I just installed a DX system in October, and I still have my oil fired baseboard system in place. (It's turned off, obviously.) But, I was wondering, as the price of oil plummets, at some point it will be cheaper for me to switch back. I live in CT, where electricity is .18 KWh.

Has anyone done their own calculations on their systems to find that magic price?

The magic number depends on the actual cop of your two systems.
Assume your dx has a cop of 4, and the old oil system has a cop of 0.85, as overall systems' efficiency indices . If you burn a gallon of oil in your old system you will have 139000 btus x .85 = 118150 btus delivered to your house.
1 KWH of electricity is equivalent to 3413 btus at a system cop of 1. At the assumed dx cop of 4, each KWH will give your house 13652 btus. You will use 8.65 KWH to replace 1 gal of heating oil. The magic number, with the above assumptions is 8.65 KWH x $.18 = $1.56 for a gal of oil.

Seems like a reasonable analysis, Masoud.

Vinman, What is the probability that oil would go low enough to have a lower operating cost than electricity AND STAY THERE? Also you might consider how well the oil furnace would operate after being idle for however long, the condition of the storage tank, condensation/water in the fuel, the condition of the exhaust stack and the fact it is a potential air leakage source, and the baseboards taking up floorspace.

I suppose if you have a 1000 gallons of oil you've already paid for, go ahead and use it up. In the meantime you can track current fuel oil prices and decide if you want to refill the tank. Good luck, Dave

Thanks for the info. I'm confident that oil will not stay like it is forever, but electricity is set to increase 2-4% a year around here. There may be a point (albeit short lived) at which it would be more economical. Sad really...

My oil furnace has an indoor 200 gallon tank that is 3/4 full. Baseboards are staying (too lazy to rip them out), and I like the idea of having two systems. (If I lose power, a generator can power my furnace.) It's also set up to be my aux system, in case the geo fails (which it has before..high pressure switch) and I'm not home for an extended period of time.

So my other question is: How much of a problem will I have with my furnace turned off? Will I need to fire up the dino juice once a week? Once a month? Is there a version of STA-BIL (the same stuff I use in my Harley over the winter) to stabilize the oil in my tank? What should I do to keep it in operating condition?

(During my last cleaning, my oil guy told me it was a bad idea to leave it turned off, but come on... look at the source!) ;)

Oil guy has a legitimate concern. There are mold-like bacteria that thrive in Diesel fuel. There are additives to combat this and other issues with long term storage of Diesel (heating oil is almost identical to Diesel)

Then get some fungicide.  Gosh.  They do make a product similar to Stabil for diesel fuel.  Several products, and some will kill the mold and other growths. 

Personally I'd hit the start button if the oil were 5 or 10 years old.  Unlike an engine, a furnace is a heck of a lot cheaper, and if it does get clogged, can be fixed relatively easily.  And it probably won't.

The other posters have a legitimate concern : your old clunker could be leaking heat into the environment, increasing the thermal load on your heat system.  If that were the case, I might think about removing it.  Otherwise, I think you have the right idea : I suspect your furnace will last a long time if it is never turned on except maybe once a year.