Bought a house with hot water heat (built 1920). Old boiler, but had it serviced and cleaned and thermostat replaced; they saw no problems with the unit.
Gas company gave us last year's usage (i.e. by previous owner). Of course the dollar cost is affected by the weather and the price of nat gas, but I was able to do get the data to calculate therms used per degree-day, which is independent of those variable factors.
We are using nat gas at a rate that is about 30% higher than the previous owner.
I know they might have had their thermostat set low (ours is at 69), but while that would make a difference, it wouldn't make such a huge difference. I have looked at the rate of gas use by the dryer and the hot water heater, and these seem normal (about 300 cu. ft per hr for the dryer, for example) and wouldn't make a big difference to the total. When all the gas-using devices are off, the gas meter dials do not move.
All our windows are closed and the house is in fact better sealed (more weatherstripping and some fiberglass batting in attic) than when we moved in, and we also insulated the hot water pipes in the basement. So we expected to be slightly ahead of the previous owner, not far behind.
All suggestions greatly appreciated.