While the average heating system lasts about 25 years, some boilers can hang on for 50 years or more,
according to Wilson, Alex and Morrill, John (1998) Consumer Guide to Home Energy Savings. 
How to determine whether to replace your old heating system
If any of the following are true, you should consider replacing your old heating system:
- Your system needs frequent repairs and your energy bills keep rising.
- Your furnace or boiler is over 15 years old.
- Your heat pump is over 12 years old.
- You have a gas furnace that doesn't feature an electronic or pilot-less ignition. If your furnace has a pilot light, it most likely was installed before 1992 and has an efficiency rating of about 65 percent. Even today's least efficient systems are 80 percent.
- You have a gas furnace with no means of preventing heated air from going up the chimney when it's turned off (such as vent dampers or an induced draft fan).
- You have an old coal burner that was switched over to oil or gas.
- You score below five on ENERGY STAR's Home Energy Yardstick. Grab your last 12 months of utility bills and take this quick test. A score below five means you're likely using more home energy and paying more for it than most other Americans.
If you have an older furnace or boiler, and you either find your house is not heating comfortably or your utility bills are too high, consider hiring a qualified home heating contractor who can check out your existing heating system and help you decide whether to install a new one. It's a big investment: paying for a professional opinion will be money well spent.
How efficient is your heating system?
The efficiency of an oil- or gas-fired heating system is determined by how well it converts fuel into heat. One measure is combustion efficiency, which tells how your system performs while it's running. An even better measure is AFUE which stands for annual fuel utilization efficiency. While combustion efficiency is equivalent to the miles per gallon your car gets zipping down the highway, the AFUE measures start-up, cool-down, and other operating losses as well — more like measuring your car's mileage for both city and highway driving
Is it cost-effective to replace your system?
To find out how much you'll save by installing a new system, check out charts compiled by the American Council for an Energy Efficient Economy. Using these charts, you can take the AFUE of your old system and the AFUEs of new systems and calculate how many dollars you'll save for every hundred dollars spent on fuel if you buy a new furnace. You can even do a simple financial calculation and figure out the return on your investment (ROI). If you don't know the AFUE of your old system, your heating system technician or an energy auditor might be able to help you figure it out. If he or she can tell you the combustion efficiency, multiply that number by .85 to get the approximate AFUE.
Before you buy
Before you make the decision whether or not to buy a new heating system, consider having a professional energy audit done. A professional energy audit will not only find sources of heat loss — your first line of defense — but the auditor can also help you decide whether you need a new heating system and what the correct size for a new one would be. Often it's more cost-effective to improve house insulation, repair or insulate ductwork, or tune up your system than it is to get a new furnace. And tightening up your house means you may need a smaller heating system, so be sure to make these changes first before considering a new furnace or boiler.
Find it! Guides to replacing your heating system
Replacing your heating system helps you go green because…
- If the 40 million households in climates with large heating requirements boosted the efficiency of their furnace or boiler from 70 to 90 percent, about 45 million tons of carbon dioxide emissions would be saved every year..
- Switching from electric to gas or oil heat would save 23 or 19 tons of CO2 per year, respectively..
Heating your home with fossil fuels not only results in a high cost to you, it also comes at a high cost to the environment. The Union of Concerned Scientists analyzed the ecological impact of the most common consumer actions and products and ranked "home heating, air conditioning, and water heating" fourth in its list of the "seven most harmful human activities" the environment.
- ENERGY STAR's Home Energy Yardstick Take this quick test and find out how your home energy costs stack up compared to other Americans.
- American Council for an Energy Efficient Economy - Heating Systems: Furnaces and Boilers Using these charts, calculate how much you'll save on fuel if you buy a new furnace, and figure out the return on your investment (ROI).
- Wilson, Alex and Morrill, John (1998), Consumer Guide to Home Energy Savings. Washington, DC: American Council for an Energy-Efficient Economy
- ENERGY STAR - A Guide to Energy-Efficient Heating and Cooling
- American Council for an Energy-Efficient Economy - Heating Systems: Furnaces and Boilers
- San Francisco Chronicle - Group's Surprising Beef With Meat Industry