There's no magic number for a setback. You just need to see how low it
gets in your house during the day without heat coming on. Set it to
that temp and furnace wont come on for XX hours. No furnace burning
fuel= saved money.

Craig wrote:
wrote...

I'm trying to save money on heating bills (i have a gas furnace), and
so i keep it pretty cold when i'm not home. I usually keep it around
60 degrees when i'm not there. But in the morning when i wake up, i
programmed the thermostat to 70 or so for a couple of hours.

Someone told me that that actually would cost more in the end, b/c the
furnace has to work harder to get the temp up from 60 to 70 everyday,
than it would to just keep it at a constant 65. But, like i said, its
a gas furnace and it heats up the house relatively quickly. Plus, my
home is older (built in 1940) and is not very efficient. So, I'm
afraid the heat would be running much more often to keep the house at
65-67 degrees throughout the day.

Any advice?

I think you're doing a good job now. Based on info from my gas company, if
true, a set-back of any amount will save money as long as the time at a
lower temp is a minimum of 8 hours. And the larger the set-back differential
and the longer the set-back period (above 8 hr.), the better for savings.
The gas company does suggest that cutting back temps for periods of under 8
hours may not result in a saving.

The folks that should worry about issues that you bring up are the heat
pump owners whose heat pump includes additional electrical resistance
heating for a quick early heat-up period.

Craig