How To tell If Furnace Heat Exchanger Is Cracked

Heat exchangers are special devices designed to transfer heat from one medium to another. A water heater heat exchanger, for instance, transfers hot water from a boiler or solar panel to the water in a reservoir tank.

The heat is transferred by conduction through the exchanger’s materials that separate the materials in question. For instance, the heat exchanger in your furnace transfers heat from the combustion chamber to the unit’s exterior, where it’s then forced into the duct system and distributed throughout your home.

This makes them a critical part of heating systems, especially furnaces. It would be impossible for the furnace to heat your home safely without the heat exchanger.

When the Heat Exchanger Cracks

Unfortunately, the heat exchanger can get damaged, and one of the most common damages happens when the heat exchanger cracks.

Cracking is typically caused by overheating, often because of insufficient airflow. When there’s poor airflow within the furnace, the heat exchanger overheats and suffers excess stress from repeated expansion and contraction. Over time, the pull and push movements against the heat exchanger walls can cause cracking near weak areas such as the bends or welds.

Poor airflow can result from dirty filters and an oversized furnace, among other factors. A clogged filter restricts airflow through the furnace, which is necessary for temperature regulation inside the heating system. The outcome is overheating inside the furnace and exchanger. Filters usually become dirty due to poor maintenance.

An oversized furnace can also cause the heat exchanger to overheat and crack. Maybe the furnace is too big for the ductwork or the housing. This can negatively impact airflow into and out of the unit, causing the heat exchanger to overheat and ultimately crack.

An oversized furnace can also cause excess humidity and condensation that may cause the heat exchanger to rust inside and out, making it more likely to crack.

Signs of a Cracked Heat Exchanger

Diagnosing a cracked heat exchanger shouldn’t be too difficult, though sometimes you may need more than a simple eye test to prove your findings. Keep an eye out for the following;

1. A Yellow Furnace Flame

A healthy furnace flame is blue and clean-burning. However, the flame changes to yellow when the heat exchanger is cracked. Why does the flame suddenly become yellow? Because of incomplete or inefficient burning.

When the heat exchange is cracked, airflow to the burning process is further compromised. As a result, the burning process won’t have enough oxygen to produce the blue, clean-burning flame you’re used to.

Your fears are most likely true if the yellow flame keeps flickering. A health gas heater flame is stable and upright. But, when the heat exchanger is damaged, excess gas escaping into the unit at pressure can create a breeze across the flame, causing it to flicker.

2. Too Much Soot 

You’re likely already aware that excessive soot is one of the main byproducts of inefficient combustion. Remember that gas is a hydrocarbon. It contains carbon atoms and hydrogen atoms. During complete combustion, the hydrogen atoms combine with oxygen atoms to form water, while carbon atoms combine with oxygen to form carbon dioxide.

However, if there isn’t enough oxygen for the combustion process, the highly heated carbon may be emitted as gaseous carbon. Gaseous carbon is black. Since a cracked heat exchanger compromises airflow inside the furnace, you can expect plenty of soot.

3. A Strong Smell 

A broken heat exchanger will also likely produce a strong, formaldehyde-like smell that is highly intoxicating. Formaldehyde is a colorless chemical with a pickle-like odor used in many manufacturing processes. It’s also commonly used in building materials and household products such as flooring, furniture, and fabric.

Cracked heat exchanges can produce a similar smell when the carbon monoxide from the incomplete combustion process mixes with other gases and surfaces. If you catch a whiff of the characteristic smell, turn off the gas immediately and call for professional help.

4. Corrosion and Cracks on the Furnace

Heat exchangers can also crack because of natural wear and tear. The repeated expansion and contraction of the metal parts can eventually take a toll on the exchanger resulting in signs of cracking.

If you suspect that the exchanger has cracked but can’t find specific evidence, stress cracks and corrosion on other parts of the furnace, also associated with wearing and tearing, would be the ultimate evidence.

Over time, the repeated expansion and corrosion due to heating and cooling causes the other metal parts of the furnace to crack. Prolonged exposure to minor traces of water vapor and chloride acid may also cause the parts to corrode.  

5. Presence of Carbon Monoxide 

Although you should never wait until you have carbon dioxide in the home to begin fixing your furnace issues, it helps to keep an eye out for the poisonous gas. And, one of the leading causes of carbon dioxide in the home during heating is a cracked heat exchanger.

The reasons are similar to when your heater produces too much soot. In healthy gas heating, the carbon binds to oxygen molecules to produce carbon dioxide – a harmless byproduct.

However, when your furnace has insufficient airflow, there aren’t enough oxygen atoms to bind to the carbon in gas to form carbon dioxide. Thus, you may have carbon monoxide instead.

6. Cracking Sounds 

Finally, you should also be confident that the heat exchanger has cracked if you hear cracking sounds when the furnace starts or stops. The reason for the crackling or rattling sounds is that the heat exchanger is made of metal. When you heat metal, it expands.

When you cool it, the material contracts. If the material has cracks, the contraction and expansion will cause the cracked surfaces to grate against each other, causing popping or cracking sounds.

The noise is highest when you start the furnace and when the appliance is going off because expansion and contraction are most pronounced at these two stages, respectively.

What to Do About a Cracked Heat Exchanger

A cracked heat exchanger is dangerous. As we’ve seen, it exposes you to soot, carbon monoxide, and a powerful formaldehyde-like odor. All three are harmful to your health.

Carbon monoxide and the formaldehyde-like smell can even make you dizzy and drive you into unconsciousness. Even death is possible. In the end, a cracked heat exchanger can also accelerate appliance wear and cause malfunction and damage.

To protect yourself, your family, and the appliance, begin by ensuring proper furnace installation and maintenance. Proper installation means installing the right-size furnace plus the requisite oxygen depletion sensor (ODS).

Additionally, it means installing the unit in an area with unimpeded airflow. Meanwhile, regular maintenance helps prevent clocked filters and blocked vents that may compromise the airflow.

Do your part by regularly inspecting the furnace, at least once every two days, to catch any signs of damage early. More importantly, schedule professional maintenance at least once before the onset of the heating season.

Never DIY Heat Exchanger Repair

If you suspect that the heat exchanger may be damaged, perhaps after confirming two or more of the signs above, call a licensed HVAC professional to diagnose and fix the problem. Don’t attempt to do it yourself. The risk is too huge.