, but a significant difference will make your furnace work hard when you crank it up in the morning. Change temp at night will also decrease the life of your furnace and fan motor. By maintaining the same temp day and night, your furnace works less overall. The same principle is true for A/C (although I don’t have that luxury).
Your house will always loose a little heat (especially if it is ancient like mine), it’s the law of thermodynamics. However, your house will not loose more heat just because the temperature differential is greater. If you want to decrease heat loss, then insulate, caulk windows, use draft dodgers, etc...This is why DH and I recent glass blocked the basement windows to reduce the heat loss from the basement. We also replaced some of the 90+ year old upstairs windows.
See this link for more info about home heat loss. http://homerepair.about.com/od/heati.../heat_loss.htm