Cracked Heat Exchanger

I have a couple of furnace questions that I would appreciate some help with:
(1) Is it possible to have CO leaking from a cracked heat exchanger on a high efficiency furnace? The induced draft fan keeps the H/E interior at a negative pressure. It seems to me that this would cause air to leak into the H/E rather than gases to leak out.
(2) For a standard efficiency furnace, have there been any fatalities caused by a cracked H/E. I did some research in Canada and couldn’t find any. There have been numerous CO related deaths due to blocked venting, cars left running in garages, barbequing indoors, etc but none that I can find due to a cracked H/E. Has there been any documented cases in the US?

Hi John, I believe that Co can still easily be drawn into the supply air due to the venturi effect within the plemumn, the draft fan does not produce a huge ammount of draft.

John I am not aware of any deaths associated with CO from this sourse however I have personally know of 2 instances of sickness caused by this issue, IMHO death is a real possibility.



Here are a couple I found in the last couple of weeks.



One factor that we should all keep in mind is the possibility that most “older” homes are not as “air tight” (windows/ doors/ walls) as newer homes with high efficiency cat 4 condensing units. You have to also consider the possibility that CO detectors and areas of the country that do not rely on gas / propane heating as much also affect statistics. Older homes in heating areas like in my area (midwest) have more Cat 1 natural draft units installed in basements, enclosed basements and closets where CO generation is more likely then cat 4 high efficiency units with combustion air coming from the outside via PVC piping. This is not to say that all sources of CO come from your FAU. The kitchen range, water heater, garage (car) and some space heaters, fire places are other sources…

So what does my city do? Tell everyone to get a CO detector! Save lives! Doesn’t take away the fact that “their” installation is potentionally dangerous but to have a warning when the concentration is in the range of the EPA or other governmental organization that sets standards states so… Deaths occur every year around here…

Check UL, insurance companies websites like State Farm and you should find numbers…

Yes it is possible to have carbon monoxide leak into the home from a heat exchanger in a inducer draft fan furnace. The pressure with in the heat exchanger is still greater then that out side it until the blower kicks in and even then the risk of pulling C02 in to the house air is present.

Yes deaths have occurred here just a few

Here’s one from About Homes. It is the best web site I’ve found for current news on CO deaths and injuries. They update it on a daily basis.


If that site doesn’t make one sit up and take action, nothing will.

Here’s a very informative thread that was posted last year…

Thanks everyone, but unless I missed it, none of these deaths were directly attributed to a cracked furnace heat exchanger. Please repeat the link if I did miss it. Thanks again.


I know there is at least one because I found it over there before. That was last December that I was looking. Their list is very extensive and you have to have a lot of time to read through every post to find that one item.

First of all furnaces do not produce CO, they produce CO2.
IF they are burning properly and getting ample make up air.
Secondly, a crack in a heat exchanger can’t let hardly any gases pass through because the opening is so small plus the pressure from the blower is much greater then the pressure inside the heat exchanger. (assuming “crack” doesn’t really mean “large hole”)

That’s not to say that a cracked heat exchanger is not potently very dangerious because it is.
As an example, say the flue starts dumping CO2 in the furnace room for whatever reason and rather than good clean make up air, the CO2 starts getting drawn in, mixed with the fuel and gets burned in the place of oxygen, the second time through the burning process produces CO.

That same thing is what happens when there is not enough makeup air at the furnace or the flue is not drawing.

That’s a dangerous statement, even with the “IF.”

Anything that burns organic fuel (such as wood, oil, gas, etc.) will produce carbon monoxide because it is a natural byproduct of the burning process. The only time that carbon monoxide would not be produced is if a system were 100% efficient. And 100% efficiency doesn’t exist on this earth outside of a laboratory, and I’m not sure that it exists in a laboratory either.

I think a Google search and a day’s worth of reading is in order.

First the Gas Research Institute 3 point inspection method states in their first step. Visual inspection: any visible crack requires replacement of the heat exchanger or the furnace.

In spite of the claims that a negative draft induced furnace will not allow carbon monoxide leak out I have found a couple of the exceptions. In spite of engineering claims nothing is 100 percent. A furnace heat exchanger is a curved surface air flowing across such surfaces can do some mighty strange things. One instance I wished I had a video camera. A visible crack above a burner section. Yet when the circulating blower came on rather than the flame blowing away from the crack it floated right up into the crack. Seeing that will happen make believe anything is possible.

Hi John
On my high efficiency furnace, the induced draft blower kicks in sometime before ignition. Therefore the H/E will be under negative pressure anytime the gas is on. Aren’t they all like that?

Most but not all. Comfortmaker at one time had one model of their furnaces that had a combustion air blower blowing air into the burner box. Rheem Ruud roof top units also use a small combustion air fan that blows air into the burner box.

As I stated in the heat exchanger inspection methods: One method does not fit all furnaces. You have to choose the inspection method for the type furnace and heat exchanger.

1). Furnaces using a draft inducer are still injecting large amounts of gas and air into the heat exchanger (around 30 cuft of air per 100,000btu of gas) this neg pressure is relative to the air around it and it is still at times greater then the air out side the heat exchanger and it will leak This is a serious and dangerous issue, leaks / cracks are dangerous. No matter what type of furnace.

The issue is when the blower kicks in and introduces additional air into the heat exchanger through the crack, that combustion is effected and the possibility of carbon monoxide exits,.

2). Furnaces that are operating normally will produce Carbon Dioxide, when combustion is effected so it is not burning properly they it can produce Carbon Monoxide. A furnace with a crack may burn clean and not produce C02, but it still must be replaced.

If you dig down deep into the chemistry of burning organic fuels, I believe you’ll find that both carbon monoxide and carbon dioxide are produced as byproducts during any burn that is not 100% efficient. And nothing that we use in our homes is 100% efficient. Carbon monoxide also reacts with oxygen and/or moisture (H20) in the air to produce carbon dioxide.

It is dangerous for anyone to state that carbon monoxide is not produced by burning organic fuels. Just do a Google and a little reading; there’s enough information out there to convince most people to go electric.

Here’s just one reference that is out there on the Internet:

**Chemical Reactions and Reaction Products **
An understanding of chemical reactions and reaction products is important to this activity. Several of the questions deal with the production or burning of fuels. In both cases, chemical reactions and reaction products are involved. In the burning of fuels, the fuel reacts with oxygen, and the resulting products are of great importance as they may be benign (like water from burning hydrogen) or they may be hazardous (like carbon monoxide from burning organic fuels). This result must be taken into account when considering alternative fuels. The production of fuels also involves chemical reactions and their products. There are several methods of producing hydrogen, a potential fuel for both fuel cells and internal combustion engines. While the use of hydrogen as fuel produces only water as a reaction product, not all methods of hydrogen production are so harmless. Hydrogen can be produced by the electrolysis of water, essentially the reverse process of that which acts in fuel cells. However, methods that produce hydrogen by breaking down hydrocarbons also are being proposed for large-scale production. These processes produce carbon dioxide as a waste product, which is a greenhouse gas. Clearly, understanding reactions and reaction products is central to sorting out the issues involved in the quest for long-term energy solutions.

Organic fuels are carbon based (the C). Carbon (the C) is the sixth most abundant element in the universe, being in many different compounds, such as food, clothes, cosmetics, and, of course, fuel. It has a melting point of 6422°F and a boiling point of 6917°F. So when burning organic fuels, carbon will be produced since it’s not going to be burned because nothing we do outside of a laboratory, a huge fire, or a nuclear reactor will be hot enough to burn the carbon. And yet carbon is used as control rods in nuclear reactors, as well as added to clays to form the “lead” in pencils, added to iron to make steel, etc.

Most (if not all) of our fuels are in the methane family, which are various combinations of carbon and hydrogen (methane - CH4, ethane - C2H6, propane - C3H8, butane - C4H10, etc.). In burning them, combine the fuel with oxygen and one gets a very powerful reaction (think the Zeppelin, which was full of hydrogen gas). In a complete burn (100% efficiency) two oxygen atoms will combine with the one carbon atom to produce carbon dioxide (CO2) and water (H2O). Incomplete combustion (anything less than 100%, even 99.99%), will produce trace amounts of carbon monoxide (CO) as well as carbon dioxide and water. You’d have to write the chemical equation to see how this works in a simple case, and then balance that equation to see how complex it can be depending on the efficiency of the burn.

The most important fact, however, is to always remember that burning organic fuels—in other words, burning anything in our homes—produces carbon monoxide. With appropriate ventilation, knowledge, and caution, it rarely becomes a problem. However, a quick read of the daily news at
will show you just how dangerous carbon monoxide is.](“”)

I apologize that you misunderstood my statement, I did not mean to suggest that furnaces can burn clean or 100% efficient, all heating appliances do have a byproduct of combustion, nor did I mean to suggest that carbon monoxide is not present in some level on a properly working furnace.

What I meant is that as long as the measured levels of CO are within the published norms then all is ok. A working furnaces produces primary carbon dioxide.

See these two good references

Both references simply reinforce what I posted.

Oxygen is needed for combustion, and when there is not enough oxygen to combine with the carbon to produce carbon dioxide (CO2; carbon and two oxygens), then one gets carbon monoxide (CO; carbon and only one oxygen). One can never fully get rid of carbon monoxide emissions when burning organic fuels because nothing we have is 100% efficient, but one can reduce carbon monoxide emissions to very, very low levels, all things considered.

The fact that burning organic fuels produces carbon monoxide as a byproduct, even if at every low levels, is why the gas company, and my company, recommend the installation of carbon monoxide detectors in all residences, particularly those with gas-burning appliances and fireplaces, and an annual inspection/servicing of those same gas-burning appliances and fireplaces. Unfortunately, rarely is that done, which is why the news at
goes on and on and on.