There are many variables which would make a 'one size fits all' formula impossible.

Bear in mind, that when you set back the temperature, the furnace is not just heating the air in your house, but all of the thermal mass. This includes the walls, floors, furniture, etc.

As the air is being heated, the process is being hindered by the TM exchanging heat with the room air, until finally, an equilibrium is reached. So, in general, the larger the house, the greater the thermal mass, and the longer time required to recover.

But, this is not necessarily bad. If you do have large TM, your furnace would be pouring out many BTUs during the proposed recovery period.

In order to take advantage of setback, a period of 6 - 8 hours should be a minimum, and probably not more than about 6 degrees. If you're going out for an hour or two, don't bother, or you can choose a smaller setback.

If you want, you can compile data to determine whether to, and how much to set back. Perhaps your system has some means to do this (such as Infinity).

V

Yes heating the thermal mass of the home...walls, counters, furniture....is something to take into account.

I'm going to hook up an 24V hour meter to the gas valve:

http://www.grainger.com/Grainger/REDINGTON-Hour-Meter-2PPV8?Pid=search

and see how long the gas valve is open during the night with the furnace turned down to 62 and add that to the recovery time in the morning to bring it back up to 70. I'll note the time the home hits 70 in the morning. Then on a similar night keep the thermostat at 70 all night and see how long the gas valve is open

As long as you are considering thermal mass, consider this:

When your furnace shuts down for the night (setback), the thermal mass is at (about) 70 degrees. In the morning, when the stat indicates 70 degrees. Your thermal mass is lagging in temperature.

I believe that in order to make an accurate assessment, you need to include the gas usage required to bring the mass back to 70 degrees.

So, if I were running the test, I would note gas usage until the furnace first turns off in the morning.

V

How are you going to correct for changes in the outside temp when you measure gas usage?

I installed a set back thermostat on my mother's oil boiler year before last. Hers is a 1903 brick Victorian, not particularly well insulated, with forced hot water and radiators. Took it from 72 to 60 in the setback.

Setback starts at 10 p.m. and it kicks back on at 7:30 so the house is warm when Mom gets up in the morning.

So far the oil/money savings have been significant, and the comfort levels don't seem to be affected.

The studies have been done. Unless you are driving a heat pump into electric strips, you will always save money by setbacks.

My father is a PhD engineer and did a ton of studies on this subject at Oak Ridge Nat Lab in the 70s under Carter. The conclusion is that setbacks always save energy and money (except in the heat pump scenario). It is basic thermodynamics. You can get into thermal mass issues but that never changes the delta T issue. The lower the delta T, the less heat loss (or gain) across a constant wall.

How are you going to correct for changes in the outside temp when you measure gas usage?

Well I know one night will not be exactly the same as another as far as temp, humidity and wind but here in Ohio, where I live, we have nights that are similar enough for government work. I'll post my findings here when they come in.

I've had my hour meter hooked up for almost a week now and am recording temps, thermostat settings and meter reads a few times a day. I'll post results in a few days.

Aslo the hour meter helps for filter changes.