The 2009/2010 Heating Season I mean. We used about 850 gallons. That's down from 1100 last winter (colder winter), 1500+ in previous winters, thanks to our Green Mt. Doom program with the new Energy Kinetics way efficient boiler and the Armstrong heat pump. We didn't have that heat pump last winter, I think it knocked down the heating oil another 20% or so.
That's not a lot of heating oil for a 4200 sf house. So I think we're getting our money's worth from this new gear. The last thing we could do (short of going geothermal) is spray foam insulation in the attic, but that would cost $4K and would only save another 150-200 gallons tops. Not worth that cost for the incremental savings. But that would bring us down into the 600-700 gallon range.
A full (upstairs two zones, basement still on baseboard from the old boiler) geothermal system would have dropped us down to 300-400 gallons (we'd still have used the boiler for hot water and to heat the basement zone where my office is located). So I think we're essentially there. 400+ gallons more than we'd use with a full geothermal conversion, @ half the capital outlay ($20K vs. $40K). Even @ $4 per gallon, that's only a difference each year of about $1500 in heating costs between this set up and geothermal
For new construction or a major renovation, geothermal is a total no-brainer. Fascinating technology, and it works. Harnesses the 50º air temps underground to run heat pumps that are indoors to heat and cool your house very efficiently. Once you're using 50º air, the SEER ratings of heat pumps start going up over 20 (our current air to air heat pump has a SEER rating of 15 or 16) and they don't use so much power.