Statistics show, two out of every 10 furnaces over 15 years old, where annual maintenance has been inadequate or ignored, will likely have a breached and potentially dangerous heat exchanger. The problem won't present itself in an easily detectible manner. Instead, as fractures in a heat exchanger worsen, increased amounts of carbon monoxide (a poisonous bi-product of furnace combustion) can find its way into your home instead of being vented outdoors. And, the odds of this occurring increase as your equipment ages.
The age of your furnace may be irrelevant
It is important to know that every furnace, no matter the age, can become unsafe. Newer government energy guidelines have mandated furnace manufacturers to increase fuel efficiency. One obvious way they've done this is to reduce the thickness of metal used in their heat exchangers to allow faster transfer of heat from the burning fuel to the indoor air.
The downside of thinner metal being used in today's heat exchangers is that, if the furnace is not sized correctly or installed properly, many of the newer furnaces have been found to fail within just a few years. Unfortunately, homeowners don't know if this has occurred until a problem is detected. For this reason, the qualifications and reputation of the installing contractor should be a determining factor when taking bids instead of price alone.
What causes a heat exchanger to fail?
The heat exchanger is the metal passage separating combustion products and gasses from the indoor air being heated. This metal is exposed to the hot flame within the furnace and is constantly expanding and contracting as the furnace heats up and cools down. The stress of this constant expansion and contraction will eventually wear the metal out. This is known as "metal fatigue". Over time, this will cause the metal in the heat exchanger to split or crack-no different than if you were to bend a paper clip back and forth until it breaks.
Horizontal furnaces (typically installed in crawlspaces below the home) and oversized furnaces (where the gas is continually turning on and off) are subject to more stress and usually wear out sooner due to their operating conditions. These types of furnaces should be inspected yearly-especially if more than ten years old.
Keeping your furnace's blower clean is critical
Dirt build-up on the blades of your furnace's blower can also contribute to early aging of your furnace's heat exchanger. Blower cleaning is critical since a build-up of dirt on the blower blades will reduce the furnace's airflow and cause it to use more electricity. The lower airflow will cause the furnace to run hotter, increasing the rate of expansion and contraction of the heat exchanger's metal. The end result is excessive metal fatigue and eventual premature failure.
How can I know if my furnace is safe?
A professional inspection is the most accurate way to know for sure if your furnace's heat exchanger is sound. Electronic "gas sniffers" can help find bad heat exchangers, but it is important that they never be used as the reason to condemn a furnace. They can be fooled and are wrong in a great many circumstances.
Many companies offer an inspection service for around $60. However, these include very little, if any, actual cleaning of the furnace, and typically take only about 20 to 30 minutes. This type of program may provide a feeling of confidence in your equipment, but they don't make your furnace run any better.
A better investment is a complete tune-up and cleaning that includes a heat exchanger inspection. When comparing prices, it is important to know that a good tune-up should take a technician anywhere from 1 to 2 hours, and will always include the removal and cleaning of the furnace blower.
If ever your heat exchanger is found to be faulty, knowing how to verify the problem may save you from unnecessary expense and grief. Unfortunately, without this knowledge, trusting homeowners can sometimes be misled by unscrupulous companies into replacing their entire furnace.
See the problem for yourself
The only true way to know for sure if your heat exchanger is cracked is to actually see the crack. If a service technician claims to be able to see a crack in your heat exchanger, he should be able to show it to you, too. Most reputable companies will insist on this.
You can also verify it for yourself. This can be done by removing the furnace's blower, or by cutting an access into the ductwork on top of the furnace, and then inserting a light into the burner area where the flame usually is. If the heat exchanger is cracked, you will see light inside the ductwork shining through the crack. Another way is to spray water on the outside of the heat exchanger. If there is a crack, you will see the water seeping through it, causing the area inside the exchanger to show a wet spot. For either test, a mirror can help you look into the different burner sections. If you still have doubts, we'll be happy to give you a second opinion.
What are my options?
If you do have a failed heat exchanger, it is serious and, if left uncorrected, can be fatal. That is why, when a failed heat exchanger is discovered, the furnace must be shut off for obvious safety reasons. Since it is against the law to repair it, the heat exchanger or the entire furnace must be replaced. Which should you choose?
First, refer to any warranty information you have relating to your furnace. However, if the failure was due to improper sizing or installation, consider that a new heat exchanger will be subject to the same conditions which led to the premature failure of the original. Under these circumstances, a new furnace may be a wiser investment. Of course, other factors such as the age and efficiency of your existing furnace should also be considered in making your decision.