Digital Programmable Thermostats
 

I got a couple of those digital programmable thermostats for Christmas. The instructions give no indication as to what the largest drop you can have during the day and still heat back up by evening without really messing with the economy of the whole thing.

Right now I have a 4 degree drop during the day while I'm not home. I figure there has to be a point where the furnace is working double duty to catch up to the nice "when I get home temp".

Anyone have any data or anecdotal information?

Steve, it probably depends on size of the house, how many zones, outside temp...etc.

Perhaps there's a formula around somewhere.....

Here, how about this:

Links that may answer your questions...
http://www.luxproducts.com/consumer_info.htm

"Programmable thermostats can save energy and save you money on utility bills. According to the the U.S. Department of Energy, a programmable thermostat can reduce heating costs by up to 35% and cooling costs by up to 25%. Your savings will largely depend on the length of your energy saving program periods and the temperatures set. For example, an energy saving period of 10 hours with the temperature set back 10° will save more than a 5 hour energy saving period with the temperature set back 5°"

Rick,

Good links, thanks. They seem to recommend a bigger swing than I currently am maintaining. It looks like I have an afternoon of reading!

Steve

Steve, IMO it is more important to keep a tighter temp variation in hot humid weather. When the humidy soaks into everything (wood, upholtery etc) it takes quite a while to remove...and that's mostly what AC does.

During the summer months I usually have the AC on 100% of the time just to keep humidity in check.

This also can depend on the type of heat you use. A heat pump can take forever to recover temperature and also will force you to use secondary heat. Oil or gas heat can recover much more quickly due to the higher duct temperature. A heat pump with gas or oil secondary heat changes things as well. The 5 to 10 degrees numbers are reasonable. And as Rick noted A/C and humidity control situations are important as well. Normal summer time settings are high 70s to 80 you certainly wouldnt want to allow the house to creep up to 90 during setback modes.