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To MarkB and 5/0.
First and foremost.NO ONE saves money on energy when its 0 or below.Setback tstat or not.These cold days are called "design days" by those of us in the industry.Every boiler or furnace runs at its max rated output.
The best way to save money on your energy dollar is to invest in an outdoor reset control for your boiler.The boilers output is regulated by outdoor temperature.It is a known fact that for every 3degrees F that you can lower a boilers setpoint and still maintain a 68 degree F indoor temp the occupants of the dwelling will see a 1% AFUE.Shoulder days account for about 40-60% of a boiler total annual run time.Do the math....That come out to about 15 to 40% annual fuel savings.
Properly designed or sized systems like the one I describe don't even require a Tstat.Its just stays steady all heat season long.In Europe these controls are LAW! They must be on all heating systems.I have this kind of system in my own home and installed many when I was in buisness.
Just my 2 cents.....
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