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Replacing an old furnace is part of home ownership. However, you also want to get the most life possible out of your investment before you pay for a new furnace. How can you tell if it is time to get a new one versus continuing to maintain your home’s current system?
Expected Furnace Lifespan
The average furnace in an American home will stay in service 15-20 years. Notably, efficiency declines usually start to occur around the 12th year.
You will find homes with furnaces that are significantly older than 25 years. Bear in mind that the homeowners have likely elected to keep maintaining the furnace even in the face of efficiency drops and growing repair costs. Even in this scenario, parts for discontinued models will likely become scarce 10 years or so after the manufacturer stops making the particular furnace type.
On balance, age is the simplest and best indicator of whether it’s time to replace a furnace. Particularly, material fatigue in the heat exchanger leads to cracks that make the system unsafe due to potential exhaust leaks. Likewise, a compromised heat exchanger will leak heat, leading to poor performance and higher heating bills. Replacing the heat exchanger can easily run north of 60% of the cost of simply replacing the furnace.
Rising Energy Bills
Wear and tear will also reduce the efficiency of many furnace components. Deteriorating seals allow warm air to escape from the system. The blower motor experiences metal wear from friction, causing it to get louder and run less efficiently. The control board may also experience a short circuit. Corrosion and fouling on burners also prevent them from burning as efficiently as possible.
The net effect of all of this is a decline in efficiency. In many cases, the furnace continues to operate well enough. However, the inefficiency will lead to rising gas bills.
Note that some furnaces do experience efficiency declines before the 12th year. For example, the furnace might not be correctly sized for the house. This leads to more frequent operating cycles that will shorten the system’s life. Consequently, you should keep an eye on your gas bills to determine if it might be time to replace your home’s furnace.
System Efficiency
The furnace in a home may not be as efficient as the best available newer models. Even if the furnace is otherwise in good operating condition for its model, it might have never been especially efficient.
Annual Fuel Utilization Efficiency (AFUE) is the industry standard for furnace efficiency ratings. Roughly speaking, older furnaces tend to have an AFUE of 56-70%. If you purchased an older house and never replaced the furnace, it might be a unit from the 20th Century that operates at this low of an efficiency level. For reference, an AFUE of 80-85% is now considered standard. A model is usually high-efficiency if it has an AFUE of 90-98.5%.
Even if the furnace in a house is relatively new, the difference between 80% and 98.5% is considerable. Many home sellers like to put in a new furnace before listing a house, but they may not necessarily opt for the top-efficiency model. If the efficiency level is below your preference, you may want to replace even a relatively new furnace.
Frequent Repairs
Regardless of the age of the furnace, increasingly frequent repairs are a sign that you might need to replace it. The rule of thumb in the HVAC industry is that replacement is presumptive whenever the repair cost exceeds 50% of the price of a new furnace installation.
Keep in mind that this includes scenarios in which multiple small costs could add up to more than 50% of the replacement price. If anything, a bunch of small repairs is more concerning because it may be a sign that the furnace is experiencing a cascade of failures. Effectively, as one part fails, it creates stress on other parts. For example, weaker airflow from a damaged blower motor might cause overheating that cracks the heat exchanger. Whenever you see multiple repairs within two years, the best move is to replace the furnace.
You should be especially concerned about heating repairs if they’re necessary after years of regular maintenance. If a well-maintained furnace that’s getting annual tune-ups is starting to see failing parts, then it’s likely approaching its mechanical limits.
Most furnaces are rated for 80,000 or more heating cycles. For reference, a normal heating season should involve 4,000 to 6,000 cycles. If your HVAC has a smart thermostat that tracks runtime hours, you can do a ballpark calculation. At the peak of heating season, a system will cycle roughly five times an hour. If the thermostat shows 8,000 hours of runtime, then the rough estimate of the cycle count would be 40,000 cycles.
Performance Problems
Performance issues are a little trickier because many of the signs that replacement is necessary overlap with signs that a system just needs a bit of care. You might notice that the furnace is short-cycling. Too-frequent cycling is a relative term. During a cold snap, eight cycles per hour can be normal. If the system is doing eight cycles per hour on a 55°F day, then something is probably wrong.
Other performance issues include insufficient air reaching far parts of your house, declining efficiency as well as hot and cold spots throughout your house. Note that these may happen due to poor air balance or a clogged furnace filter. Even a buggy thermostat can cause performance issues.
Performance problems should clear up, however, once you do appropriate maintenance. If the system still has problems after clean filters, recalibration and a tune-up, then you have to consider the possibility that you need to replace the furnace.
Flickering Flames
Natural gas should have a steady blue color during combustion. If you look into the chamber during operation and see yellow or orange flames, this is a sign of incomplete combustion. It is a safety risk because carbon monoxide tends to build up more in houses with furnaces that aren’t burning off enough gas.
The burners might not be getting enough oxygen. In the best-case scenario, soot has accumulated on the burners. One of our technicians can usually clean the soot and get the system back to safe and efficient operation.
A more concerning scenario is the possibility of a cracked heat exchange. Leaks can draw air away from the burners, preventing the flame from maintaining its target temperature. Furnace replacement is likely the most cost-efficient solution.
Noisy Operation
Furnaces get noisier as they age. You may notice sounds like:
- Banging
- Squealing
- Clanking
In many cases, lubricating the moving parts or replacing a belt is enough to resolve the issue.
However, be extra cautious if you hear a thumping sound at the start of each heating cycle. This is a sign that gas buildup is occurring, and it is dangerous. You should strongly consider replacing the furnace if it experiences hard ignitions.
A/C Plus Heating & Cooling serves Murfreesboro, TN, and the surrounding areas. We stake our reputation on prompt and courteous service, backed by a 100% customer satisfaction guarantee. People trust us because we have been in business since 1997.
Contact A/C Plus Heating & Cooling today if you have questions about replacing the furnace in your Murfreesboro home.
