6 Cracked Heat Exchanger Symptoms: Bad Heat Exchanger? (Checklist)

A bad furnace heat exchanger is a nightmare. We many times forget that a heat exchanger is the biggest part of the furnace. It’s very understandable why everybody fears a cracked heat exchanger in a furnace.

Here’s are two simple reasons why the issue of a cracked heat exchanger is so unnerving:

  1. Release of carbon monoxide (CO); serious safety hazard. As we’ll explain further on, a furnace heat exchanger (made out of metal) is the only thing standing between you and the flue gas. Flue gas – a side product of gas combustion – contains carbon monoxide that is very harmful, even deadly. Any crack in the heat exchanger leaves you vulnerable to toxic gases.
  2. Hard-to-detect cracked heat exchanger symptoms. It’s very difficult (but utterly important) to detect the symptoms of the furnace’s cracked heat exchanger. The furnace seems to be working fine, and carbon monoxide is an invisible killer. Only a furnace technician can use a probe (or disassemble the furnace to get to the heat exchanger) to actually check the surface of the heat exchanger for cracks. Symptoms of a cracked heat exchanger are indirect only; soot build-up, external cracks, a strange smell coming from a furnace, etc.

The most important thing you can do is adequately detect the furnace heat exchanger crack symptoms. We will look at 6 symptoms of a cracked heat exchanger (both primary and secondary; in the case of high-efficiency gas furnaces). You can go one by one and check if your furnace actually has a cracked heat exchanger.

symptoms of a cracked heat exchanger
This is how a furnace heat exchanger looks like (with copper tubes). Even a small crack in 1 of the multitude of tubes can cause serious problems.

If you detect that a heat exchanger is bad, you have to immediately shut off your furnace. We will also explain what to do in the event of a heat exchanger (professional assistance, replacing the heat exchanger vs replacing the furnace entirely).

Note: It is absolutely necessary for you to have a working carbon monoxide detector. In the event of a cracked heat exchanger, it’s the best alarm system you can possibly have.

To adequately detect the symptoms of a cracked gas furnace heat exchanger, we need to first understand what a heat exchanger is and what does it do. Let’s start with looking at exactly what is the function of a heat exchanger before we check the 6 most common symptoms of a bad heat exchanger:

What Is A Heat Exchanger?

A heat exchanger is a furnace part that transfers the heat generated by gas combustion to air. In most simple terms, this is the very part that heats the air.

Gas furnaces can use several types of heat exchangers like shell and tube heat exchanger, plate and shell heat exchanger, and finned tube heat exchanger. They all have the same working principle.

In most cases, the heat exchanger is basically a set of tubes with repeated loops. All these tubes are made out of metal like copper and nickel.

Here is how heat exchangers work in a furnace:

  1. In the combustion chamber, gas (natural gas, propane, even oil) is combusted. This creates extremely hot flue gases.
  2. Hot flue gases travel via heat exchanger’s tubes and heat the metal.
  3. On the other side of the metal, the cold indoor air comes into contact with the heated metal which heats the air.
  4. In standard-efficiency furnaces, you only have 1 heat exchanger (primary heater exchanger). About 70-80% of heat is transferred to air via the primary heat exchanger and the rest of the gas flue (containing dangerous gases like carbon oxide (CO), sulfur dioxide (SO2), and nitrous oxide (NOx) are safely released outdoors via the exhaust vent.
  5. In high-efficiency furnaces, you have 2 heat exchangers (primary and secondary heat exchangers). The primary heat exchanger transfers 70-80% of heat and the secondary heat exchanger transfers an additional 12-18% of the heat to the air. The rest is safely disposed of via the exhaust vent.

Now, with a cracked heat exchanger, not all of the generated flue gas will be safely disposed of via the exhaust vent. That’s why a crack in the heat exchanger will start leaking the dangerous flue gases out of the furnace and inside your home.

Wear and tear is the most common reason for a cracked heat exchanger.

Note: Crack in the primary heat exchanger is the most dangerous. That’s where most of the heat is exchanged and where the pressure of the flue gas is the highest. Crack in the secondary heat exchanger is still serious; there the pressure is lower and the gas condenses. A faulty secondary heat exchanger drainage is also the most common reason for furnace leaking water.

Here’s the key:

If you have a cracked heat exchanger, you have to detect it. The real question is this:

“How do you actually detect a bad heat exchanger?”

This leads us to check for the most common symptoms of heat exchanger being cracked:

6 Cracked Heat Exchanger Symptoms

Symptoms of a failed primary or secondary heat exchanger can be, unfortunately, rather subtle. We have to adequately check for all the potential signs of having a cracked heated exchanger in our furnace.

That’s why regular furnace maintenance and inspection are so important. The key to checking for signs of a bad heat exchanger is to detect the crack early on and fix it accordingly.

To help you out, we are going to list the 6 most common furnace signals that point to a cracked heat exchanger (with explanations):

#1 Soot Or Black Carbon Build-Up (Cracked Heat Exchanger Leads To Incomplete Combustion)

A cracked furnace exchanger leads to incomplete gas combustion. Incomplete combustion will generate small or large amounts of soot (depends on small or big heat exchanger, time of running a gas furnace after the crack was made).

Eventually, you will start seeing a black carbon buildup within the furnace. This is the first thing that is noticed during furnace maintenance when suspecting a cracked heat exchanger. You will see a thick layer of soot; a clear sign of incomplete combustion and a symptom of a cracked heat exchanger.

If you want to check for soot inside the furnace, you have to first disassemble the furnace. This usually means you have to remove all the key parts to get to the gas chamber.

It is not recommended for you to do this alone. A certified furnace professional will know how to disassemble and, more importantly, correctly reassemble the furnace and check if there is a soot build-up in your furnace.

#2 Cracked Furnace (If Outside Is Cracked, Changes Are Heat Exchanger Is Cracked)

The simplest way to visibly detect a heat exchanger crack is to check the entire framework of the furnace.

If your furnace is cracked on the outside, chances are the internal parts are not intact as well. This also includes issues with the heat exchanger.

Why does a heat exchanger crack?

Actually due to simple wear and tear over the years. Gas furnaces can last for 15-20 years. However, during that time, they are exposed to severe stresses that can cause cracks prior to a supposed lifetime of the furnace.

As we know from earlier, the heat exchanger is made out of metal (mostly copper). It has to handle extreme changes in temperatures; handling anything from the extremely hot gas flue to temperatures below room temperature.

These temperature swings extract and contract all components of a furnace. If these temperature changes cracked the exterior of the furnace, there is a very good chance that the heat exchanger – the part that has to deal with the most extreme hot/cold swings – will be cracked.

#3 Corrosion (Symptom Of A Failed Secondary Heat Exchanger)

Corrosion is a chemical reaction between a metal and, in most cases, water. Corrosion is basically a sign of prolonged moisture and can be one of the symptoms of a failed secondary heat exchanger.

Let’s explain this one:

Standard-efficiency gas furnaces only have the primary heat exchanger. They vent out warm flue gas. High-efficiency gas furnaces, on the other hand, vent out cold flue gases. That’s because the heat is additionally extracted from the flue gas inside the secondary heat exchanger. That’s also where the condensation happens due to colder gas.

If the high-efficiency furnace operates as it is supposed to, the PVC drain pipe will adequately drain the accumulated condensate and this water will be disposed of.

However, if anything goes wrong with either the drainage system or, more seriously, the secondary heat exchanger – a crack would be the worst – the moisture will, in time, start to corrode the exterior of the furnace.

If you see corrosion, this may be a subtle sign of a cracked secondary heat exchanger.

On top of that, a crack if either the primary or secondary heat exchanger can release chloride gases (part of the gas flue). This chlorine gas is not only dangerous to humans, but it will also corrode the furnace very quickly. A corroded furnace might be a sign of serious internal problems, like a heat exchanger crack.

#4 Furnace Smells Like Formaldehyde (Most Easily Detectable Symptom That A Heat Exchanger Is Cracked)

Most of the symptoms of a cracked heat exchanger are indirect and barely noticeable if you don’t know what to look for. One of the most noticeable ones, however, is a bad smell coming from a furnace.

In the event of an issue with a heat exchanger, a formaldehyde-like odor might start coming from your furnace. This is a very distinct furnace smell, very closely related to a crack in the primary heat exchanger.

Detection of the furnace smelling like formaldehyde can be direct and indirect. If you are lucky, you will immediately detect the distinct smell of the formaldehyde and know to suspect a cracked heat exchanger.

If you’re unlucky, you might start experiencing unpleasant bodily symptoms. Headache, for example, is one of the symptoms of exposure to formaldehyde.

If you can detect a formaldehyde leak (both directly or indirectly), you should immediately shut off your furnace and call a certified furnace professional to help you out. You should, of course, mention the formaldehyde odor which will point the professional to the one clear culprit: a cracked heat exchanger.

#5 Furnace Leaking Water (Unlikely Sign Of Heat Exchanger Crack But It Can Happen)

As we have covered in an article about ‘Why Is My Furnace Leaking Water?’ here, there are several reasons why this might be happening. In more than 80% of cases, we see this in high-efficiency condensing furnaces. It is a nuisance but it mostly relates to drainage system problems.

In standard-efficiency furnaces (ones with only the primary heat exchanger, no condensation, and AFUE rating over 90), there should be no water anywhere.

If there is water coming out of a standard-efficiency furnace, there is a good possibility that the faulty primary heat exchanger is to blame.

In this case, it’s best to call an HVAC professional. It might be a cracked heat exchanger or some other serious issue, and it’s almost impossible to adequately diagnose it on your own.

#6 Unpleasant Physical Signs Especially When You’re Near A Faulty Heat Exchanger Furnace

Now, a cracked heat exchanger can leak all kinds of toxic gases inside your home. Gases like nitrous oxide and sulfur dioxide can cause a very diverse negative effects on humans.

This range from headaches, nausea, nose and eye irritation, and even insomnia. These are very non-specific indicators that you may have a cracked heat exchanger.

The key part is this:

These potential side effects of harmful gases from your furnace only occur when you’re near a furnace or in your home. As soon you’re out of the door, the symptoms cease because you or your family members are no longer exposed to harmful gases.

If you suspect your furnace might be at fault, it’s better to schedule immediate furnace maintenance.

What To Do If You Suspect A Heat Exchanger Crack?

The best precaution, of course, is regular furnace maintenance. An annual or even biannual maintenance might be costly but it can prolong the lifespan of your furnace and reduce the likelihood of unwanted and harmful consequences of a cracked heat exchanger.

If you do suspect a crack in your heat exchanger, you should call a professional immediately. Don’t delay. A cracked heat exchanger is not an easy-to-solve issue.

Here are 3 scenarios of what to do in regards to heat exchangers:

  1. In the best scenario, there is nothing wrong with your heat exchanger.
  2. Your heat exchanger is cracked. If you have a new furnace (less than 10 years old), a professional will most likely recommend you to replace the heat exchanger. The average cost of replacing the heat exchanger is from $400 to $1,500. That includes labor and is a financially good investment.
  3. Your heat exchanger is cracked. If you have an older furnace (10+ years old; with below 90 AFUE), it’s probably best to think about replacing the whole furnace. Modern furnaces have higher energy efficiency (you can read more about high-efficiency gas furnaces with a 90+ AFUE rating here). These furnaces will have lower running costs and make for a smart investment, considering your current furnace only has a few years left (15-20 years is an average lifespan for a gas furnace).

In all cases, if you need help, you should turn to your local HVAC company for advice. You can also fill this form here, and you’ll get up to 4 free quotes from HVAC experts in your area on the matter.

Again, it is essential for you to have your carbon monoxide detector working. It’s your first line of defense against a pierced heat exchanger.

We hope all of this was helpful.

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