The furnace should be on its down dedicated 15A circuit and probably has a 10A fuse at the power switch on the furnace itself, thus it should have never been able to draw more than 1200 watts max without blowing it. High power bills can come from a lot of things. Do you use any space heaters and are all your house lights incandescent? I would go around the house with a good multimeter and check voltages at all outlets. Anything in your garage that might be sapping power?  With the state of D-troit infrastructure I'd not be surprised if the power coming to your house is very unstable and not clean.
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