My Heat Exchanger's Cracked.... Why Should I Care?

We can see the light at the end of the winter tunnel.  There is still a little bit of snow on the ground from last week, but the warmer weather and rain from the weekend has almost erased all signs of winter from the landscape.  The brown grass is just about the only evidence this isn't a late spring or early fall morning.   So why am I thinking about heat exchangers?  Well, we've been dealing with a lot of them recently and it has become obvious to me that there is a lot of confusion and misinformation when dealing with this subject.  

As a homeowner, getting the news that your heat exchanger is cracked is a bit of a harsh blow. It's not unlike a doctor telling you about the "C" word.  You know it's serious and you know you need to do something about it.  Some people choose to live in denial of the seriousness of a cracked heat exchanger.  They observe that their furnace is running just fine and was doing so before we arrived and will most likely continue doing so after we leave.  The reality is that their furnace may be presenting a very real danger to their home and family and they need to address it before something terrible happens.  

It isn't hard to see these cracks - Clamshell Style HX

It isn't hard to see these cracks - Clamshell Style HX

I'm not going to say that every cracked heat exchanger is a life or death emergency and that it is spewing levels of Carbon Monoxide into your home that can cause sickness or death.  I am, however, going to say that every single cracked heat exchanger does need to be replaced, either through a complete furnace replacement or changing out the heat exchanger.  If I can go back to my cancer analogy, you would never expect a doctor to say "Yeah, we found some cancer, but it's really in a non-critical area and you're feeling just fine, so we're not going to do anything about it.  Go on living your life and we'll deal with this when it really becomes a problem." While you might not get rushed into emergency surgery to remove the offending tumor and surrounding tissue, you can be assured that there will be a treatment in your future.  In the same way, a cracked heat exchanger needs to be replaced.  There is no repairing it, and ignoring the problem simply isn't a solution.

So let's get back to basics for a minute here and talk about what a heat exchanger is and what it does.  At its simplest level, a heat exchanger allows for the exchange of heat between two fluids without the fluids mixing.  In the case of a forced air furnace, it is two different air streams. The return air from the home and the hot exhaust from the combustion of gas are seperated by sheet metal or tubing that typically snakes back and forth to allow for the maximum amount of air to pass over it and the most heat to be exchanged between the two streams.  

Other types of heat exchangers exist in the HVAC world as well -- we can have refrigerant and air in an air conditioning or heat pump system, or air and water, like in a hot water boiler.  The reason we want to keep the fluids seperate is obvious in the other examples - we don't want our refrigerant to leak out of an air conditioning system so we keep it within the coils and linesets and we don't want water leaking out of our boilers because that would be a hot mess.  Why do we care about keeping the combustion air and house air separate in a furnace?  What's the big deal, anyways?  

Tube Style Heat Exchanger Example

Tube Style Heat Exchanger Example

Inside your furnace or boiler there is actually a fire burning, not unlike your oven.  Natural gas or propane is being burned and the byproduct of that process is energy in the form of heat, along with varying amounts of water vapor, carbon dioxide, as well carbon monoxide, pure carbon (soot) and some Nitrogen Oxides (NOx).  If you haven't already decided you wouldn't like those things to be in your house in any great quantities, let me tell you that almost none of them are desirable in large quantities in your home and the air you breathe.  With the exception of some additional water vapor in the home during the winter months, CO2, CO, soot, and NOx are certainly what I would call pollutants and all of them can cause sickness or be fatal in high enough quantities.  So why aren't we worried about ovens and stoves inside our homes?  Two reasons for that - they aren't typically used enough or have a high enough output to worry about the levels they create, and you're supposed to use a vent hood when you cook and bake.  Everyone does that, right?  So, we can all agree that we want our heat exchanger to keep the bad air away from the good air we breathe.

So why do heat exchangers crack in the first place?  They're not supposed to, right? Heat exchangers are designed to provide safe operation of the furnace during its life expectancy - typically 15-18 years.  However, there are many factors that can accelerate the wear and tear on a heat exchanger that they fail much sooner, sometimes in spectacular fashion.  So what causes them to fail?  

Sometimes it's just age and use.  Where we live, furnaces see nearly 900 hours of use every winter.  If we assume the average heating cycle is 15 minutes long, that heat exchanger is heating up and cooling down over 3,000 times per year.  Try this experiment - take a paper clip, stretch it out and see how many times you can bend it back and forth before it breaks.  So, sometimes heat exchangers just wear out with time and age and need to be replaced with a new furnace.  Sometimes that's 15 years, sometimes it's 20 and sometimes they never crack and have other expensive repairs or the efficiency gain is enough to warrant replacment of the furnace.



More commonly, the cause of a cracked heat exchanger is misapplication or poor maintenance of the furnace.  All too often, furnaces are grossly oversized or used incorrectly which causes the heat exchanger to fail prematurely.  Think of our paperclip example again.  This time fold it completely in half for each "cycle" and see how many times you can do it.  I promise you it will be far fewer times.  When a furnace is oversized, a number of problems can exist including short cycling and cycling on limit.  Both of those cause the heat exchanger to heat up and cool down far more often than if the furnace was closer to the proper size for the home or, better yet, was able to use a lower stage or modulate its heat output for smaller loads.

Another heat exchanger killer is airflow, or more specifically, a lack of it.  The primary cause of low airflow in a furnace is a dirty filter.  It can be hard to remember to change that filter or at least check it every month (which is why we recommend an air cleaner that can go for much longer without changing) If your filter gets clogged up with dust and dirt and the air simply can't get through, your furnace will cook itself to death.  The heat simply won't be able to get out and the furnace will likely cycle on its high limit until it ceases to function or you change the filter.  Poor airflow can also be the result of poor ductwork design or installation or simply not having enough ductwork to support the size of the furnace.  Meanwhile, your heat exchanger will go through a lot of stress when it is constantly overheated and cooled back down.  Lack of airflow also leads to much longer heating cycles, so not only is the heat exchanger hotter than it needs to be, it also has to endure the pain for a great time.

Sometimes it's easy to see the crack in the heat exchanger and understand why it's a problem, but what about when cracks are just forming or what if it doesn't seem like it's a critical area of the heat exchanger?  It is our opinion and one shared by the American Gas Association (AGA) and AHRI that any crack, hole or other failure of the heat exchanger is reason to replace the heat exchanger or the unit.  Sometimes the cracks like to hide like these:


But we can see why these are a cause for concern when we simply shine a light inside the heat exchanger and see this:

photo 2.JPG

Our recommendation on finding a crack in a heat exchanger will always be to replace the heat exchanger or the furnace it is in because your family's health and safety is far too important to make guesses and assumptions about how dangerous this crack may or may not be.  It's like the doctor that finds cancer - it might not be good news, but it needs to be shared and dealt with.

If you would like to have your furnace inspected and cleaned it's not too late in the season. Head over to our Specials page for money saving coupons too.