Causes and Signs of a Cracked Heat Exchanger

The heat exchanger is a part of the furnace that heats up the air that warms up your house. The heat exchanger consists of a series of tubes inside the furnace, and the furnace blower forces air over the heat exchanger to heat it up.

Sometimes, however, the heat exchanger tubes crack and become a fire hazard. Apart from the risk of fire, a cracked heat exchanger can also introduce poisonous carbon monoxide in your indoor air. Both of these dangers mean you should know what causes heat exchangers to crack and the telltale signs that mean your heat exchanger is cracked.

Why Heat Exchangers Crack

Here are some of the reasons a heat exchanger may crack.


All heat exchangers eventually fail, even those of the best furnaces; the cracking of a heat exchanger is a natural part of aging. As previously mentioned, the heat exchanger is a serious or set of metal tubes; metal expands when heated and contracts when cool. The heat exchanger metal weakens with each expansion and contraction.

After years of use, the tiny damages eventually add up to serious damage that cause the metal to crack. Therefore, you should expect your heat exchanger to crack if your furnace is nearing the end of its useful life.


An overheated heat exchanger may crack even if it’s still relatively new. The crack may appear when the heat becomes too much for the metal material to handle. Here are some of the reasons a furnace, and by extension the heat exchanger, may overheat:

  • Dirty air filter: Dirty air filters restrict air supply to the furnace and makes the furnace overwork and overheat.
  • Air duct blockage: Air duct damage or blockage causes the furnace to overwork in an attempt to force through the required air.
  • Electrical malfunction: An electrical malfunction can cause overheating in the furnace just as it can in any other electrical appliance.

You can help prevent overheating in a newer heat exchanger if you maintain the air filter, air ducts, and electrical components of your furnace.

Telltale Signs of a Cracked Heat Exchanger

Learn the warning signs that mean you have a cracked heat exchanger on your hands. Watch out for the following.

Too Much Soot in the Furnace’s Interior

Soot is actually carbon compounds that are byproducts of incomplete combustion. Too much soot in the interior of the furnace may point to a cracked heat exchanger because the furnace won’t burn its fuel cleanly if its heat exchanger is cracked. The soot is the product of this incomplete combustion.

Dirty or Yellow Flame

If your furnace is operating normally and burning its fuel efficiently, it will produce a hot flame that is bluish in color. However, the blue color will change to a dirty or yellowish color if the furnace burner is not running as usual. A yellow flame is not a definitive sign of a cracked heat exchanger, however, because the furnace burner may also produce the same color of flame if the furnace is dirty.

Physical Damage to the Exterior Components

A visual inspection of the exterior components of a furnace may also tell you whether the heat exchanger has cracked. You should particularly watch out for signs of corrosion, hairline cracks, or opening of seams. Since furnace damage typically begins from the interior and extends outwards, you are likely dealing with a crack if the exterior of the furnace has signs of damage.

At JC Heating & Cooling, we have the tools, expertise, and resources to give you a correct diagnosis and solution if you suspect a crack. If we confirm the crack, then we will advise you either to replace the heat exchanger or the whole furnace. If your furnace has a problem, contact us today and we will help you out as soon as possible.