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The beauty of a 90+ GAS furnace is that it uses plastic pipe for a chimney AND to connect directly to outdoor air. Now you can shut your house up tight as a drum and air to the furnace becomes a none issue.
It's called "direct venting". The same can be done with oil but you can't use plastic for fluepipe. The reason you can use plastic pipe is that so much heat has been removed from the fire that the exhaust is only about 100*F. They are called "condensing furnaces" because the water in the flue gas actually condenses out and has to be drained off.
Sad part. Given the BTU per $ value of NG vs oil an 86% oil BOILER (we can't achieve 86% with an oil furnace, them crazy Germans are working on it though) will still outperform a 90+ gas appliance. NG is actually much more expensive per BTU than oil. And then theres PROPANE! DO NOT listen to Hank Hill! Propane is the LEAST efficient / MOST EXPENSIVE of ALL the fossil fuels.
Let's do the math.
Here's what you do. Find out how much a "Therm" of natural gas costs in your area. A Therm = 100,000 BTUs. Compare that to the price of a gallon of oil. A gallon of oil = 140,000 BTUs.
When we talk about the "Efficiency" of a furnace or boiler we are talking about how much of those BTUs make it into the house vs how many go up the chimney. So a 90% furnace using natural gas will give
90,000 BTUs of heat per therm to the house, the other 10,000 go up the chimney.
An 86% oil boiler will send
120,400 BTUs to the house and 19,600 up the chimney. Generally a gallon of oil and a therm of NG have always cost about the same. I don't know if that still holds true actually.
If anyone wants to post the figures for their area, I'd be interested in seeing them.