Myth #1  -  A furnace with a cracked heat exchanger will definitely produce carbon monoxide and poses an immediate danger.   (Wrong!)

There's no "definitely" about it. 

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.

What usually happens in the worst case, is the burner flame inside the heat exchanger may 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 slightly.  But, and here's the important part, 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.

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 small hole or crack is simply repeating what they heard someone else say.  Their information is not based on fact.

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 buy something.

Here's where it starts getting ugly.

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 problem.

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 the home.

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, mean the equipment or heat exchanger should be replaced immediately.

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 presure that pushes air through the duct work out the registers also pushes against the OUTSIDE of the heat exchanger and forces air into it! 

But, there's 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 in a heat exchanger does NOT 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.

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.