Not so cheap heat

Been running an bit of a mostly uncontrolled experiment here. We got a new furnace two weeks ago, and the house seemed so much warmer. Even with the thermostat set at 67, it was comfortable all over (except maybe for the girls' bedroom, which has no source of heat and faces West, into the wind). The new equip. is more efficient, so I was thinking, hey, forget all the work of keeping a fire in the stove and actually heat the house with the furnace. What a novel idea! So, here we are 2 weeks later and I finally get around to checking the oil tank, only to find that I've burned about $250 worth of heating oil!! Holy moly! Damn the chilly house and oven-like living room (so comfy to fall asleep in though), I'm bring firewood up and lighting the stove!