Cracks In Furnaces
All About Homes by John Schnedier
Q: In order to prepare for the winter, I had my forced air furnace serviced by a repairman. The unit is probably 30 years old and it seemed to be working fine. However, the repairman stated that there was a crack in the combustion chamber, and the furnace needed to be replaced. When I called PG&E to check the furnace, their guy said that it is fine. I want to be sure I have a safe furnace, but I don’t want to buy a new unit unless it is absolutely necessary. Who should I believe?
— H.J., Moraga
A: The crack that you are speaking of can be very serious and is one that occurs in the combustion chamber of the furnace. The combustion chamber is the metal box that surrounds the gas burners serving the furnace. When the furnace is turned on, the burners light, creating flames that heat the chamber. The top of the chamber is opened so that the hot gasses from the burning process can be vented directly to the exterior through the metal flue pipe. The walls of the combustion chamber keep the flue gasses from entering the house, or the air stream that blows into the interior rooms.
When a furnace is turned on, the flames from the gas burners heat the walls of the combustion chamber. This heating causes the metal to expand. When the furnace shuts off, the metal cools down, allowing the metal to contract. This constant expansion and contraction of the metal over the years causes it to fatigue and sometimes crack. The average life of most combustion chambers (before they crack or otherwise fail) is 18-22 years.
The danger of a cracked combustion chamber is that it can allow flue gasses to enter the house presenting a toxic hazard to the occupants. This condition is most easily illustrated using a forced air furnace as an example. When you turn on a forced air furnace, the gas burners ignite and the flames begin to heat the chamber. Once the chamber reaches a specific temperature (usually between 120 to 150 degrees), the blower turns on and draws air from inside the house, and blows it past the outside of the combustion chamber. This is where the air picks up heat and carries it to the rooms of the house.
Depending upon the location of a crack in a combustion chamber, it can allow flue gasses to mix with the interior air and be circulated throughout the house. The main concern with this happening is that the occupants of the house can inhale carbon monoxide. A crack near the burner can allow air to blow into the combustion chamber where the flame is burning. This can cause the flame to “roll out” of the protected chamber and onto the face of the furnace, burning the controls or anything combustible. This presents a fire and electrical hazard.
Any deflection or unusual movement of the burner flame is physical evidence of an unsafe condition. If the air entering the combustion chamber through the crack were to temporarily blow out the flame, unburnt gas could rise to the top of the chamber, ignite and cause an explosion. Whether or not there is flame deflection, any crack in a combustion chamber should be evaluated for safety.
In your particular case, you first need to determine whether or not there actually is a crack in the combustion chamber. If your repairman saw a crack, did the PG&E representative see it? Is it possible that either PG&E missed seeing the crack, or that there is no crack present. At this point the most important thing is to have the two inspectors meet at the property and re-inspect the furnace. I’m sure that both individuals will be more than willing to establish its true condition.
If there is a crack, then there is reason for concern. However, this does not always guarantee that PG&E will red tag the furnace. Sometimes a crack will not present an immediate danger. If the crack is tight and does not let air through the combustion chamber, PG&E will not red tag and disconnect the furnace.
This is because their primary concern is the safe operation of the unit at the time of their inspection. They are not in position to disconnect a consumer’s furnace unless there is current danger present. And without flame deflection, it is harder to establish the immediate danger of operating the furnace with a crack.
Your repairman is conducting his inspection under different circumstances. He is required to provide any information that might affect the safe operation of the furnace to a consumer.
A crack in the combustion chamber will eventually open and get worse with continued use of the furnace, creating a hazard at any future time. Failure to report this kind of information would put a lot of liability on the repairman.
Based on the age of your furnace, it is a good candidate for a cracked chamber. Unfortunately, a cracked chamber cannot be repaired or welded; it must be replaced. If a replacement chamber cannot be obtained, then an entire new furnace will have to be purchased.
The truth is that your furnace lasted much longer than the industry average. The one good thing about a new furnace is that it will be more energy efficient, and effectively lower your monthly energy bills.