How to Tell if Furnace Heat Exchanger Is Cracked?

A cracked heat exchanger usually spells the end of the life of a furnace or at least an expensive repair – and it can be a dangerous problem too.

The danger is the result of carbon monoxide (CO), which can enter the warm air ductwork through a cracked heat exchanger and be blown into the air you breathe. That’s why we start with the CO danger and move on to testing that might indicate a cracked heat exchanger. Use the Navigation list to find the exact information – or simply read through the article, which will take 4-7 minutes.

Carbon Monoxide Danger

What happens when a heat exchanger goes bad? Nothing good.

CO is odorless, but it may be accompanied by the exhaust gases leaking from your furnace that you can smell. So, if the air coming from your furnace and out your supply vents smells like exhaust, it is an indication that something is wrong and should be investigated further.  

Also, there is nothing about a cracked heat exchanger that will cause a change in the sounds of your heating system. In short, there are not many symptoms that would lead you to believe that you have this potentially dangerous problem. One possible physical symptom is that of unexplained nausea, headaches, or a tightness across the forehead. If these symptoms are present, get out of the house or building and into fresh air!

How to Tell if the Heat Exchanger is Cracked

Normally, diagnosing a cracked furnace heat exchanger is the job of a trained HVAC technician. However, if you are somewhat handy with simple hand tools, are not skittish about removing side panels on your furnace, and don’t mind getting your hands a little dirty – you might be able to determine if you have this problem by some of the tests below. A few tests require special equipment that are not usually part of the DIYer’s toolbox.

Here are some ways to tell if you may have a cracked heat exchanger.

Indications by Alert Mechanism

One alert is a detector every home should have near the furnace; the other is a response within the furnace.

1. Carbon Monoxide Detector Goes Off

If your CO detector operates, it is telling you that the CO level of the air in your home is above an acceptable level. This may be caused by a cracked furnace heat exchanger. The first thing to do is to open doors and windows to get fresh air – unless you’re already feeling ill, and if so, you should get everyone out of the house.

If you aren’t ill, turn off the power to the furnace either using the switch at the furnace or the circuit on the electrical panel.

If you are one of those somewhat handy people mentioned above, you might try some of the visual tests listed below. Otherwise, contact a reputable HVAC company to send out a technician to investigate further.

Did you know? Many homeowners call 911 when their carbon monoxide detector goes off. This is an option, especially if anyone in the household is feeling sick or is drowsy. Those people need medical attention!

When the detector goes off before anyone is feeling ill, then shutting down the furnace and calling a furnace pro is a good choice. Most technician carry an advanced CO monitor in their truck that they use to probe inside the furnace while it is running. It’s a more accurate carbon monoxide test and can detect levels lower than most Kidde or other brand carbon monoxide detectors detect.

You might try this: While a tripped CO detector is nothing to ignore, some homeowners reset the detector and/or change its batteries first. If it continues to go off, there is a CO emergency that should not be ignored. The safe approach is always to err on the side of caution and have your furnace heat exchanger checked for cracks.

2. Flame Rollout Switch Tripped

The flame rollout switch is a small sensor located above the burners behind one of the side panels of your furnace. The flame rollout switch is designed to operate if excess heat is coming back into the area containing control components as a result of the flames from the burners “rolling out”. This means they are being pushed backwards from where they are supposed to go, which is directly into the heat exchanger. If the flame rollout switch trips, it will shut off your furnace until it is manually reset by pushing a button on it. You might try resetting it, but if the switch trips again after being reset, an HVAC technician should be contacted – you might have a cracked heat exchanger.

Indications by Visual Inspection

HVAC technicians use mirrors with bendable handles cameras to visually inspect the heat exchanger without having to disassemble much of the furnace.

However, short of this type of inspection, there might be visual clues that the heat exchanger is cracked.

1. White Powdery Substance at Joints of the Flue. Normally, if there is any substance at the joints of the flue, which is the pipe leading to the outdoors, it should be dark, and slightly sooty. A white powdery substance (like chalk) is a sign of bad combustion, which can be traced back to a cracked heat exchanger.

2. Flame Blowback at Burners. To observe whether this is happening in your furnace, the side cover must be removed that allows you to see the burners. The blower cover at the bottom of the furnace should remain on to prevent the blower from affecting the burners from the outside. When the burners and the blower are on, the flames should be mostly blue and flowing straight into the heat exchanger with no distortion. Flame spreaders between the burners cause a small “flared” flame on each side of adjoining burners – this is normal. Distorted flames and/or the presence of a significant amount of orange or yellow flame is a good indication that the heat exchanger is cracked.

3. Static Pressure Test. With the gas turned off and the blower on, hold a burning match in front of each burner where it normally projects its flame into the heat exchanger. Use long tongs or a piece of wire to get the match directly in front of the burners. If the flame of the match becomes distorted or blown out by air coming back, this probably indicates a cracked heat exchanger.

The last two tests are most often done by certified technicians, since they have the right equipment.

4. Condensation Leak. Do you have a high-efficiency furnace? That’s a furnace with an AFUE or efficiency of 90% or higher. These furnaces have two heat exchangers to do a superior job of getting as much heat as possible out of the exhaust gases before they leave your home. In fact, they transfer out so much heat that the moisture in the exhaust condenses (water is a byproduct of the combustion of natural gas, propane and oil). If the secondary heat exchanger is cracked, the condensation might drip out onto the ground rather than entering the drain pain and drain. If you see water where it shouldn’t be and you have any of these other indications of a cracked heat exchanger, you might have found the reason. There are other causes of condensation leaking too, usually a plugged condensate line or failed condensate pump.

5. Inspection Camera. If you have an inspection camera with a flexible probe, it can be inserted into the heat exchanger with the burners removed. This view will enable you to inspect from the inside. It can also be inserted into the hole where the limit switch is mounted to view the outside top of the heat exchanger. Additionally, with the blower removed, the outside bottom of the exchanger can be visually inspected with the camera. Cracks, holes, or the presence of a white, chalky substance indicates a bad heat exchanger. Even the smallest cracks are a problem, so look closely and don’t ignore any damage you see.

6. Combustion Check. This is done with a combustion analyzer, which is a battery powered, handheld instrument. It can measure and interpret the flue temperature and pressure, oxygen and carbon monoxide levels, inlet temperature, draft, combustion efficiency, and excess air. This is accomplished by drilling a small hole in the flue where the gases from the burners are leaving the building after passing over the heat exchanger. The sampling probe is inserted into the flue through the hole. HVAC technicians will have access to information which tells them what numbers are within acceptable ranges for each different test. One or more of these tests can indicate a cracked heat exchanger.

Cracked Heat Exchanger Response

If any of these tests seems to indicate that you have a cracked heat exchanger, you should contact a competent HVAC company to decide whether it will be best to replace the heat exchanger or the entire furnace. Because a heat exchanger will only be faulty after years of service and indicates that the metal is severely fatigued or corroded, it is not a good idea to repair it.

How to Prevent a Cracked Heat Exchanger

OK, let’s say you have the furnace checked out, and the cause of the problem was not a bad heat exchanger. But this article has you thinking about the dangers and the cost – heat exchanger replacement runs as high as $2,000 or more, and you should strongly consider replacing the entire furnace.

What causes heat exchanger failure? Restricted airflow and dirt are the major causes. Address those, and you can help prevent a cracked heat exchanger. Here are simple but effective methods.

1. Keep the filter changed. Check it monthly during AC and heating season, and change it when it needs it. A dirty filter causes the temperature inside a furnace to get high enough to cause damage. See our Guide on How to Know When to Change your Furnace / AC Filter.

2. Use a filter with a MERV rating appropriate for your system. A MERV rating of 6, 8 or 10 is usually sufficient for homes where nobody has breathing issues like asthma or severe allergies. The higher the MERV rating, the denser the filter is, generally speaking. MERV ratings of 12 and higher can start to restrict airflow in some furnaces when the blower has to work harder to circulate air. When this happens, heat can back up and create internal furnace temperatures that can damage including a cracked heat exchanger.

changing a folded dirty air filter in the HVAC furnace system

We’ve created a Guide to the Best Furnace Filters for cleaner air and excellent airflow.

3. Have your furnace cleaned annually. This is good practice for furnace longevity, and it might prevent a cracked heat exchanger. Why? Because dust and hair build up inside any furnace. When that dust gets really hot and combusts, the extra heat can cause the metal of the heat exchanger to crack or separate.

Heat Exchanger Replace/Repair?

That’s the question – if the heat exchanger is bad, should the furnace be repaired or should the furnace be replaced?

Heat exchangers have warranties of 20 years, in most cases, and “Lifetime” in others. However, labor costs aren’t covered, and a heat exchanger takes 4-8 hours to replace, so your cost adds up.

We recommend getting several written estimates for both – heat exchanger replacement and complete furnace replacement. Make your decision based on the age of the furnace – the older it is, the more it makes sense to replace it. Consider how long you intend to live in that home – the longer you plan to stay, the more value you’ll get from a new furnace. 

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