Cracked Fireboxes. Causes and Solutions!
What is a firebox or what is a heat exchanger? These two different words are nomenclature meaning the thin layer of metal that separates the good air that you breathe and the bad air comes from burning natural gas in a forced air heater or burning propane in a forced air heater. When natural gas is burning, it emits carbon dioxide, water vapor, carbon monoxide, nitrogen oxides and sulfur dioxide. While carbon dioxide (CO2) and water (H2O) not harmful to us carbon monoxide (CO), nitrogen oxides(NO, NO2, etc.) and sulfur dioxide(SO2) are. During the burning process, the firebox contains these poisons and steers them towards the vent, which takes them up and out the roof safely away from you and your family. About 170 people a year die from non-automobile related carbon monoxide poisoning. For safety facts and carbon monoxide facts from the Consumer Product Safety Commission click on this link for a pdf.
Flames and flame patterns are important for furnace safety
-1. The tiki torch flame
This is carbonizing flame which can soot up the firebox and vent through the roof. Left unchecked it can cause flue fires.
-2. Too much fuel
This flames creates flames the whip up into the firebox and can touch the metal spot overheating it. This flames burns holes in fireboxes.
-3. Too much oxygen
This can cause overheating the same as too much fuel.
-4. The perfect blue flame
This is what the furnace is designed to run on.
Not all fireboxes are 100% metal. The Carrier corporation had an idea to presumably save money by bonding metal fireboxes (there are usually several different distinct fireboxes in each furnace) and bond them together using a polymer (a type of plastic like substance). The polymer rapidly broke down in many furnaces and opened them up to a Class Action lawsuit in which somewhere in the neighborhood of $10,000,000 exchanged hands. Carrier denied any wrongdoing and any defective construction, but you can judge the exchange of $10,000,000 anyway you want.
Heat exchangers corrode from the chemicals in the emissions of the burning gases. The water moisture present tends to corrode those fireboxes not protected by coating. American Standard and Trane both have heat exchangers that are steel coated with aluminum. This certainly slows down the rusting (hence the name of the web site “the Rusty Firebox” and gives a greater lifetime of usability to these protected fireboxes than to ordinary run of the mill tubular or stamped steel fireboxes.
The number one reason for firebox failure is age. A furnace can cycle on and off upwards of 10,000 times a year. That means that the firebox goes from dead cold (lets take a temperature of 70 degrees Fahrenheit) and is subjected to the natural gas flames which burn at 900 to 1500 degrees Fahrenheit. As the firebox heats up it warms the air on the outside of the firebox, the air that you breathe, and this triggers the fan which turns on from temperature, time or a combination of time and temperate. Most furnaces run at about 140 degrees air temperature.
The heat exchanger is subjected to a temperature rise of about 900 degrees in minutes and does so upwards of 10,000 times year. This rapid increase in temperature subjects the metal in the box to expansion and corresponding contraction. This expansion and contraction wears the heat exchanger out making it brittle and eventually the metal of the heat exchanger cracks. The crack cannot be welded or repaired. It must be replaced as any welding will simply fail because of the expansion and contraction and the brittle nature of the much worn metal of the cracked heat exchanger.
The number two reason for heat exchanger failure is rust. Older furnaces have pilot lights. If you have a forced air furnace with a pilot light, you have a very old furnace and it should be replaced because of safety concerns and efficiency. The money that you will save on your heating bill, bear in mind that the longer your furnace operates the higher your electric bill will be as well, will end up paying for the new furnace.
Those pilot lights emit the carbon monoxide, nitrogen oxides, sulfur dioxide and water vapor just like their bigger cousin the main burners. This constant supply of moisture and acidic gas eats away at the firebox just above the flame and causes a rusted through section that can allow the products of combustion into the air that you and your family breathe.