Myth #1 - A furnace with a cracked heat exchanger will definitely produce carbon
monoxide and poses an immediate danger. (Wrong!)
Residential 80% furnaces made in the last 30 years with air-conditioning coils on
the discharge (outlet) of the furnace do NOT allow carbon monoxide to enter the indoor air stream.
Any person that insists that the gasses from inside the heat exchanger are going to get into the
home's air stream because of a hole or crack is simply repeating what they heard someone else say. They've never
tested this and they don't understand how air flows through a furnace. Their information is not based on fact. If combustion
gasses could move through a heat exchanger crack to the indoor air stream, it would defy the laws of physics.
(See thecautionary note about 90% units.)
My concern is that everyone concentrates on the heat
exchanger in the furnace and ignores or overlooks the circumstances in a home that actually
cause carbon monoxide poisoning. Those issues have NOTHING to do with the heat exchanger,
cracked or otherwise. Simple things like blocked or broken flues, open return-air panning or open return-air ducts
near the furnace or insufficient combustion air are the causes for the majority of carbon monoxide issues.
Howard Leonard is
president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and
application training for service technicians. He can be reached at 602-943-2517.
Sidebar: CO Article Correction
Information was added to the CO article [“Hidden Problems Can Cause Carbon Monoxide,” October 10, page 10] that needs to be
clarified. Cracked heat exchangers don’t cause CO. This is a
common misunderstanding in our industry. Cracked heat exchangers cause one of two
1. Pushing more air into the
combustion chamber; or
2. Pulling more air into and
through the combustion chamber.
Although both of these situations, for other reasons, are a hazard, in either
case, the amount of secondary air is increased, which leans the flame out and reduces CO.
Publication date: 11/26/2001
Note that the article is dated November 2001, so this isn't "new" information. It's been known
since the invention of forced-air furnaces.
Very few, if any, furnaces with a typical small crack or hole in the heat
exchanger show elevated levels of carbon monoxide. Even if the carbon monoxide levels become elevated, the CO
goes up the flue with the rest of the products of combustion. (See this Laclede Gas FAQ, question 8.)
Here's an excerpt from an October 1, 2004 Contracting Business Article titled
"Carbon Monoxide: Let's Stop The Madness!" that clearly makes the case that cracked heat
exchangers do not make carbon monoxide because of the cracks. Read the article because it highlights a number of
areas about carbon monoxide and is directed at the technicians who service gas equipment.
chairman and CEO of National Comfort Institute, a national training, and certification organization focused on
air diagnostics, CO and combustion, and IAQ. He can be reached at 800/633-7058, atdomg@
I would like to know who started this myth. CO is produced as a result
of incomplete combustion. A cracked heat exchanger might allow products of combustion to enter the air stream.
In some rare cases, it can cause flame impingement when air from the blower enters the heat
I'm not advocating that we shouldn't replace defective furnaces — we should. The point is: incomplete
combustion is what produces CO, not cracks.
MYTH # 2
is a passing grade — kind of like getting a D on an exam. Code doesn't provide protection for every situation.
We've seen thousands of installations that passed code with flying colors, yet producing obscenely dangerous
levels of CO.
MYTH # 3
not enough room on this page to cover this pervasive myth, soI'll challenge you to prove or
disprove this one for yourself. There are a number of other myths about CO.
Symptoms of a Cracked Heat Exchanger
A heat exchanger crack in a residential furnace large enough to cause trouble
usually upsets a steady flame or causes the burner flames to roll out the front of the burner openings.
This can cause a fire or burned wiring and should trip the "rollout switches", which are temperature sensors
located near the front and above the burners.
If the crack or split is really big, it may keep a burner from lighting correctly (or blow it out) and
cause a delayed or concusive ignition. In really bad cases the concusion can blow the access panels off the front
of the furnace. On older furnaces with standing pilots, a big crack might cause the pilot flame to be blown out,
which turns into a no-heat situation.
Many times, smaller cracks cause the burner flames inside the heat exchanger to waver back and forth and be blown
against the inside of the heat exchanger. At this point, the carbon monoxide output of the flame is raised.
But, unless something else is wrong with the furnace, the additional carbon monoxide simply goes up the flue
with the rest of the products of combustion. It never enters the home or poses a danger to the occupants. It
will affect furnace efficiency but it doesn't get inside the home.
A heat exchanger is a metal
boxor set of tubes
inside anothermetal box (the outside cabinet of the
Having said all that, anytime the crack or hole is large enough that it forces the
burner flames out the front of the heat exchanger, it is a dangerous situation that should be corrected
immediately. This is really what the heat exchanger inspection is all about - to make sure that the furnace is
What is a Heat Exchanger?
A forced-air gas furnace (upflow or horizontal) is a metal cabinet that has
another, smaller metal box inside of it. The cabinet is usually insulated and designed so that the outer
surfaces are always cool so that it can be placed next to combustible wall surfaces without creating a
The smaller metal "box" inside the furnace cabinet is the heat exchanger.
(See the figure above.) The heat exchanger is where the gas burners are located. The heat exchanger is
designed to be heated by the burner flames while the furnace blower moves room air across it. Under
normal conditions, when the furnace is running, the air coming into blower from the house (return air) is
generally 60 to 68 degrees F. As the air passes through the furnace, the heat exchanger warms the
air. The discharge air coming out of the furnace and moving through the duct work will be somewhere between
40 and 90 degrees hotter than the return air.
The heat exchanger is what separates air flow of the fuel burning process from the
blower-driven air that moves through your house. This is a box-in-a-box arangement. When the furnace blower
is running, it pushes air through inside of the outer cabinet across the outside of the heat exchanger.
The force of the blower-driven air is much greater than the air pressures
inside the heat exchanger. As a result, any crack or hole or split results in air being pushed INTO the heat
exchanger. There is no way air will move from inside the heat exchanger to the indoor air stream.
This applies to residential gas furnaces with an air-conditioning coil mounted on the discharge. The same
theory does not apply to commercial furnaces, duct furnaces, roof top units or residential furnaces with an
open supply air discharge. They are configured differently and have different operating characteristics.
Temperature Rise - Why Should You Care?
The increase in temperature between the return air and discharge air is called
"temperature rise". This number is printed on the ratings tag (where the model and serial numbers are)
and is used by technicians to make sure that the proper amount of air is moving through the furnace. If the
furnace blower is moving too slow, the temperature rise will be higher than the rating. If the blower is
moving air too quickly through the furnace, the discharge temperature will be too low.
The temperature rise rating dictates the "recommended" and
safe range of discharge temperatures for your furnace. A high temperature rise puts additional stress on the
heat exchanger, results in higher energy losses through the duct work and can cause a lot of other
problems. If the temperature rise is too low, the discharge air might feel cool when coming out of a
register, areas of the house may be hard to heat or problems with condensation in the flue or on the inside
of the heat exchanger may occur.
The furnace blower is sized by the manufacturer to push a certain amount of air
through the furnace cabinet. That amount of air is based on the heat output of the gas burners and is a
design decision made by a manufacturer's engineering team. The design takes into account the "average" or
expected resistance of the duct work and the air-conditioning coil. They refer to it as total system
resistance. When you allow your air filters to get dirty or have an
air-conditioning coil coated with fuzz, those "averages" are out the window and the furnace is forced to
run at higher than normal temperatures.
Simplified Sequence of Operation.
On a call for heat from your thermostat, the furnace ignition system starts. On really old
furnaces with standing pilots, this simply meant that the gas valve opened and the gas was ignited
by the pilot flame.
On newer furnaces (built in the last 20+ years) the igntion system
either used a spark or a glow coil to ignite the gas. Some systems would first spark
and ignite a pilot, then the pilot would ignite the main burner.
Other systems would use a glow coil or hot surface igniter to directly light the main burner. In
all cases, there is an electronic module that "supervises" the system and determines whether
ignition was successful and whether the furnace should continue to operate.
Hot Surface Igniter
Once the main burner was lit and producing heat, the furnace's
main blower would kick on. (The blower would be energized by either a temperature
control or a timer that would wait a pre-selected period of time before bringing the blower on. The
idea was to have the furnace warm up to reduce the amount of cold air blown into the home when the
furnace first started.)
When the blower is running, it produces enough force to push air
through the furnace, through the air-conditioning coil and out through the duct work.
This "force" surrounds the outside of the heat exchanger and is always putting air pressure on the
outer cabinet and on the heat exchanger shell, from the outside of the heat exchanger.
Because of this pressure, any hole or crack in the heat exchanger will allow air INTO the heat
exhanger. The air may blow the main burner flame around or cause it to roll out of the front of the
heat exchanger. But, there's no chance that air from inside the exchanger can ever make its way
into the home's indoor air stream. (That would defy the laws of physics and common sense.)
Furnace Indoor Blower
Some mechanics have argued that the speed of the air moving across the heat exchanger would
create a "venturi" effect and draw products of combustion from inside the heat exchanger into the
blower air stream. That might be a reasonable theory if there were no a-coil sitting on the furnace
and the duct work was oversized. Under those conditions there would be very little resistance to
air flow through the furnace and the venturi effect might work.
This is what the venturi effect looks like
in a stove-top burner. As raw gas shoots into the burner tube, it
draws in primary air around the gas orifice. The primary air and gas mixture combine with secondary air
being drawn in around the burner. The venturi effect depends on the velocity of the gas moving through
the burner tube.
However, anytime there is an a-coil in the discharge air stream, the venturi
theory falls apart. That's because the a-coil converts some of the velocity pressure to static pressure inside the
furnace cabinet. When that happens, the inside of the furnace is pressurized with respect to the inside of the heat
exchanger so there is no chance for the venturi effect to occur. Air is always pushed into the
heat exchanger whenever the blower is running. It doesn't matter where the crack is, or its shape;
when the blower is on, air gets pushed into the heat exchanger through any crack or hole.
The test most-often used to determine if there's a problem with the clam-shell or
sectional heat exchanger is called a "candle test". Basically they slowly move a lit candle inside the heat
exchanger and watch for the flame to be blown around or actually get blown out. If the flame waivers or is blown
out, they say the heat exchanger has a "flame disturbance" and needs to be replaced.
When you ask "well is it dangerous"? They affirm that you
could definitely have a problem with carbon monoxide entering the house. Congratulations
... you've just been "sold".
Because you have a "bad" heat exchanger ... you're going to buy something. Might
be a new furnace, or just a heat exchanger (labor extra, of course), but you're going to have to
The question is, what is the
definition of "bad"?
Is it a small crack or hole that doesn't affect operation? Or, is it a crack that
may open up to become a bigger crack that causes the burner flames to roll out the front of the
Heat Exchanger Cut-a-way Drawing
On the Other Hand, here is a tubular heat exchanger problem that
no visual inspection, even a camera on stick, would detect.
This kind of "heat exchanger breach" slips past almost everyone. It never causes an issue, though, because it has very little effect on equipment operation.
The furnace operated, had NO rollout or other symptoms, except for slightly low draft as measured at the draft
inducer. The pressure switch stayed closed when the draft inducer was running, so the furnace stayed on. The
furnace was replaced when the air-conditioner had to be replaced. That's when the heat exchanger problem was
Keep in mind, this isn't just a problem with Rheem, almost all manufacturers have produced tubular heat
exchangers where this issue showed up.
Magnehelic Air Pressure Meter
This situation would have been uncovered when watching combustion analyser readings as the blower
came on. Using a combustion analyzer gives a much clearer picture of how the furnace is operating. It's another
reason why you need to call a reputable heating-cooling contractor.
don't shop for clean & check services by price.
Generally, you'll get the opposite of what you want. You'll get
the guy chasing little cracks versus someone who has the knowhow and test equipment to really check for
safety issues. The guy chasing cracks needs to find problems and "replace parts" to make
Here's where it starts getting
Most gas utility companies have a standing policy (some actually written into
local ordinances) that require service mechanics to "red-tag" and shut-down furnaces with a heat exchanger crack. To avoid potential liability,
most service companies have adopted similar policies.
In thirty years of business, I've never seen or heard of a utility employee or a
service company mechanic using a calibrated CO
tester to see if the furnace in question posed any kind of threat. They simply state that having the
crack is bad enough, and shut-off the family's heat!
In many cases, the utility company will come out with a standard environmental CO
meter designed to measure CO concentrations in a room. Once they establish that the CO level is indeed
elevated, they'll tell the homeowner to contact a private service company, because there is a problem with
the furnace. They never usually ask about whether the stove or oven is on, or whether there are other gas
appliances running in the house. Basically, the utility performs a cursory glance at the furnace, water
heater and flue or chimney, and then bows out.
In areas of the country where the utility service department also sells equipment and does
installations, it's a different story. They may condemn and red-tag the furnace, sell the homeowner a new furnace
and actually do the installation. In this case, the homeowner relies totally on one organization. In either case,
the family still is forced to endure a cold home with their heat shut off simply because of a minor
I've been on dozens of service calls where the gas utility told the customer to
replace their furnace because of a crack in the heat exchanger that had NOTHING to do with the high CO levels
in the house. In most cases, elevated CO was tracked down to the use of attic or window fans at inappropriate
times, limited combustion air, or blocked flues or flue caps or other appliances that had been operating in
A cracked heat exchanger
needs to be addressed.
It is usually an indicator of more trouble in the future, and should NOT be taken lightly. If it is a small hole
or crack, then it is a minor issue that needs to be corrected - generally by replacing the furnace or heat
exchanger. Some holes or small openings may have been there for years. Other splits or cracks, especially big ones
that blow the flame around and out the front of the heat exchanger, mean the equipment or heat exchanger should be
If you want to give your service
technician a bad day, ask him how the carbon monoxide is supposed to get through the hole or crack in the heat
exchanger if the blower is pushing air around and over the heat exchanger and that's what he's using to check its
integrity. The same blower presure that pushes air through the duct work and out the registers also pushes against
the OUTSIDE of the heat exchanger and forces air into it!
This is why
heat exchangers need to be checked.
When the blower runs, it pushes air INTO the heat exchanger
and causes the burner flames to rollout, as shown here.
Note that small cracks or holes do NOT cause rollout.
They may cause the burner flames to move around a little,
but they do not have an appreciable affect on combustion.
In many cases the cracks have no effect on the burner flames or combustion.
Here's a video that shows what a small rollout looks
like. The problem is caused by a heat exchanger that has split open between the halves of the clam
shell. When the blower comes on, it forces air into those splits and causes the burner flames to rollout into the
front of the heat exchanger.
Accidental Proof - Here's what a heat exchanger looks like. It is worth watching this 10 minute
This video is by a home inspector and clearly shows how a heat exchanger is positioned in a furnace
and how all the hoopla over hairline cracks is bogus. The intent of the video was to show home
inspectors that there's no way they can reliably identify heat exchanger cracks.
What he really does, is show in great detail that a hairline crack is convenient
reason to condemn equipment and make a sale. He doesn't say that, but it is plainly evident in the
video. The crack location, in this case, is in a position that would not affect combustion or the burner
flames. It is a "fatigue" crack that probably shows up in many heat exchangers. Two or three years
after the heat exchanger is replaced, it is probable that a crack will appear in the same location. I imagine
that the techician that spotted the crack had to lift the air-conditioning coil off the furnace and used a remote
camera to even see it.
A small crack or hole that is NOT causing a rollout or
other problems is NO reason to put a family in the cold, risk water
pipe freeze ups, and cause stress over a situation that may have existed for five or ten years and never caused a
problem! A pencil size hole or hairline crack in a heat exchanger does NOT, in my opinion, justify shutting off the
heat on someone in the the middle of winter.
In years past, with older furnaces, a heat exchanger crack or small slit could turn into a big,
gaping split once the heat exchanger warmed up. At that point the blower could force the burner flame out the front
of the furnace and cause a fire or burn up wiring. A heat exchanger hole could also blow out a pilot or keep a row
or two of burners from igniting and cause an explosion. These are big openings that cause the problems, not a
little quarter-inch hole or two-inch crack.
Service mechanics will always argue that there's no guarantee that the split or crack won't widen during
operation and that is why it is not safe to leave it running. That's true, but it is also true that a split or
crack could open up at any time when the furnace it running. They could have checked the heat exchanger, not
discovered any problems, and the crack may develop before the mechanic gets to the end of the driveway.
I don't have firm numbers to back up this next statement, so this is just a guess.
But in my experience looking at furnaces, I'd guess that at least 10% of all the gas furnaces in
operation today have heat exchanger cracks. Most go unnoticed because they don't affect operation and do NOT
cause increases in carbon monoxide formation.
If you have a heat exchanger crack, you need to replace the heat exchanger or the furnace. This is
a requirement, is part of the Fuel Gas Codes and is ultimately not negotiable.
My concern is the fact that mechanics walk past other potential
carbon monoxide poisoning issues to get to heat exchanger inspection. Then, when they find a minor
crack, they immediately shut down the furnace with a disregard for the circumstances, outside
temperature or needs of the homeowner.
High Efficiency Furnace - Cautionary Note
This entire discussion about heat exchangers applies to residential steel, sectional and
tubular primary heat exchangers.
It does NOT apply to the secondary heat exchangers in high efficiency (90%+ AFUE) furnaces
or the Lennox Pulse furnace or commercial equipment. When a secondary heat exchanger develops a leak or crack,
carbon monoxide is a possibility.
If you have a 90%+ furnace, make sure your service contractor has a calibrated carbon monoxide meter, knows how
to use it, and will test your furnace with it. There is some kind of "interaction" between the secondary and
primary heat exchangers that creates turbulance that somehow negates the static pressure around the secondary heat
exchanger. This allows carbon monoxide to exit the secondary and get picked up in the indoor air stream, I
What I know for sure is that secondary heat exchangers that have cracks or leaks and are partially or fully
blocked will cause a foul rotten egg smell at the exhaust pipe. Carbon monoxide may also detected around the heat
exchanger in the bottom of the furnace. If you smell sulphur or rotten eggs at the furnace exhaust, have your
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