Cracks in heat exchangers are an often overlooked and misdiagnosed problem. Below is a description of some basic problems associated with cracked heat exchangers and reasons why they can be cracked and not condemned for repair or replacement. With the design of the heat exchangers many cracks are almost impossible to find with the naked eye. Removal of ductwork and/or blower assembly from the furnace would usually be required to visually locate a crack. Symptoms will not always be apparent even if a crack is visible.
If the oil burner is burning clean and is not being affected by the leak i.e. excess or lack of air than it is possible to have no noticeable signs of a crack. However, it is possible to have a crack that can fall or split open dramatically. I have seen an entire "chunk" of sheet metal which started as "spider" cracking where multiple fine cracks that do not pass any significant air can meet together and create an opening your fist could fit through in a split second. A failure of this type would be rare, but dramatic and dangerous, capable of burning down a house. On a less dramatic scale realize that a crack starts small and grows. Have you ever seen a windshield with a small crack? It can be there for years and suddenly one day it can spread so fast you can watch it grow! So in reality it would be possible for a heat exchanger to be cracked, have no opening that air can pass through and have no noticeable symptoms.
Once a crack begins to grow longer there is a much greater probability it will open as the metal warps. Now that air passes through the crack, the burner is still likely to start ok with no symptoms. Then when the blower starts, the air from the blower passing by the crack or blowing into the crack can cause either a negative or positive pressure in the combustion chamber. Negative pressure will typically cause symptoms such as soot on supply registers and smells on shut down. Positive pressure will typically cause a loss of draft up the chimney and soot around the furnace barometric damper, inspection doors, etc. and soot is often found in the air filter or on the return filter openings.
I mentioned the burner start-up under normal conditions. Let’s say the filter is dirty or the ducts restricted in some way and the furnace overheats and cycles on the high limit. Now the burner will shut off, leaving the fan running. When the furnace cools down enough to allow the burner to restart; now we have the chance of a significant problem to show up! Negative pressure could cause delayed ignition, with oil smells and oil mist could even be drawn into the ductwork! Positive pressure could cause delayed ignition and a loud bang or a long sooty rumbling start-up.
The first visible sign of a crack or leak between the house air and the combustion chamber would be visible through the inspection port and would be a change in the burner flame when the blower comes on. The change can be very subtle to the eye, but can be detected quite easily with a draft gauge in the breech (over the fire combustion area).
A crack or hole visible in the combustion liner can be mistaken for a heat exchanger crack and should be inspected carefully with a mirror to make sure it is not a replaceable part before condemning a furnace or heat exchanger.
The Trainingman 2005