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.  

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.

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 assume that 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.  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.  That's OK, except for the "red tag" policy. 

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.

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.  But, there's NO reason to put a family in the cold, risk water pipe freeze ups, and cause a fair amount of stress over a situation that may have existed for five or ten years and never caused a problem!

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 carbon monoxide formation.

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