Carbon monoxide (CO) is a colorless, odorless gas that can kill. If your furnace tech says you have a cracked heat exchanger it doesn’t mean that your house is filling with carbon monoxide unless your CO detector is going off. If a gas furnace is burning clean and has a good supply of oxygen to the burner it could be that it’s not producing CO to unsafe levels. But never take that chance!

Carbon monoxide is created when fossil fuels such as natural gas or propane have incomplete combustion or don’t burn completely. Even though CO is colorless, odorless, tasteless and initially nonirritating, it will cause flu like symptoms and it can kill you.

I’ve gone to a lot of service calls to work on gas furnaces that had a rather caustic odor. The flame had that dirty yellow color like the second image below. I didn’t have a CO detector but I knew those products of combustion going up the chimney were not what you would want in the air you breath.

So, yes it is possible for you furnace to create CO and your CO detector not detect any CO. It could all be going up the chimney, unless you have a cracked heat exchanger. More about that as you read.

When your gas appliances are burning a nice clean blue flame they emit carbon dioxide (CO2) and water vapor (H2O). Two of the five main components of the air we breath. Be sure you understand the difference between CO2 and CO.

Carbon monoxide is created when a gas appliance is burning dirty. Instead of a clean burning blue flame with yellow tips, a burner will have a more orange colored flame

Understanding how carbon monoxide is created is a valuable way to help us all understand how to keep what they call the silent killer from stalking your hallways.

That really sounds a little dramatic but we saved the lives of an entire family once. They had been all but overcome by carbon monoxide poisoning when we arrived at there home to do service.

Carbon Monoxide Poisoning Has Flu Like Symptoms

  • Muscle Weakness
  • Dizziness
  • Dull Headache
  • Nausea or Vomiting
  • Confusion
  • Chest Pain
  • Upset Stomach

So if everyone in the house has flu like symptoms be very suspicious of carbon monoxide being present. Have more than one CO detector in your home and keep them up to date.

While doing a lot of research on the internet I noticed a lot of misinformation about CO. You don’t have to look far to find someone, mostly heating contractors, saying, that if you have a crack in your heat exchange you are filling your house with CO.

That’s not entirely true.

Like mentioned before the main products of combustion of clean burning appliances that burn natural gas, propane or fuel oil are:

  • Carbon Dioxide
  • Water Vapor

Notice that carbon monoxide is not in the list. But carbon dioxide and water vapor are two of the five main parts that make up the ATMOSPHERE OF EARTH

Nitrogen (N2)78.08%​
Oxygen (O2)20.95%​
Argon (Ar)0.93%
Neon, Helium, Krypton0.0001%​
Carbon dioxide (CO2) 0.038%​
Water vapor (H2O) 0-4%​
Methane (CH4) trace
Sulfur dioxide (SO2) trace
Ozone (O3) trace
Nitrogen oxides (NO, NO2, N20) trace

VIsion Learning

Be aware of the heat exchanger scam. Some companies will claim you have a cracked heat exchanger to get the replacement job.

If you are being told you have a cracked heat exchanger get another opinion. Look for those companies in your area who will give a free second opinion.

A lot of companies would jump at the chance to get your work and do it honestly.

So alarmists will tell you that a cracked heat exchanger is filling your house with carbon monoxide, but now you know a little more than they think they know or are willing to admit about carbon monoxide .

That doesn’t mean when your heat exchanger is cracked that you don’t need a new furnace. It just means you shouldn’t get unnecessarily excited. After it’s been cracked for how long?

A closed up home will fill with dangerous levels of carbon monoxide if it is being created. It’s obviously a big concern if it’s the dead of winter and you’re stuck with no other source of heat.

That’s why maintenance done in seasons other than in winter is so important. I can’t say this too often, have more than one up to date CO detector or be on your way to get some.

CO detectors in your home need to be in locations that you will be sure to hear when sleeping. Don’t depend on your dog or cat to wake you.

How Do You Know If Carbon Monoxide Is Present?

Carbon monoxide detectors for the home have been mentioned (repeatedly) and they do an excellent job of detecting for the open areas of your home, but they are meant to be used at a distance from the source of appliances that have the potential to produce CO. Not as a detector of the source.

In other words, residential CO Detectors that you buy at Home Depot or Ace Hardware are not tools to wave in front of your gas furnace or appliance to see if they are leaking CO.

If you would rather check at the source for CO then there are a few detectors that are made to be used at the appliance or furnace if you will. These are available on Amazon and range in prices depending on brand and functions. You can take a peek here if you feel so inclined.

Pro-Lab makes a DIY test kit for less than 15 bucks that claims to be more sensitive than in home detectors. Amazon sells it. The reviews are not OK. Seems you need to be aware that product could be over expiration date or that you may need to pay an additional $40 to have your kit tested.

What level of CO is acceptable

As CO levels increase and remain above 70 ppm, symptoms become more noticeable and can include headachefatigue and nausea. At sustained CO concentrations above 150 to 200 ppm, disorientation, unconsciousness, and death are possible.


Still more sophisticated equipment is available for testing and to locate the source of any CO. A significant investment would be required which makes it more reasonable to hire the job out to a company who does CO testing on a regular basis.

If your current service company hasn’t invested in CO detecting equipment or tools then you should call around and find one and get prices.

You do have options. Perhaps your supplier of fuel has a service department that will check for CO.

Does A Cracked Heat Exchanger Leak Carbon Monoxide

Depending on the type of heat exchanger and where the crack is located on the heat exchanger itself. Sensing a Even which way the crack is open to the air flow.

Some gas furnaces have an inducer motor that assists the flue gases through the heat exchanger to the flue pipe. The flue pipe should do the rest of the job of exhausting due to the natural draft it should have.

So the answer to does a cracked heat exchanger leak CO is maybe. Too many maybe’s to take chances on.

Lets not bet our life on it.

Can I Run My Furnace With A Cracked Heat Exchanger?

You say your technician shut off the gas to your furnace because your heat exchanger is cracked and now you want to turn it back on because it’s cold and you don’t have the cash for a new furnace or even a repair.

Presuming that the furnace in question is creating carbon monoxide, the answer is, are you dead sure you want to turn it back on?

You might as well do your barbecuing indoors. Briquettes create CO until they are completely out cold.

Go out right now and find a good meter and test it until you’re sure.

The heat exchanger below was in a business. Nobody complaining about symptoms related to CO poisoning. The blower in the furnace was situated in a way that it blew right into that big crack that you see. It blew the flam right out. The cracks are not always in the same place and don’t have the same reaction to the blower coming on.

The blower can have no reaction to the flame sometimes depending on where the crack is. The blower can actually suck on the flame. Those are the dangerous ones if CO is being created.

Residential gas furnaces come with basically two types of heat exchangers, clamshell and tubular. Again, the location of the crack on the heat exchanger has a lot to do with whether or not CO goes into the airstream.

Is Flame Rollout A Symptom Of A Cracked Heat Exchanger

The rollout switch on the furnace above had tripped. When I reset the furnace to see the problem, the burner lit and the flame looked good, until the fan came on. When the flame came on the flame flashed back toward me. I shut it off and tore it apart, exposing the heat exchanger as you see it above.

You could do a search on google and see great examples of cracked heat exchangers.

This heat exchanger was under warranty and it was an extremely easy replacement.

The cause was lack of maintenance. Restrictive filters that were too small. The temperature rise across the heat exchanger was much higher than the furnace was rated for.

Temperature rise on most gas furnaces shouldn’t exceed 70 degrees. That’s the difference between the temperature of the air entering the furnace and the temp of the air leaving the furnace.

If this concerns you please read my post on what filter you should use in your furnace.

Remember. On some cracked heat exchangers that I have found, some only make a slight deflection on the flame and some make no deflection at all.

So it just varies as to where the crack is, which direction the crack cracked in relation to the air flow and the flue draft and the inducer motor and on and on.

Can I Fix A Carbon Monoxide Leak In A Furnace?

Of course you can. Replace the heat exchanger and address the problem as to why the original heat exchanger failed.

Their have been some faulty heat exchangers manufactured just like any other product.

If yours is faulty the manufacturer will most likely not supply a replacement or they have made an improvement.

This is one of those situations when you have to compare the age of the furnace with the cost of repair. Replacing a heat exchanger under warranty is best if you have that option.

Some are very difficult to replace and will consume a lot of labor expense.

If you replace the heat exchanger out of warranty what will fail next? A fan motor or an inducer motor?

You could be throwing money away when compared to a new furnace with a heat exchanger that would be completely under warranty for probably the next 20 years or even lifetime. And the other parts for several years also.

I’ve been asked if the crack could be welded. In most cases absolutely not. In a few cases maybe, but you’re dealing with distressed metal. Weakened metal, by overheating, expansion and contraction over years of use and it just doesn’t make sense to risk the obvious.

It would probably crack again, if not in the same place then in another. These heat exchangers are subject to being heated and cooled over and over again. That expansion and contraction is the core of their demise.

How Much Should It Cost To Replace My Heat Exchanger?

I have to admit the heat exchanger pictured above only cost my customer right at $500 to replace under warranty.

It was such an easy job. An 90% horizontal installation. Stand up and working at eye level. Just remove the gas valve and the panel which held the heat exchanger.

Then put the new one in place, put the gas valve back on and a little PVC pipe work and it was done in about 4 hours.

If your unit is under warranty, then labor only should be under 8 hours at a cost of anywhere from $75 per hour to well over $100 per hour. Probably more now with inflation and covid.

If you have to add in the price of a heat exchanger plus shipping you could be adding another $1500 to $3000.

Quite the range, but it’s all dependent upon the type of heat exchanger, the manufacturers markup and the markup of the dealer.

Shipping can be very expensive.

Prices vary immensely even within the same cities or areas.

As with any expensive repairs you should get a couple of bids. Ask pertinent questions such as do you check for temperature rise and static pressure. If they say no or it’s not necessary then get another quote from another company.

In Related Topics: CO Has Some Useful Applications

  • In fuel gas mixtures for metal fabrication
  • Chemicals such as acids, alcohols and esters us CO
  • Reduction of ores to manufacture metal carbonyls
  • Very pure CO is used in semiconductors and electronics

This one is scary. There seems to be demand for fresh muscle foods and to extend their shelf-life. Evidently the food packaging industry developed a way to use CO for meat and seafood packaging. But, it is not allowed in many countries. Let’s hope ours is one of them that it’s not allowed.

You’ll really like this article by Science Clarified They give some history on carbon dioxide, some words to know and some physiological effects relative to our topic here.

It was widely used as one of the components of industrial and household fuels which empowered the Industrial Revolution before other sources of fossil fuel could be widely exploited. It was made by alternatively blowing air then steam over red hot coals. The coals plus air is exothermic while the coals plus steam is endothermic, so you could maintain the coal embers at a fairly steady level. The combustible products were a mixture of hydrogen and carbon monoxide (about 50/50). It was called “water gas” and converted the solid fuel coal to a gas fuel which could be distributed by pipeline. It is, of course, very toxic, and it also burns with a very noisy flame. I can remember as a lad that you always knew if the oven was on because of the noise it made, and talking in a school chemistry lab with Bunsen Burners on was near impossible. When North Sea gas was introduced in Europe a mssive conversion of houesehold gas appliance burners was necessary because of the radically different burning properties of gas containing a high hydrogen content. It was also used to fuel vehicles. I can remember public vehicles with a water gas generator on tow during WW2.

Clive Holloway, former Full Professor at York University (1968-2003)

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  1. Had tech out to service my natural gas furnace today. Furnace original to house built in 1999. Was informed that there were 4-5 cracks in heat exchanger. Tech determined that the unit was emitting a carbon monoxide read of 0-5 ppms . Would not condemn the unit, said that it was not servicable not repairable, strongly recommended that it not be used the rest of the season. Live in Idaho, still have March and April winter levels of weather to look forward to. Very naive about this appliance and subject matter. Am I really at the point of the having to fully replace the unit.

    1. Ruth, I’d get a second opinion. I don’t know the age or model. Some warranties for heat exchangers go up to 20 years or so. Get your model number and serial number and do some research. A lot to consider here. In some areas it’s necessary to condemn a furnace with a cracked heat exchanger. I’d be real careful about sleeping with the house a shut up at night. Sure the ppms count is low. I’d just be careful. Chad

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