Cracked heat exchangers are not just expensive - they can be dangerous as hell. First, background. A gas-fired furnace (oil, too) has two different air-handling systems. The one most people think about is the blower that moves that wonderful warm air through the house on days where the temps, like today, are in the single digits.
The second system is the air supply for the burning of the gas. The furnace needs this air for combustion. The resulting flue gases are a mix of the air, minus a goodly portion of oxygen, and the fuel, minus a goodly portion of the chemical energy. That's where the heat comes from.
To get it into the house, we need to move the heat from the combustion cycle to the air-handling equipment. That's the role that the heat exchanger plays. It's a shell of metal that contains the noxious gases of combustion. The walls of the exchanger heat up and transfer the heat to the air driven by the blower.
The old furnaces from the 1960's and early 70's had thick shells and some of those old Lennox's can be hard to kill with anything short of a bazooka. Newer furnaces, especially the ultra-high efficiency condensing furnaces, don't last nearly as long. The new ones have much thinner shells and, to create greater surface area for the heat transfer, lots of dimples in the surface. Consequently, they tend to suffer metal fatigue faster than the old furnaces. Failures in heat exchangers have been reported in units that are only 15 years old.
Back to the noxious gases. What happens when the heat exchanger cracks? Well, we no longer have a separation between the gases and our interior air, the atmosphere that we breathe. The flue gases escape into the living spaces. The principal gas? Carbon monoxide.
The reading on my handy little toy there indicates that the furnace is releasing 16 parts per million of carbon monoxide. As a reference, at levels above 9 parts per million, carbon monoxide begins to affect health. At 35 parts per million, it becomes toxic. These are based on 8 hour exposure cycles, about the amount of time you spend (hopefully) sleeping.
There are a couple of things that you can do to protect yourself. First, have the furnace serviced on a regular basis and make sure you specify that the heat exchanger is inspected for cracks. While you would think that it would be routine, my experience in this area is that it isn't.
Second, install carbon monoxide detectors. They are relatively inexpensive. In older homes, it is perfectly acceptable to use either the plug-in type or ceiling mounted battery operated detectors. In newer homes in Washington State, they are required to be hard-wired into the home.
A special note for people selling homes in Washington State - you are legally required to furnish the home with detectors when you sell your home. This applies even if you are doing it as a for-sale-by-owner (FSBO).
If you are a buyer, I strongly recommend asking the inspector if he uses a carbon monoxide detector during his inspection. They're not foolproof - even if I get acceptable readings on mine, I'll request the heat exchanger be checked by an HVAC specialist - but they are very useful. A good inspector should have one.
Finally, what can be done about cracked heat exchangers? Replacement of the furnace, generally. That's the part that is expensive. It beats being one of the 500 people who die every year from CO poisoning, though.