When we purchased our house (Ottawa Ontario, probably same or colder than where you are) it had a 30 year old oil heater. Best guess was that it was roughly 65% efficient when new, and it was a long way from new. The house also had an electric hot water tank well past it's prime.
The day we took posession we had the oil heater and wate rtank pulled, and replaced with a new high efficiency Natural Gas furnace and mid-efficiency hot water tank (apparently there are no true high efficiency hot water tanks short of an in-line, and we don't use enough hot water to justify the cost difference).
Total cost of the job (including running a natural gas line into the house) was $7k cnd.
So, was it worth it? Yes. Our neighbour is still using the old furnace in an identical house. Her heating bill was $2200/year, plus electricty for the hot water. Our bill for both heating and hot water was $700. That's a $1500 a year savings. After our first year living next to her she got the natural gas furnace installed ;)
as I understand it, there are three basic types of high efficiency natural gas furnace - A/C motor, D/C motor, Modular.
The A/C motors are the cheapest.
The D/C motors cost a few hundred more than the A/C and use just as much natural gas, but the motor is much more efficient and they use way less electricity (a friend who heats a very large house saw a $500 reduction in her electricity bill by moving from an old AC to a new DC furnace).
The most expensive are the fully computerized Modular furnaces. While slightly more efficient (1-2%), the owner of the heating company suggested we stay away from them for one simple reason - they are very sophisticated computer systems, and the local furnace repair guy probably has a one or two year diploma in the mechanical HVAC systems. If it breaks down (and someday it will), do you want one that can be fixed by a monkey (meaning you can probably do it, and the trained guy certainly can) or the one that needs a computer whiz (where most fixing is simply replacing 'most likely' parts until it works again)?
So there you go, my suggestion, go with a D/C motor high efficiency natural gas furnace, add a natural gas hot water tank, and watch your bills drop away.
