Have you heard this one recently, “You have a cracked heat exchanger and your furnace has to be replaced, today.” Unless you are one of the top 3% of people who have all the money, replacing the furnace is going to hurt the pocket book if you have not planed for it, but it has to be done.
The heat exchanger, transfers heat from the hot flue gas to your home’s indoor air through the surfaces of the heat exchanger. If the heat exchanger has failed then it will leak those flue gases into our house. Every time the furnace heats and cools the heat exchanger will expand and contract. Moving the metal surfaces in and out every time. This eventually leads to metal fatigue and can split open the metal. Just like when you take a piece of metal and bend it back and forth several times, it will eventually crack and break off.
Corrosion can also cause the heat exchanger to leak. The corrosion normally comes from the flue gases when allowed to cool and condense inside the heat exchanger. The condensate is very corrosive to metal from the carbonic acids. Rust can work its way through the metal surfaces from the interior out. Standing pilot flames a known to cause interior rusting because the gases from the flame condense inside the heat exchanger. During the summer if the pilot is left on the pilot will rust out the furnace. The pilot does not get hot enough to dry out the heat exchanger and the main flame does not come until the cooler weather. Small rust hole can be very hard to find but they can leak large amounts of gas. Testing the heat exchanger with sulphur or a smoke bomb can relieve leaks in the heat exchanger that cannot be seen from an external inspection.
The hot flue gas products of combustion are mostly harmless. Natural gas and propane produce mostly CO2, nitrogen and water when burning correctly. These are harmless unless they build up. CO2 level up to 2500 PPM may cause a headache but are not toxic. Most burner even in good running order will produce small amounts of CO and NOX. CO (Carbon Monoxide) “The Silent Killer” is toxic. If the burners are rusted or dirty and not burning properly it can produce lethal levels of CO 2000PPM+ in the flue gas. Even when the burners are in good shape, the flue gas contains NOX, which affects your lungs in very small amounts, 1/2 PPM or 500 PPB.
That is why the furnace must be shut off if the heat exchanger has failed….