Although gas-fired furnaces and boilers are by far the most common type of home heating system, oil furnaces remain popular in areas with limited access to natural gas. As of May 1, 2013, the minimum AFUE for newly installed non-weatherized oil furnaces is 83 percent, weatherized furnace regulations take effect on Jan. 1, 2015 and will be set at 78 percent.
Oil-fired furnaces have lower annual fuel utilization efficiency (AFUE) ratings than gas furnaces on average, although some models can achieve an AFUE of 95 percent. If you have an older oil furnace in your home, replacing it with a higher efficiency model will result in lower fuel costs and fewer carbon emissions than an older, less energy efficient oil furnace. It's estimated that upgrading an older 56 percent AFUE oil furnace to a 90 percent AFUE furnace will reduce CO2 emissions by 2.5 tons per year.
While gas-fired furnaces have many advantages over oil-fired furnaces, the fact is that there are households where gas is simply not an option. One big downside is the volatile price of oil versus natural gas, though a higher-efficiency furnace will at least use less oil, cushioning the shock when prices go up. High efficiency oil furnaces no longer qualify for a federal tax credit; but rebates or credits may be available from state governments or utility companies in some areas. For a complete, state-by-state list of credits and rebates, visit the Database of State Incentives for Renewables & Efficiency (DSIRE) website.
This article from the U.S. Department of Energy advises consumers on choosing the right furnace. It also discusses retrofitting options, and has a chart that shows the cost savings in replacing an older furnace.
Review: Furnaces and Boilers, U.S. Department of Energy, June 24, 2012
2. American Council for an Energy Efficient Economy
This site discusses the benefits in upgrading to a more high-efficiency furnace or boiler. It includes oil-powered systems as well as gas. There is also a brief guide to deciding if you may need to replace your older system.
Review: Heating, Editors of American Council for an Energy-Efficient Economy, Not Dated
These are the most efficient oil-fired boilers of 2014 according to the U.S. Department of Energy. All of the oil-fired boilers here have an AFUE rating of better than 90 percent. Annual operating costs are also detailed.
Review: ENERGY STAR Most Efficient 2014 - Boilers, U.S. Department of Energy, Not Dated
In this article, gas furnaces are compared to oil; and gas comes out as a clear winner. However, if you don't have the choice of switching to gas, the author points out that upgrading any older furnace to a higher efficiency model will save in fuel costs.
Review: Gas vs. Oil: Which Furnace Is Better?, Donna Boyle Schwartz, Not Dated