What Are the Causes of the Firewall to Crack in a Furnace?

A furnace's firewall prevents it from overheating the area around it.

A firewall is a barrier that is intended to isolate an area of high temperatures. Furnace firewalls in the home include firewalls that form the exterior of a furnace's case and walls that isolate the furnace from other rooms or separate rooms from one another. A firewall can crack occur for a number of reasons and it needs repair or replacement before the furnace is safe to use again.

Improper Installation

A furnace firewall can crack during or shortly after installation if it is installed improperly. This can occur if the firewall is placed in the wrong position so that its heat-resistant surface doesn't line up with the furnace's heat exchanger or flame, where the heat is most intense. Installing a free-standing firewall without using the right materials and procedures can make it subject to cracking, especially if the wall is made to support more weight than it is capable of holding.

Physical Damage

A furnace firewall's cracks may come from physical damage that has nothing to do with the furnace's performance or temperature. For example, a furnace in a basement can suffer firewall cracks if objects fall against it during an accident or shelf collapse. Accidentally striking a furnace's firewall with a tool or stored items can also cause a crack to develop or create a weak spot that is prone to cracking in the future.

Material Failure

Furnace firewalls are made of heat-resistant materials that are specifically designed to withstand high temperatures. However, some firewall materials are not suitable for all applications. Using the wrong materials can result in a firewall that cracks as a furnace cycles on and off, causing the wall's material to expand and contract continuously. Concrete products, metals and synthetic fibers that go into furnace firewalls all have maximum heat tolerances; installers need to be aware of these so they select firewalls that won't crack.

Inspection and Repair

Cracked furnace firewalls may show few symptoms of damage until they allow a fire to spread out of the furnace's casing or they damage items in the surrounding area. Inspect furnaces and interior firewalls regularly for cracks, which are among the first indicators of a problem. The beginning and end of the heating season are good times to inspect the furnace and all firewalls. A cracked furnace firewall may require replacement to offer sufficient protection. Repairing a portion of a cracked wall or patching the crack is only an option if the crack isn't an integral part of the wall's heat-resistant surface. Cracked flashing, flooring or roofing around a firewall is replaceable when the wall itself is intact.

References

• PDH Online: Understanding Firewall Basics
• Advanced Fiber Technologies: AFT Fire Wall Testing Procedure