Heating is the largest energy expense in most homes, accounting for around two-thirds of annual energy bills in colder parts of the country. Reducing your energy use for heating, therefore, provides your single most effective way to reduce your home’s contribution to global environmental problems. Heating systems in the United States spew over a billion tons of CO 2 into the atmosphere each year and about 12% of the nation’s sulfur dioxide and nitrogen oxides.

Should I replace my existing heating system?

This can be a difficult question. If you heat with electric resistance heat, rising electricity prices may force you to switch to a gas, oil, or heat pump system that is more affordable. If you currently have a gas- or oil-fired furnace or boiler, the decision to replace it depends on its age, condition, and performance.

If your furnace or boiler is old, worn out, inefficient, or significantly oversized, the simplest solution is to replace it with a modern high-efficiency model. Old coal burners that were switched over to oil or gas are prime candidates for replacement, as are gas furnaces without electronic (pilotless) ignition or a way to limit the flow of heated air up the chimney when the heating system is off (vent dampers or induced draft fan).

A typical heating system will last about 25 years, though some boilers can last twice that long. Your heating system technician or energy auditor may be able to help you evaluate your existing system and decide whether replacement is a good idea.

If you know how efficient your existing heating system is (AFUE), it's pretty easy to calculate the savings you will get by replacing it. The chart below will help you determine potential savings resulting from replacement of your existing system. Your heating service technician or energy auditor may be able to help determine the AFUE of your present system. If you were only provided with the combustion efficiency, you can estimate the AFUE by multiplying the combustion efficiency by 0.85. The numbers in the chart assume that both the old and new systems are sized properly; savings will be greater than indicated if the old system is too large.

Dollar Savings per $100 of Annual Fuel Cost

To determine savings, find the horizontal row corresponding to the old system's AFUE, then choose the number from that row that is in the vertical column corresponding to the new system's AFUE. That number is the projected dollar savings per hundred dollars of existing fuel bills. For example, if your present AFUE is 65% and you plan to install a high-efficiency natural gas system with an AFUE of 90%, then the projected saving is $27 per $100. If, say, your annual fuel bill is $1,300, then the total yearly savings should be about $27 x 13 = $351.